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  • 1. Andersen, S. J.
    et al.
    Witha, B.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Sørensen, Jens Nørkær
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Mikkelsen, R. F.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Quantifying variability of Large Eddy Simulations of very large wind farms2015Inngår i: Wake Conference 2015, 2015, s. 012027-, artikkel-id 012027Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Large Eddy Simulations are inherently dynamic as the largest scales are resolved and the smallest scales are modeled temporally. This raises challenges for simulations including very large scales such as atmospheric flows, which require very long simulation times. Simple averages fail at capturing these dynamics and potentially yield misleading interpretations concerning the capabilities of different models when tested in blind tests or in benchmarking exercises such as Wakebench, where results from different flow models are compared. This article will present results from very large wind farm simulations using Actuator Disc (AD) and Line (AL) models for two different turbine spacings with turbulent inflow. The results of each numerical flow model include a certain variability, and it will be examined if different models result in comparable probability distributions.

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  • 2.
    Andersen, S.J.
    et al.
    Technical University of Denmark.
    Sørensen, Jens
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Technical University of Denmark.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Mikkelsen, Robert
    Technical University of Denmark.
    Comparison of Engineering Wake Models with CFD Simulations2014Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 524, s. 012161-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The engineering wake models by Jensen [1] and Frandsen et al. [2] are assessed for different scenarios simulated using Large Eddy Simulation and the Actuator Line method implemented in the Navier-Stokes equations. The scenarios include the far wake behind a single wind turbine, a long row of turbines in an atmospheric boundary layer, idealised cases of an infinitely long row of wind turbines and infinite wind farms with three different spacings. Both models include a wake expansion factor, which is calibrated to fit the simulated wake velocities. The analysis highlights physical deficiencies in the ability of the models to universally predict the wake velocities, as the expansion factor can be fitted for a given case, but with not apparent transition between the cases. 1.

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  • 3.
    Andersen, Soren Juhl
    et al.
    Tech Univ Denmark, DTU Wind Energy, DK-2800 Lyngby, Denmark.
    Sorensen, Jens Norkaer
    Tech Univ Denmark, DTU Wind Energy, DK-2800 Lyngby, Denmark.
    Mikkelsen, Robert
    Tech Univ Denmark, DTU Wind Energy, DK-2800 Lyngby, Denmark.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Statistics of LES Simulations of Large Wind Farms2016Inngår i: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016), IOP PUBLISHING LTD , 2016, artikkel-id 032002Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Numerous large eddy simulations are performed of large wind farms using the actuator line method, which has been fully coupled to the aero-elastic code, Flex5. The higher order moments of the flow field inside large wind farms is examined in order to determine a representative reference velocity. The statistical moments appear to collapse and hence the turbulence inside large wind farms can potentially be scaled accordingly. The thrust coefficient is estimated by two different reference velocities and the generic C-T expression by Frandsen. A reference velocity derived from the power production is shown to give very good agreement and furthermore enables the very good estimation of the thrust force using only the steady C-T-curve, even for very short time samples. Finally, the effective turbulence inside large wind farms and the equivalent loads are examined.

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  • 4.
    Arnqvist, Johan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Olivares-Espinosa, Hugo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Investigation of Turbulence Accuracy When Modeling Wind in Realistic Forests Using LES2019Inngår i: Progress In Turbulence Viii / [ed] Orlu, R Talamelli, A Peinke, J Oberlack, M, SPRINGER INTERNATIONAL PUBLISHING AG , 2019, s. 291-296Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This study presents an evaluation of wind field simulations, in neutral atmospheric conditions, above a heterogeneous forest. The calculations were performed with Large-Eddy Simulation (LES) code OpenFOAM, with explicit modelling of the forest through drag coefficient and forest density. The findings indicate that a large modelling domain is needed in order to reproduce the measurements in different wind directions, since the effect of far upwind forest characteristics influence the wind and turbulence profiles. It is further shown that even though the low resolution of the LES simulations lead to slightly misrepresented single point turbulence characteristics, two point turbulence characteristics are well predicted due to spatial filtering of the small scales.

  • 5.
    Asmuth, Henrik
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Janßen, Christian F.
    Institute for Fluid Dynamics and Ship Theory, Hamburg University of Technology, Am Schwarzenberg-Campus 4, 21073 Hamburg, Germany.
    Olivares-Espinosa, Hugo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Wall-modeled lattice Boltzmann large-eddy simulation of neutral atmospheric boundary layers2021Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 33, nr 10, s. 105111-105111Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The lattice Boltzmann method (LBM) sees a growing popularity in the field of atmospheric sciences and wind energy, largely due to itsexcellent computational performance. Still, LBM large-eddy simulation (LES) studies of canonical atmospheric boundary layer flows remainlimited. One reason for this is the early stage of development of LBM-specific wall models. In this work, we discuss LBM–LES of isothermalpressure-driven rough-wall boundary layers using a cumulant collision model. To that end, we also present a novel wall modeling approach,referred to as inverse momentum exchange method (iMEM). The iMEM enforces a wall shear stress at the off-wall grid points by adjustingthe slip velocity in bounce-back boundary schemes. In contrast to other methods, the approach does not rely on the eddy viscosity, nor doesit require the reconstruction of distribution functions. Initially, we investigate different aspects of the modeling of the wall shear stress, i.e.,an averaging of the input velocity as well as the wall-normal distance of its sampling location. Particularly, sampling locations above the firstoff-wall node are found to be an effective measure to reduce the occurring log-layer mismatch. Furthermore, we analyze the turbulence statis-tics at different grid resolutions. The results are compared to phenomenological scaling laws, experimental, and numerical references. Theanalysis demonstrates a satisfactory performance of the numerical model, specifically when compared to a well-established mixed pseudo-spectral finite difference (PSFD) solver. Generally, the study underlines the suitability of the LBM and particularly the cumulant LBM forcomputationally efficient LES of wall-modeled boundary layer flows.

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  • 6.
    Asmuth, Henrik
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Janßen, Christian F.
    Hamburg University of Technology, Institute of Fluid Dynamics and Ship Theory, Hamburg .
    Olivares-Espinosa, Hugo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Assessment of Weak Compressibility in Actuator Line Simulations of Wind Turbine Wakes2020Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 1618, artikkel-id 062057Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The trend of increasing rotor diameters and tip-speeds has brought about concerns of non-negligible compressibility effects in wind turbine aerodynamics. The investigation of such effects on wakes is particularly difficult when using actuator line models (ALM). This is because crucial regions of the flow, i.e. the direct vicinity of the blade, are not simulated but represented by body forces. To separately assess the impact of compressibility on the wake and the ALM itself, we conduct large-eddy simulations (LES) where the forces of the ALM are prescribed and based on the local sampled velocity (standard procedure), respectively. The LES are based on the weakly-compressible Lattice Boltzmann Method (LBM). Further to the comparison of (near-)incompressible to compressible simulations we investigate cases with artificially increased compressibility. This is commonly done in weakly-compressible approaches to reduce the computational demand. The investigation with prescribed forces shows that compressibility effects in the wake flow are negligible. Small differences in the wake velocity (of max. 1%) are found to be related to local compressibility effects in the direct vicinity of the ALM. Most significantly, compressibility is found to affect the sampled velocity and thereby accuracy of the ALM.

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  • 7.
    Asmuth, Henrik
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi. Carl von Ossietzky Univ Oldenburg, Inst Phys, ForWind Ctr Wind Energy Res, D-26129 Oldenburg, Germany..
    Korb, Henry
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    How Fast is Fast Enough?: Industry Perspectives on the Use of Large-eddy Simulation in Wind Energy2023Inngår i: WAKE CONFERENCE 2023 / [ed] Bottasso, C Schepers, G Larsen, G Meyers, J Uzol, O Chatelain, P Aubrun, S Leweke, T, Institute of Physics Publishing (IOPP), 2023, Vol. 2505Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The use of graphics processing units (GPUs) has facilitated unprecedented performance gains for computational fluids dynamics in recent years. In many industries this has enabled the integration of large-eddy simulation (LES) in the engineering practice. Flow modelling in the wind industry though still primarily relies on models with significantly lower fidelity. This paper seeks to investigate the reasons why wind energy applications of LES are still an exception in the industrial practice. On that account, we present a survey among industry experts on the matter. The survey shows that the large runtimes and computational costs of LES are still seen as a main obstacle. However, other reasons such as a lack of expertise and user experience, the need for more validation, and lacking trust in the potential benefits of LES reveal that computational efficiency is not the only concern. Lastly, we present an exemplary simulation of a generic offshore wind farm using a GPU-resident Lattice Boltzmann LES framework. The example shows that the runtime requirements stated by a large part of the respondents can already now be fulfilled with reasonable hardware effort.

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  • 8.
    Asmuth, Henrik
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Navarro Diaz, Gonzalo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Madsen, Helge Aagard
    DTU Wind Energy, Technical University of Denmark, Frederiksborgvej 399, 4000, Roskilde, Denmark.
    Branlard, Emmanuel
    National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO, 80401, USA.
    Meyer Forsting, Alexander R.
    DTU Wind Energy, Technical University of Denmark, Frederiksborgvej 399, 4000, Roskilde, Denmark.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Jonkman, Jason
    National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO, 80401, USA.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Wind Turbine Response in Waked Inflow: A Modelling Benchmark Against Full-Scale Measurements2022Inngår i: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 191, s. 868-887Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Predicting the power and loads of wind turbines in waked inflow conditions still presents a major modelling challenge. It requires the accurate modelling of the atmospheric flow conditions, wakes of upstream turbines and the response of the turbine of interest. Rigorous validations of model frameworks against measurements of utility-scale wind turbines in such scenarios remain limited to date. In this study, six models of different fidelity are compared against measurements from the DanAero experiment. The two benchmark cases feature a full-wake and partial-wake scenario, respectively. The simulations are compared against local pressure forces and inflow velocities measured on several blade sections of the downstream turbine, as well as met mast measurements and standard SCADA data. Regardless of the model fidelity, reasonable agreements are found in terms of the wake characteristics and turbine response. For instance, the azimuth variation of the mean aerodynamic forces acting on the blade was captured with a mean relative error of 15–20%. While various model-specific deficiencies could be identified, the study highlights the need for further full-scale measurement campaigns with even more extensive instrumentation. Furthermore, it is concluded that validations should not be limited to integrated and/or time-averaged quantities that conceal characteristic spatial or temporal variations.

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  • 9.
    Asmuth, Henrik
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Olivares-Espinosa, Hugo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Actuator line simulations of wind turbine wakes using the lattice Boltzmann method2020Inngår i: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 5, nr 2, s. 623-645Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The high computational demand of large-eddy simulations (LESs) remains the biggest obstacle for a wider applicability of the method in the field of wind energy. Recent progress of GPU-based (graphics processing unit) lattice Boltzmann frameworks provides significant performance gains alleviating such constraints. The presented work investigates the potential of LES of wind turbine wakes using the cumulant lattice Boltzmann method (CLBM). The wind turbine is represented by the actuator line model (ALM). The implementation is validated and discussed by means of a code-to-code comparison to an established finite-volume Navier–Stokes solver. To this end, the ALM is subjected to both laminar and turbulent inflow while a standard Smagorinsky sub-grid-scale model is employed in the two numerical approaches. The resulting wake characteristics are discussed in terms of the first- and second-order statistics as well the spectra of the turbulence kinetic energy. The near-wake characteristics in laminar inflow are shown to match closely with differences of less than 3 % in the wake deficit. Larger discrepancies are found in the far wake and relate to differences in the point of the laminar-turbulent transition of the wake. In line with other studies, these differences can be attributed to the different orders of accuracy of the two methods. Consistently better agreement is found in turbulent inflow due to the lower impact of the numerical scheme on the wake transition. In summary, the study outlines the feasibility of wind turbine simulations using the CLBM and further validates the presented set-up. Furthermore, it highlights the computational potential of GPU-based LBM implementations for wind energy applications. For the presented cases, near-real-time performance was achieved using a single, off-the-shelf GPU on a local workstation.

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  • 10.
    Asmuth, Henrik
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Olivares-Espinosa, Hugo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    The Actuator Line Model in Lattice Boltzmann Frameworks: Numerical Sensitivity and Computational Performance2019Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 1256, artikkel-id 012022Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The growing use of large-eddy simulations for the modelling of wind farms makes the need for efficient numerical frameworks more essential than ever. GPU-accelerated implementations of the Lattice Boltzmann Method (LBM) have shown to provide significant performance gains over classical Navier-Stokes-based computational fluid dynamics. Yet, their use in the field of wind energy remains limited to date. In this fundamental study the cumulant LBM is scrutinised for actuator line simulations of wind turbines. The numerical sensitivity of the method in a simple uniform inflow is investigated with respect to spatial and temporal resolution as well as the width of the actuator line’s regularisation kernel. Comparable accuracy and slightly better stability properties are shown in relation to a standard Navier-Stokes implementation. The results indicate the overall suitability of the cumulant LBM for wind turbine wake simulations. The potential of the LBM for future wind energy applications is clarified by means of a brief comparison of computational performance.

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  • 11.
    Breton, Simon-Philippe
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nilsson, K.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Olivares-Espinosa, H.
    Masson, C.
    Dufresne, L.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Study of the influence of imposed turbulence on the asymptotic wake deficit in a very long line of wind turbines2014Inngår i: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 70, s. 153-163Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The influence of imposed turbulence on the development of the flow along a long row of wind turbines is studied, in search for an asymptotic wake deficit state. Calculations are performed using EllipSys3D, a CFD code that solves the Navier-Stokes equations in their incompressible form using a finite volume approach. In this code, the Large-Eddy Simulation technique is used for modeling turbulence, and the wind turbine rotors are represented as actuator discs whose loading is determined through the use of tabulated airfoil data by applying the blade-element method. Ten turbines are located along a row and separated from each other by seven rotor diameters, which is representative of the distance used in today's offshore wind farms. Turbulence is pre-generated with the Mann model, with imposed turbulence intensity levels of 4.5% and 8.9%. The aim with this study is to investigate features of the flow that depend solely on imposed turbulence and the presence of wind turbine rotors. For this reason, the turbines are isolated from their environment, and no effect from the presence of the atmospheric boundary layer is modeled, i.e., a non-sheared inflow is used. Analysis of the characteristics of the flow as a function of the position along the row of turbines is performed in terms of standard deviation of the velocity components, turbulence kinetic energy, mean velocity, and power spectra of the axial velocity fluctuations. The mean power production along the row of turbines is also used as an indicator. Calculations are performed below rated power, where a generator torque controller implemented in EllipSys3D renders it possible for the turbines to adapt to the flow conditions in which they operate. The results obtained for the standard deviation of the velocity components, turbulence kinetic energy, power and mean velocity as functions of downstream distance show that an asymptotic wake state seems close to be reached, in the conditions tested, near the end of the 10 turbine row. Significant changes towards this state are seen to happen faster when imposing turbulence in the domain. Power spectra of the axial velocity fluctuations are shown to provide interesting information about the turbulence in the flow, but are found not to be useful in determining if an asymptotic wake state is reached.

  • 12.
    Breton, Simon-Philippe
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Olivares-Espinosa, H.
    Masson, C.
    Dufresne, L.
    Comparative CFD study of the effect of the presence of downstream turbines on upstream ones using a rotational speed control system2014Inngår i: Science of Making Torque from Wind 2012, 2014, Vol. 555Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The effect of a downstream turbine on the production of a turbine located upstream of the latter is studied in this work. This is done through the use of two CFD simulation codes, namely OpenFOAM and EllipSys3D, which solve the Navier-Stokes equations in their incompressible form using a finite volume approach. In both EllipSys3D and Open Foam, the LES (Large Eddy Simulation) technique is used for modelling turbulence. The wind turbine rotors are modelled as actuator disks whose loading is determined through the use of tabulated airfoil data by applying the blade-element method. A generator torque controller is used in both simulation methods to ensure that the simulated turbines adapt, in terms of rotational velocity, to the inflow conditions they are submited to. Results from both simulation codes, although they differ slightly, show that the downstream turbine affects the upstream one when the spacing between the turbines is small. This is also suggested to be the case looking at measurements performed at the Lillgrund offshore wind farm, whose turbines are located unusually close to each other. However, for distances used in today's typical wind farms, this effect is shown by our calculations not to be significant.

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  • 13.
    Breton, Simon-Philippe
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Olivares-Espinosa, Hugo
    École de technologie supérieure, Department of Mechanical Engineering, Montréal, Canada.
    Masson, Christian
    École de technologie supérieure, Department of Mechanical Engineering, Montréal, Canada.
    Dufresne, Louis
    École de technologie supérieure, Department of Mechanical Engineering, Montréal, Canada.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Study Of The Influence Of Atmospheric turbulence On The Asymptotic wake Deficit In A very Long Line Of Wind Turbines2013Inngår i: Proceedings of the 2013 International Conference on Aerodynamics of Offshore Wind Energy Systems and Wakes (ICOWES 2013), Denmark, 2013, s. 420-434Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The influence of atmospheric turbulence on the development of the flow along a long row of wind turbines is studied, in search for an asymptotic wake deficit state. Calculations are performed using EllipSys3D, a CFD code that solves the Navier-Stokes equations in their incompressible form using a finite volume approach. In this code, the Large Eddy Simulation technique is used for modelling turbulence, and the wind turbine rotors are represented as actuator disks whose loading is determined through the use of tabulated airfoil data by applying the blade-element method.

    Ten turbines are located along a row and separated from each other by seven rotor diameters, which is representative of the distance used in today’s offshore wind farms. Turbulence is pregenerated with the Mann model, with imposed turbulent levels of 4.5 and 8.9%. The turbines are in this study isolated from their environment, as no effect from the ground is modeled. This makes the proposed study of the asymptotic wake state behavior easier. Analysis of the characteristics of the flow as a function of the position along the row of turbines is performed in terms of turbulence intensity, mean velocity, and power spectra of the velocity fluctuations. Power production along the row of turbines is also used as an indicator.

    Calculations are performed below rated power, where a generator torque controller implemented in EllipSys3D renders it possible for the turbines to adapt to the inlet conditions in which they operate.

    The results obtained for the turbulence intensity, power and mean velocity as a function of downstream distance show that an asymptotic wake state seems close to be reached near the end of the 10 turbine row. They also show a certain dependency on the imposed level of turbulence. Uncertainties obtained in the power spectra of the velocity fluctuations suggest that further investigation is necessary.

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  • 14.
    Breton, Simon-Philippe
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Shen, W.Z.
    Technical University of Denmark, Energivej, Building 414, 2800 Kgs. Lyngby, Denmark.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Validation of the actuator disc and actuator line techniques for yawed rotor flows using the New MEXICO experimental data2017Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Experimental data acquired in the New MEXICO experiment on a yawed 4.5m diameter rotor model turbine are used here to validate the actuator line (AL) and actuator disc (AD) models implemented in the Large Eddy Simulation code EllipSys3D in terms of loadingand velocity field. Even without modelling the geometry of the hub and nacelle, the AL and AD models produce similar results that are generally in good agreement with the experimental data under the various configurations considered. As expected, the AL model does better at capturing the induction effects from the individual blade tip vortices, while the AD model can reproduce the averaged features of the flow. The importance of using high quality airfoil data (including 3D corrections) as well as a fine grid resolution is highlighted by the results obtained. Overall, it is found that both models can satisfactorily predict the 3D velocity field and blade loading of the New MEXICO rotor under yawed inflow.

    Fulltekst (pdf)
    fulltext
  • 15.
    Breton, Simon-Philippe
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Sumner, J.
    Dawson Coll, Dept Phys, Montreal, PQ, Canada..
    Sörensen, J. N.
    DTU Wind Energy, Lyngby, Denmark..
    Hansen, K. S.
    DTU Wind Energy, Lyngby, Denmark..
    Sarmast, S.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    A survey of modelling methods for high-fidelity wind farm simulations using large eddy simulation2017Inngår i: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, nr 2091, artikkel-id 20160097Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    Large eddy simulations (LES) of wind farms have the capability to provide valuable and detailed information about the dynamics of wind turbine wakes. For this reason, their use within the wind energy research community is on the rise, spurring the development of new models and methods. This review surveys the most common schemes available to model the rotor, atmospheric conditions and terrain effects within current state-of-the-art LES codes, of which an overview is provided. A summary of the experimental research data available for validation of LES codes within the context of single and multiple wake situations is also supplied. Some typical results for wind turbine and wind farm flows are presented to illustrate best practices for carrying out high-fidelity LES of wind farms under various atmospheric and terrain conditions. This article is part of the themed issue 'Wind energy in complex terrains'.

  • 16.
    Diaz, Gonzalo Pablo Navarro
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Otero, Alejandro Daniel
    Univ Buenos Aires, Fac Ingn, Buenos Aires, Argentina.;Computat Simulat Ctr CSC CONICET, Buenos Aires, Argentina..
    Asmuth, Henrik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Sørensen, Jens Nørkaer
    Tech Univ Denmark, Dept Wind & Energy Syst, Lyngby, Denmark..
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Actuator line model using simplified force calculation methods2023Inngår i: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 8, nr 3, s. 363-382Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    To simulate transient wind turbine wake interaction problems using limited wind turbine data, two new variants of the actuator line technique are proposed in which the rotor blade forces are computed locally using generic load data. The proposed models, which are extensions of the actuator disk force models proposed by Navarro Diaz et al. (2019a) and Sorensen et al. (2020), only demand thrust and power coefficients and the tip speed ratio as input parameters. In the paper the analogy between the actuator disk model (ADM) and the actuator line model (ALM) is shown, and from this a simple methodology to implement local forces in the ALM without the need for knowledge of blade geometry and local airfoil data is derived. Two simplified variants of ALMs are proposed, an analytical one based on Sorensen et al. (2020) and a numerical one based on Navarro Diaz et al. (2019a). The proposed models are compared to the ADM using analogous data, as well as to the classical ALM based on blade element theory, which provides more detailed force distributions by using airfoil data. To evaluate the local force calculation, the analysis of a partial-wake interaction case between two wind turbines is carried out for a uniform laminar inflow and for a turbulent neutral atmospheric boundary layer inflow. The computations are performed using the large eddy simulation facility in Open Source Field Operation and Manipulation (OpenFOAM), including Simulator for Wind Farm Applications (SOWFA) libraries and the reference National Renewable Energy Laboratory (NREL) 5 MW wind turbine as the test case. In the single-turbine case, computed normal and tangential force distributions along the blade showed a very good agreement between the employed models. The two new ALMs exhibited the same distribution as the ALM based on geometry and airfoil data, with minor differences due to the particular tip correction needed in the ALM. For the challenging partially impacted wake case, both the analytical and the numerical approaches manage to correctly capture the force distribution at the different regions of the rotor area, with, however, a consistent overestimation of the normal force outside the wake and an underestimation inside the wake. The analytical approach shows a slightly better performance in wake impact cases compared to the numerical one. As expected, the ALMs gave a much more detailed prediction of the higher-frequency power output fluctuations than the ADM. These promising findings open the possibility to simulate commercial wind farms in transient inflows using the ALM without having to get access to actual wind turbine and airfoil data, which in most cases are restricted due to confidentiality.

    Fulltekst (pdf)
    FULLTEXT01
  • 17.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Baltscheffsky, M.
    WeatherTech Scandinavia AB, Uppsala, Sweden.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Söderberg, S.
    WeatherTech Scandinavia AB, Uppsala, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    The Long distance wake behind Horns Rev I studied using large eddy simulations and a wind turbine parameterization in WRF2017Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 854, artikkel-id 012012Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The aim of the present paper is to obtain a better understanding of long distance wakes generated by wind farms as a first step towards a better understanding of farm to farm interaction. The Horns Rev I (HR) wind farm is considered for this purpose, where comparisons are performed between microscale Large Eddy Simulations (LES) using an Actuator Disc model (ACD), mesoscale simulations in the Weather Research and Forecasting Model (WRF) using a wind turbine parameterization, production data as well as wind measurements in the wind farm wake. The LES is manually set up according to the wind conditions obtained from the mesoscale simulation as a first step towards a meso/microscale coupling.

    The LES using an ACD are performed in the EllipSys3D code. A forced boundary layer (FBL) approach is used to introduce the desired wind shear and the atmospheric turbulence field from the Mann model. The WRF uses a wind turbine parameterization based on momentum sink. To make comparisons with the LESs and the site data possible an idealized setup of WRF is used in this study.

    The case studied here considers a westerly wind direction sector (at hub height) of 270 ± 2.5 degrees and a wind speed of 8 ± 0.5 m/s. For both the simulations and the site data a neutral atmosphere is considered. The simulation results for the relative production as well as the wind speed 2 km and 6 km downstream from the wind farm are compared to site data. Further comparisons between LES and WRF are also performed regarding the wake recovery and expansion.

    The results are also compared to an earlier study of HR using LES as well as an earlier comparison of LES and WRF. Overall the results in this study show a better agreement between LES and WRF as well as better agreement between simulations and site data.

    The procedure of using the profile from WRF as inlet to LES can be seen as a simplified coupling of the models that could be developed further to combine the methods for cases of farm to farm interaction.

    Fulltekst (pdf)
    fulltext
  • 18.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Environment and Climate Change Canada.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Impact of Wind Veer and the Coriolis Force for an Idealized Farm to Farm Interaction Case2019Inngår i: Applied Sciences, E-ISSN 2076-3417, Vol. 9, nr 5, artikkel-id 922Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The impact of the Coriolis force on the long distance wake behind wind farms is investigated using Large Eddy Simulations (LES) combined with a Forced Boundary Layer (FBL) technique. When using the FBL technique any mean wind shear and turbulent fluctuations can be added with body forces. The wind shear can also include the mean wind veer due to the Coriolis force. The variation of the Coriolis force due to local deviations from the mean profile, e.g., from wakes, is not taken into account in the FBL. This can be corrected for with an extra source term in the equations, hereon defined as the Coriolis correction. For a row of 4 turbines it is shown that the inclusion of the wind veer turns the wake to the right, while including the Coriolis correction turns it to the left. When including both wind veer and Coriolis correction the impact of wind veer dominates. For an idealized farm to farm interaction case, two farms of 4 * 4 turbines with 6 km in between, it can be seen that when including wind veer and the Coriolis correction a approximately 3% increase in the relative production for a full wake direction can be seen and only a slightly smaller increase can be seen when including only wind veer. The results indicate that FBL can be used for studies of long distance wakes without including a Coriolis correction but efforts need to be taken to use a wind shear with a correct mean wind veer.

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    fulltext
  • 19.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Byrkjedal, Øyvind
    Kjeller Vindteknikk.
    Lindvall, Johannes
    Kjeller Vindteknikk.
    Simulating wind farms in the Weather Research and Forecast model, resolution sensitivities.2015Konferansepaper (Annet vitenskapelig)
    Fulltekst (pdf)
    fulltext
  • 20.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Lindvall, J.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Wake downstream of the Lillgrund wind farm - A Comparison between LES using the actuator disc method and a Wind farm Parametrization in WRF2015Inngår i: WAKE CONFERENCE 2015, 2015, s. 012028-, artikkel-id 012028Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Simulations of the Lillgrund wind farm (located between Malmo and Copenhagen) are performed using both Large Eddy Simulation (LES) and mesoscale simulations in WRF. The aim is to obtain a better understanding of wakes generated by entire wind farms in order to improve the understanding of farm to farm interactions. The study compares the results from the two used models for the energy production and the wake characteristics downstream of the wind farm. A comparison is also performed with regards to the production data from the Lillgrund wind farm which has been filtered to be comparable to the case used in the simulations. The studied case, based on a prerun in WRF without any wind farm, has an inflow angle of 222 +/- 2.5 deg, a wind speed at hub height of 9.8 m/s and a near neutral atmosphere. A logarithmic wind shear is used in LES and the turbulence intensity is 5.9%. The WRF simulations use a parameterization for wind farms. The wind farm is treated by the model as a sink of the resolved atmospheric momentum. The total energy extraction and the electrical power are respectively proportional to specified thrust and power coefficients. The generated turbulent kinetic energy are the difference between the total and the electrical power. The LES are performed using the EllipSys3D code applying the actuator disc methodology for representing the presence of the rotors. Synthetic atmospheric turbulence is generated with the Mann model. Both the atmospheric turbulence and the wind shear are introduced using body forces. The production was found to be better estimated in LES. WRF show a slightly higher recovery behind the farm. The internal boundary layer is for the compared simulation setups higher in LES while the wake expansion is about the same in both models. The results from the WRF parameterization could potentially be improved by increasing the grid resolution. For farm to farm interaction a combination of the two methods is found to be of interest.

    Fulltekst (pdf)
    fulltext
  • 21.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Mikkelsen, R.
    Hansen, K. S.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Analysis of long distance wakes of Horns Rev I using actuator disc approach2014Inngår i: Science of Making Torque from Wind 2012, 2014Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The wake recovery behind the Horns Rev wind farm is analysed to investigate the applicability of Large Eddy Simulations (LES) in combination with an actuator disc method (ACD) for farm to farm interaction studies. Periodic boundary conditions on the lateral boundaries are used to model the wind farm (as infinitely wide), using only two columns of turbines. The meteorological conditions of the site are taken into account by introducing wind shear and pre-generated synthetic turbulence to the simulation domain using body forces. Simulations are carried out to study the power production and the velocity deficit in the farm wake. The results are compared to the actual power production as well as to wind measurements at 2 km and 6 km behind the wind farm. The simulated power production inside the farm shows an overall good correlation with the real production, but is slightly overpredicted in the most downstream rows. The simulations overpredict the wake recovery, namely the wind velocity, at long distances behind the farm. Further studies are needed before the presented method can be applied for the simulation of long distance wakes. Suggested parameters to be studied are the development of the turbulence downstream in the domain and the impact of the grid resolution.

    Fulltekst (pdf)
    fulltext
  • 22.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Analysis of long distance wakes behind a row of turbines – a parameter study2014Inngår i: Science of Making Torque from Wind, 2014, s. 012152-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Large Eddy Simulations (LES) of the long distance wake behind a row of 10 turbines are conducted to predict wake recovery. The Navier-Stokes solver EllipSys3D is used in combination with the actuator disc concept. Neutral atmospheric conditions are assumed in combination with synthetic turbulence using the Mann method. Both the wind shear profile and turbulence are introduced into the flow field using body forces. Previous simulations using the same simulation method to model the Horns Rev wind farm showed a higher wake recovery at long distances compared to measurements. The current study investigates further the sensitivity to parameters such as the grid resolution, Reynolds number, the turbulence characteristics as well as the impact of using different internal turbine spacings. The clearest impact on the recovery behind the farm could be seen from the background turbulence. The impact of the wind shear on the turbulence level in the domain needs further studies. A lower turbulence level gives lower wake recovery as expected. A lower wake recovery can also be seen for a higher grid resolution. The Reynolds number, apart from when using a very low value, has a small impact on the result. The variation of the internal spacing is seen to have a relatively minor impact on the farm wake recovery.

    Fulltekst (pdf)
    fulltext
  • 23.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Environm & Climate Change Canada, Dorval, PQ, Canada.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Large-eddy simulations of the evolution of imposed turbulence in forced boundary layers in a very long domain2020Inngår i: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 23, nr 6, s. 1482-1493Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The technique of using imposed turbulence in combination with a forced boundary layer in order to model the atmospheric boundary layer is analyzed for a very long domain using large-eddy simulations with different combinations of prescribed velocity profiles and pregenerated turbulence fields based on the Mann model. The ambient flow is first studied in the absence of wind turbines. The velocity profiles undergo a transition throughout the domain with a velocity increase of 10% to 15% close to the ground far downstream in the domain. The turbulence characteristics close to the turbulence plane are, as expected, similar to those of the added Mann turbulence. The turbulence will then undergo a transition throughout the domain to finally reach a balance with the shear profile at a certain downstream distance. This distance is found to depend on the turbulence level of the added Mann turbulence planes. A lower Mann turbulence level generally results in a shorter "balancing" distance. Secondly, a row of 10 turbines is imposed in the simulations at different distances from the plane of turbulence in order to determine how the distance affects wake conditions and power production levels. Our results show that a "balancing" distance is needed between the turbulence plane and the first turbine in the row in order to ensure nonchanging ambient conditions throughout the turbine row. This introduces an increase in the computational costs. The computational cost for the forced boundary technique is normally lower compared with using precursor simulations, for longer domains; however, this needs to be verified further.

    Fulltekst (pdf)
    fulltext
  • 24.
    Eriksson, Ola
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nilsson, Karl
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Large-eddy simulations of wind farm production and long distance wakes2015Inngår i: Wake Conference 2015, 2015, s. 012022-, artikkel-id 012022Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The future development of offshore wind power will include many wind farms built in the same areas. It is known that wind farms produce long distance wakes, which means that we will see more occasions of farm to farm interaction, namely one wind farm operating in the wake of another wind farm. This study investigates how to perform accurate power predictions on large wind farms and how to assess the long distance wakes generated by these farms. The focus of this paper is the production's and wake's sensitivity to the extension of the grid as well as the turbulence when using Large-eddy simulations (LES) with pregenerated Mann turbulence. The aim is to determine an optimal grid which minimizes blockage effects and ensures constant resolution in the entire wake region at the lowest computational cost. The simulations are first performed in the absence of wind turbines in order to assess how the atmospheric turbulence and wind profile are evolving downstream (up to 12,000 m behind the position where the turbulence is imposed). In the second step, 10 turbines are added in the domain (using an actuator disc method) and their production is analyzed alongside the mean velocities in the domain. The blockage effects are tested using grids with different vertical extents. An equidistant region is used in order to ensure high resolution in the wake region. The importance of covering the entire wake structure inside the equidistant region is analyzed by decreasing the size of this region. In this step, the importance of the lateral size of the Mann turbulence box is also analyzed. In the results it can be seen that the flow is acceptably preserved through the empty domain if a larger turbulence box is used. The relative production is increased (due to blockage effects) for the last turbines using a smaller vertical domain, increased for a lower or narrower equidistant region (due to the smearing of the wake in the stretched area) and decreased when using a smaller turbulence box (due to decreased inmixing) The long distance wake behind the row is most impacted by the use of a smaller turbulence box, while the other simulation setups have less influence on these results. In summary, the results show the importance of having relatively large extensions of the domain, large extensions of the equidistant region and especially large extensions of the turbulence box.

    Fulltekst (pdf)
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  • 25.
    Forsting, Alexander R. Meyer
    et al.
    Tech Univ Denmark, DTU Wind & Energy Syst, Frederiksborgvej 399, DK-4000 Roskilde, Denmark..
    Navarro Diaz, Gonzalo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Segalini, Antonio
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Andersen, Soren J.
    Tech Univ Denmark, DTU Wind & Energy Syst, Anker Engelunds Vej 1, DK-2800 Lyngby, Denmark..
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    On the accuracy of predicting wind-farm blockage2023Inngår i: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 214, s. 114-129Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    To assess the uncertainty in blockage quantification, this study proposes a comparison of farm blockage predictions from wind-tunnel experiments, Reynolds Averaged Navier-Stokes based simulations using multiple numerical setups, and analytical models. The influence of the numerical setup is demonstrated to be small if a consistent definition of blockage (able to sort out systematic errors) is used. The effect of domain confinement and turbulence intensity is investigated assessing their range of variability. Different analytical models performed similarly in comparison to the numerical data, demonstrating the best accuracy for realistic spacing between the turbines and supporting their use as reliable engineering tools.

    Fulltekst (pdf)
    fulltext
  • 26.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Aird, Jeanie A.
    Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, USA.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Körnich, Heiner
    Swedish Meteorological and Hydrological Institute, Norrköping, Sweden.
    Barthelmie, Rebecca J.
    Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, USA.
    Pryor, Sara C.
    Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Brief communication: On the definition of the low-level jet2023Inngår i: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 8, nr 11, s. 1651-1658Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Low-level jets (LLJs) are examples of non-logarithmic wind speed profiles affecting wind turbine power production, wake recovery, and structural/aerodynamic loading. However, there is no consensus regarding which definition should be applied for jet identification. In this study we argue that a shear definition is more relevant to wind energy than a falloff definition. The shear definition is demonstrated and validated through the development of a European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis (ERA5) LLJ climatology for six sites. Identification of LLJs and their morphology, frequency, and intensity is critically dependent on the (i) vertical window of data from which LLJs are extracted and (ii) the definition employed.

    Fulltekst (pdf)
    fulltext
  • 27.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Aird, Jeanie
    Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, USA.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Körnich, Heiner
    Swedish Meteorological and Hydrological Institute, Norrköping, Sweden.
    Vakkari, Ville
    Finnish Meteorol Inst, PB 503, FI-00101 Helsinki, Finland.;North West Univ, Chem Resource Beneficiat, Atmospher Chem Res Grp, PB X6001,Potchefstroom Campus, ZA-2520 Potchefstroom, South Africa.
    Barthelmie, Rebecca J
    Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, USA.
    Pryor, Sara C
    Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Machine Learning Methods to Improve Spatial Predictions of Coastal Wind Speed Profiles and Low-Level Jets using Single-Level ERA5 DataManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Observations of the wind speed at heights relevant for wind power are sparse, especially offshore, but with emerging aid from advanced statistical methods, it may be possible to derive information regarding wind profiles using surface observations. In this study, two machine learning (ML) methods are developed for predictions of (1) coastal wind speed profiles and (2) low-level jets (LLJs) at three locations of high relevance to offshore wind energy deployment; the U.S. Northeastern Atlantic Coastal Zone, the North Sea, and the Baltic Sea. The ML models are trained on multiple years of lidar profiles and utilize single-level ERA5 variables as input. The models output spatial predictions of coastal wind speed profiles and LLJ occurrence. A suite of nine ERA5 variables are considered for use in the study due to their physics-based relevance in coastal wind speed profile genesis, and the possibility to observe these variables in real-time via measurements. The wind speed at 10 m a.s.l. and the surface sensible heat flux are shown to have the highest importance for both wind speed profile and LLJ predictions. Wind speed profile predictions output by the ML models exhibit similar root mean squared error (RMSE) with respect to observations as is found for ERA5 output. At typical hub heights, the ML models show lower RMSE than ERA5 indicating approximately 5% RMSE reduction. LLJ identification scores are evaluated using the Symmetric Extremal Dependence Index (SEDI). LLJ predictions from the ML models outperform predictions from ERA5, demonstrating markedly higher SEDIs. However, optimization utilizing the SEDI results in a higher number of false alarms when compared to ERA5.

  • 28.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Arnqvist, Johan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Kornich, Heiner
    Swedish Meteorol & Hydrol Inst, SE-60176 Norrköping, Sweden..
    Vakkari, Ville
    Finnish Meteorol Inst, PB 503, FI-00101 Helsinki, Finland.;North West Univ, Chem Resource Beneficiat, Atmospher Chem Res Grp, PB X6001,Potchefstroom Campus, ZA-2520 Potchefstroom, South Africa..
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Looking for an Offshore Low-Level Jet Champion among Recent Reanalyses: A Tight Race over the Baltic Sea2020Inngår i: Energies, E-ISSN 1996-1073, Vol. 13, nr 14, artikkel-id 3670Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    With an increasing interest in offshore wind energy, focus has been directed towards large semi-enclosed basins such as the Baltic Sea as potential sites to set up wind turbines. The meteorology of this inland sea in particular is strongly affected by the surrounding land, creating mesoscale conditions that are important to take into consideration when planning for new wind farms. This paper presents a comparison between data from four state-of-the-art reanalyses (MERRA2, ERA5, UERRA, NEWA) and observations from LiDAR. The comparison is made for four sites in the Baltic Sea with wind profiles up to 300 m. The findings provide insight into the accuracy of reanalyses for wind resource assessment. In general, the reanalyses underestimate the average wind speed. The average shear is too low in NEWA, while ERA5 and UERRA predominantly overestimate the shear. MERRA2 suffers from insufficient vertical resolution, which limits its usefulness in evaluating the wind profile. It is also shown that low-level jets, a very frequent mesoscale phenomenon in the Baltic Sea during late spring, can appear in a wide range of wind speeds. The observed frequency of low-level jets is best captured by UERRA. In terms of general wind characteristics, ERA5, UERRA, and NEWA are similar, and the best choice depends on the application.

    Fulltekst (pdf)
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  • 29.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Arnqvist, Johan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Nilsson, Erik O.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Shapkalijevski, Metodija
    Swedish Meteorol & Hydrol Inst, Norrköping, Sweden..
    Thomasson, August
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Pettersson, Heidi
    Finnish Meteorol Inst, Helsinki, Finland..
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Classification and properties of non-idealized coastal wind profiles - an observational study2022Inngår i: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 7, nr 3, s. 1183-1207Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28-300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.

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  • 30.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Körnich, Heiner
    Swedish Meteorological and Hydrological Institute, Norrköping, Sweden.
    Vakkari, Ville
    Finnish Meteorological Institute, Helsinki, Finland; Atmospheric Chemistry Research Group, Chemical Resource Beneficiation, North-West University, Potchefstroom, South Africa.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    The smoother the better?: A comparison of six post-processing methods to improve short-term offshore wind power forecasts in the Baltic Sea2021Inngår i: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 6, nr 5, s. 1205-1226Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    With a rapidly increasing capacity of electricity generation from wind power, the demand for accurate power production forecasts is growing. To date, most wind power installations have been onshore and thus most studies on production forecasts have focused on onshore conditions. However, as offshore wind power is becoming increasingly popular it is also important to assess forecast quality in offshore locations. In this study, forecasts from the high-resolution numerical weather prediction model AROME was used to analyze power production forecast performance for an offshore site in the Baltic Sea. To improve the AROME forecasts, six post-processing methods were investigated and their individual performance analyzed in general as well as for different wind speed ranges, boundary layer stratifications, synoptic situations and in low-level jet conditions. In general, AROME performed well in forecasting the power production, but applying smoothing or using a random forest algorithm increased forecast skill. Smoothing the forecast improved the performance at all wind speeds, all stratifications and for all synoptic weather classes, and the random forest method increased the forecast skill during low-level jets. To achieve the best performance, we recommend selecting which method to use based on the forecasted weather conditions. Combining forecasts from neighboring grid points, combining the recent forecast with the forecast from yesterday or applying linear regression to correct the forecast based on earlier performance were not fruitful methods to increase the overall forecast quality.

    Fulltekst (pdf)
    fulltext
  • 31.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Körnich, Heiner
    Swedish Meteorological and Hydrological Institute, Norrköping, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    A Single-Column Method to Identify Sea and Land Breezes in Mesoscale-Resolving NWP Models2023Inngår i: Weather and forecasting, ISSN 0882-8156, E-ISSN 1520-0434, Vol. 38, nr 6, s. 1025-1039Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    One of the most prominent mesoscale phenomena in the coastal zone is the sea-breeze/land-breeze circulation. The pattern and its implications for the weather in coastal areas are well described, and with mesoscale-resolving operational NWP models the circulation can be captured. In this study, a straightforward method to identify sea and land breezes based on the change in wind direction in the column above a grid point on the coastline is presented. The method was tested for southern Sweden using archived output from the HARMONIE-AROME model with promising results, describing both the seasonal and diurnal cycles well. In areas with a complex coastline, such as narrow straits, the concept of the land–sea breeze becomes less clear, and several ways to address this problem for the suggested method are discussed. With an operational index of the sea and land breezes, the forecaster can better understand and express the weather situation and add value for people in the coastal zone. Further, the indices can be used to study systematic biases in the model and to create climatologies of the sea and land breezes.

    Fulltekst (pdf)
    FULLTEXT01
  • 32.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Körnich, Heiner
    Swedish Meteorol & Hydrol Inst, Norrköping, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    A Single-Column Method to Identify Sea and Land Breezes in Mesoscale-Resolving NWP Models2023Inngår i: Weather and forecasting, ISSN 0882-8156, E-ISSN 1520-0434, Vol. 38, nr 6, s. 1025-1039Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    One of the most prominent mesoscale phenomena in the coastal zone is the sea-breeze/land-breeze circula-tion. The pattern and its implications for the weather in coastal areas are well described, and with mesoscale-resolving operational NWP models the circulation can be captured. In this study, a straightforward method to identify sea and land breezes based on the change in wind direction in the column above a grid point on the coastline is presented. The method was tested for southern Sweden using archived output from the HARMONIE-AROME model with promising results, describing both the seasonal and diurnal cycles well. In areas with a complex coastline, such as narrow straits, the concept of the land-sea breeze becomes less clear, and several ways to address this problem for the suggested method are discussed. With an operational index of the sea and land breezes, the forecaster can better understand and express the weather situation and add value for people in the coastal zone. Further, the indices can be used to study systematic biases in the model and to create climatologies of the sea and land breezes.

    Fulltekst (pdf)
    fulltext
  • 33.
    Hallgren, Christoffer
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Körnich, Heiner
    Swedish Meteorological and Hydrological Institute, Norrköping, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Ville, Vakkari
    Finnish Meteorological Institute, PB 503, FI-00101 Helsinki, Finland.;North West Univ, Chem Resource Beneficiat, Atmospher Chem Res Grp, PB X6001,Potchefstroom Campus, ZA-2520 Potchefstroom, South Africa.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    The winds are twisting: analysis of strong directional shear across the rotor plane using coastal lidar measurements and ERA5Manuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The change of wind direction with height (the directional shear) affects both the power production from a wind turbine, wake effects and aerodynamic loading. In this study, a climatology of the relative occurrence of strong directional shear over Scandinavia is created using 43 years of hourly ERA5 data covering the height range of a modern wind turbine and at wind speeds of operation. It is shown that strong directional shear (≥15° over the rotor) is occurring 20-30% of the time over land and 10-25% of the time over the extended Baltic Sea. The height of the atmospheric boundary-layer and the wind speed at hub height are identified as the most important predictors for strong directional shear, with low boundary-layer heights and weak winds being the main causes. Associated with this, a strong land-sea seasonality is observed. Further, ERA5 is validated against lidar soundings from two coastal sites, both indicating a major underestimation in the distribution of the directional shear in ERA5. Especially in strongly stratified boundary-layers ERA5 struggles, with 25% of the data having errors exceeding 24° and 28° for Östergarnsholm and Utö respectively.

  • 34.
    Hanssen-Bauer, O. W.
    et al.
    Inst Energy Technol IFE, Inst Veien 18, N-2007 Kjeller, Norway..
    de Vaal, J. B.
    Inst Energy Technol IFE, Inst Veien 18, N-2007 Kjeller, Norway.;Norwegian Univ Sci & Technol, Dept Civil & Environm Engn, Hogskoleringen 7a, N-7034 Trondheim, Norway..
    Tutkun, M.
    Inst Energy Technol IFE, Inst Veien 18, N-2007 Kjeller, Norway..
    Asmuth, Henrik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Stenbro, R.
    Inst Energy Technol IFE, Inst Veien 18, N-2007 Kjeller, Norway..
    Dependence of wind turbine loads on inlet flow field2020Inngår i: Science of making torque from wind (TORQUE 2020), pts 1-5, IOP Publishing , 2020, Vol. 1618, artikkel-id 062065Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In wind farm simulations, the inflow wind field plays a crucial role in the accuracy of both power production, structural load predictions and the turbulent wake development behind wind turbines. Three different inflow wind field generation techniques, namely the Mann model, a reduced order based model described herein and LES data, are used in this study to characterise the relation between the inflow and the structural response of the wind turbine. In addition, the wake development under different inflow conditions are studied. The turbulence statistics of the reduced-order model and the LES data are similar to each other while the Mann turbulence has different turbulence profiles and spectral characteristics. An in-house developed aeroelastic code, 3Dfloat, is used for structural response analysis. The differences between the inflow fields are mainly attributed to the turbulence intensity profiles, and differences in their spectral characteristics.

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  • 35.
    Hanssen-Bauer, O. W.
    et al.
    Inst Energy Technol, Inst Veien 18, N-2007 Kjeller, Norway..
    Doubrawa, P.
    Natl Renewable Energy Lab, Golden, CO 80401 USA..
    Madsen, H. Aa
    Tech Univ Denmark, Dept Wind Energy, Riso Campus, DK-4000 Roskilde, Denmark..
    Asmuth, Henrik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Jonkman, J.
    Natl Renewable Energy Lab, Golden, CO 80401 USA..
    Larsen, G. C.
    Tech Univ Denmark, Dept Wind Energy, Riso Campus, DK-4000 Roskilde, Denmark..
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Stenbro, R.
    Inst Energy Technol, Inst Veien 18, N-2007 Kjeller, Norway..
    Comparison of three DWM-based wake models at above-rated wind speeds2023Inngår i: WAKE CONFERENCE 2023 / [ed] Bottasso, C Schepers, G Larsen, G Meyers, J Uzol, O Chatelain, P Aubrun, S Leweke, T, Institute of Physics Publishing (IOPP), 2023, Vol. 2505Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this study we investigate three mid-fidelity wind turbine wake models based on the dynamic wake meandering (DWM) model principle, and compare their performance with a reference dataset, produced with large-eddy simulations using the actuator line model. The models are compared with respect to flow field, power, and loads on a row of four 5MW reference turbines experiencing above-rated wind conditions. In general, the DWM models show fairly good agreement with large-eddy simulation for the time-averaged flow fields, blade forces and power, with increasing differences along the turbine row. Also when comparing fatigue loads of blade root moments, the differences between the models increase further into the row, with deviations up to 25 % of the reference case. However, while the development in blade root moment fatigue along the turbine row is predominantly driven by the energy content at the frequency corresponding to the turbine's rotational period (1P) for the DWM models, the large-eddy simulation results suggest that the key drivers for the blade root and tower loads are the increase in meandering and energy at higher frequencies (> 1P) deeper into the turbine row. For the tower loads, the DWM models highly underestimate the fatigue for the waked turbines. From these results, we suggest priorities for future model developments so that robust model implementations can be used in wind farm design and operation.

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  • 36.
    Ivanell, Stefan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Arnqvist, Johan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Avila, Matias
    Barcelona Supercomp Ctr, Barcelona, Spain.
    Cavar, Dalibor
    Tech Univ Denmark, Wind Energy Dept, Lyngby, Denmark.
    Aurelio Chavez-Arroyo, Roberto
    Natl Renewable Energy Ctr CENER, Pamplona, Spain.
    Olivares-Espinosa, Hugo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Peralta, Carlos
    Wobben Res & Dev MS GmbH, Bremen, Germany.
    Adib, Jamal
    Wobben Res & Dev MS GmbH, Bremen, Germany.
    Witha, Bjoern
    ForWind Carl von Ossietzky Univ Oldenburg, Oldenburg, Germany.
    Micro-scale model comparison (benchmark) at the moderately complex forested site Ryningsnäs2018Inngår i: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 3, nr 2, s. 929-946Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This article describes a study in which modellers were challenged to compute the wind field at a forested site with moderately complex topography. The task was to model the wind field in stationary conditions with neutral stratification by using the wind velocity measured at 100 m at a metmast as the only reference. Detailed maps of terrain elevation and forest densities were provided as the only inputs, derived from airborne laser scans (ALSs) with a resolution of 10 m x 10 m covering an area of 50 km x 50 km, that closely match the actual forest and elevation of the site. The participants were free to apply their best practices for the simulation to decide the size of the domain, the value of the geostrophic wind, and every other modelling parameter. The comparison of the results with the measurements is shown for the vertical profiles of wind speed, shear, wind direction, and turbulent kinetic energy. The ALS-based data resulted in reasonable agreement of the wind profile and turbulence magnitude. The best performance was found to be that of large-eddy simulations using a very large domain. For the Reynolds-averaged Navier-Stokes type of models, the constants in the turbulence closure were shown to have a great influence on the yielded turbulence level, but were of much less importance for the wind speed profile. Of the variety of closure constants used by the participating modellers, the closure constants from Sogachev and Panferov (2006) proved to agree best with the measurements. Particularly the use of C-mu approximate to 0.03 in the k-epsilon model obtained better agreement with turbulence level measurements. All except two participating models used the full detailed ground and forest information to model the forest, which is considered significant progress compared to previous conventional approaches. Overall, the article gives an overview of how well different types of models are able to capture the flow physics at a moderately complex forested site.

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    FULLTEXT01
  • 37.
    Ivanell, Stefan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Carlen, Ingemar
    Nilsson, Karl
    Sarmast, Sasan
    Odemark, Ylva
    Andersen, Sören Juhl
    Tenneler, Görkem
    Eriksson, Ola
    Breton, Simon-Philippe
    Åkervik, Espen
    Gravdahl, Arne
    Fransson, Jens
    Mikkelsen, Robert
    Sörensen, Jens
    Henningson, Dan
    Optimization of Large Wind Farms, The Nordic Consortium, Activity report 2009-2012: Elforsk rapport 13:122013Rapport (Annet vitenskapelig)
  • 38.
    Ivanell, Stefan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Leweke, T.
    Sarmast, Sasan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Quaranta, H. U.
    Mikkelsen, R. F.
    Sorensen, J. N.
    Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry2015Inngår i: Wake Conference 2015, 2015, s. 012018-, artikkel-id 012018Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Results from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes.

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  • 39.
    Ivanell, Stefan
    et al.
    Högskolan på Gotland, Institutionen för kultur, energi och miljö. Kungliga Tekniska högskolan.
    Nilsson, Karl
    Kungliga Tekniska högskolan.
    Mikkelsen, Robert
    Technical University of Denmark.
    A technical Note - Methods used in wake simulations of floating turbines2012Rapport (Annet vitenskapelig)
  • 40.
    Ivanell, Stefan
    et al.
    Högskolan på Gotland, Institutionen för kultur, energi och miljö. Kungliga Tekniska högskolan.
    Sørensen, Jens
    Technical University of Denmark.
    Mikkelsen, Robert
    Technical University of Denmark.
    Troldborg, N.
    Technical University of Denmark.
    Okulov, V.
    Technical University of Denmark.
    Simulation and Modelling of Turbulent Wind Fields in Wind Farms: DTU Mechanical Engineering contribution to TOPFARM Work Package 12011Rapport (Annet vitenskapelig)
  • 41.
    Juhl Andersen, Søren
    et al.
    Tech Univ Denmark, DTU Wind Energy, DK-2800 Lyngby, Denmark.
    Breton, Simon-Philippe
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Environm & Climate Change Canada, 2121 Route Transcanadienne, Dorval, PQ H9P 1J3, Canada.
    Witha, Björn
    Energy & Meteo Syst GmbH, Oskar Homt Str 1, D-26131 Oldenburg, Germany.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Nørkaer Sørensen, Jens
    Tech Univ Denmark, DTU Wind Energy, DK-2800 Lyngby, Denmark.
    Global trends in the performance of large wind farms based on high-fidelity simulations2020Inngår i: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 5, nr 4, s. 1689-1703Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A total of 18 high-fidelity simulations of large wind farms have been performed by three different institutions using various inflow conditions and simulation setups. The setups differ in how the atmospheric turbulence, wind shear and wind turbine rotors are modeled, encompassing a wide range of commonly used modeling methods within the large eddy simulation (LES) framework. Various turbine spacings, atmospheric turbulence intensity levels and incoming wind velocities are considered. The work performed is part of the International Energy Agency (IEA) wind task Wakebench and is a continuation of previously published results on the subject. This work aims at providing a methodology for studying the general flow behavior in large wind farms in a systematic way. It seeks to investigate and further understand the global trends in wind farm performance, with a focus on variability.

    Parametric studies first map the effect of various parameters on large aligned wind farms, including wind turbine spacing, wind shear and atmospheric turbulence intensity. The results are then aggregated and compared to engineering models as well as LES results from other investigations to provide an overall picture of how much power can be extracted from large wind farms operating below the rated level. The simple engineering models, although they cannot capture the variability features, capture the general trends well. Response surfaces are constructed based on the large number of aggregated LES data corresponding to a wide range of large wind farm layouts. The response surfaces form a basis for mapping the inherently varying power characteristics inside very large wind farms, including how much the turbines are able to exploit the turbulent fluctuations within the wind farms and estimating the associated uncertainty, which is valuable information useful for risk mitigation.

    Fulltekst (pdf)
    FULLTEXT01
  • 42.
    Kleusberg, E.
    et al.
    Royal Inst Technol, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish E Sci Res Ctr SeRC, KTH Mech, SE-10044 Stockholm, Sweden.
    Mikkelsen, R. F.
    Tech Univ Denmark, DTU Wind Energy, DK-2800 Lyngby, Denmark.
    Schlatter, P.
    Royal Inst Technol, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish E Sci Res Ctr SeRC, KTH Mech, SE-10044 Stockholm, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Royal Inst Technol, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish E Sci Res Ctr SeRC, KTH Mech, SE-10044 Stockholm, Sweden.
    Henningson, D. S.
    Royal Inst Technol, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish E Sci Res Ctr SeRC, KTH Mech, SE-10044 Stockholm, Sweden.
    High-Order Numerical Simulations of Wind Turbine Wakes2017Inngår i: WAKE CONFERENCE 2017, IOP PUBLISHING LTD , 2017, artikkel-id 012025Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Previous attempts to describe the structure of wind turbine wakes and their mutual interaction were mostly limited to large-eddy and Reynolds-averaged Navier Stokes simulations using finite volume solvers. We employ the higher-order spectral-element code Nek5000 to study the influence of numerical aspects on the prediction of the wind turbine wake structure and the wake interaction between two turbines. The spectral-element method enables an accurate representation of the vortical structures, with lower numerical dissipation than the more commonly used finite-volume codes. The wind-turbine blades are modeled as body forces using the actuator-line method (ACL) in the incompressible Navier Stokes equations. Both tower and nacelle are represented with appropriate body forces. An inflow boundary condition is used which emulates homogeneous isotropic turbulence of wind-tunnel flows. We validate the implementation with results from experimental campaigns undertaken at the Norwegian University of Science and Technology (NTNU Blind Tests), investigate parametric influences and compare computational aspects with existing numerical simulations. In general the results show good agreement between the experiments and the numerical simulations both for a single turbine setup as well as a two-turbine setup where the turbines are offset in the spanwise direction. A shift in the wake center caused by the tower wake is detected similar to experiments. The additional velocity deficit caused by the tower agrees well with the experimental data. The wake is captured well by Nek5000 in comparison with experiments both for the single wind turbine and in the two-turbine setup. The blade loading however shows large discrepancies for the high-turbulence, two-turbine case. While the experiments predicted higher thrust for the downstream turbine than for the upstream turbine, the opposite case was observed in Nek5000.

    Fulltekst (pdf)
    fulltext
  • 43.
    Kleusberg, E.
    et al.
    Royal Inst Technol, KTH Mech, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish ESci Res Ctr SeRC, SE-10044 Stockholm, Sweden.
    Sarmast, Sasan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Schlatter, P.
    Royal Inst Technol, KTH Mech, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish ESci Res Ctr SeRC, SE-10044 Stockholm, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Royal Inst Technol, KTH Mech, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish ESci Res Ctr SeRC, SE-10044 Stockholm, Sweden.
    Henningson, D. S.
    Royal Inst Technol, KTH Mech, Linne FLOW Ctr, SE-10044 Stockholm, Sweden;Royal Inst Technol, Swedish ESci Res Ctr SeRC, SE-10044 Stockholm, Sweden.
    Actuator line simulations of a Joukowsky and Tjaereborg rotor using spectral element and finite volume methods2016Inngår i: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016), 2016, artikkel-id 082011Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The wake structure behind a wind turbine, generated by the spectral element code Nek5000, is compared with that from the finite volume code EllipSys3D. The wind turbine blades are modeled using the actuator line method. We conduct the comparison on two different setups. One is based on an idealized rotor approximation with constant circulation imposed along the blades corresponding to Glauert's optimal operating condition, and the other is the Tjareborg wind turbine. The focus lies on analyzing the differences in the wake structures entailed by the different codes and corresponding setups. The comparisons show good agreement for the defining parameters of the wake such as the wake expansion, helix pitch and circulation of the helical vortices. Differences can be related to the lower numerical dissipation in Nek5000 and to the domain differences at the rotor center. At comparable resolution Nek5000 yields more accurate results. It is observed that in the spectral element method the helical vortices, both at the tip and root of the actuator lines, retain their initial swirl velocity distribution for a longer distance in the near wake. This results in a lower vortex core growth and larger maximum vorticity along the wake. Additionally, it is observed that the break down process of the spiral tip vortices is significantly different between the two methods, with vortex merging occurring immediately after the onset of instability in the finite volume code, while Nek5000 simulations exhibit a 2-3 radii period of vortex pairing before merging.

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  • 44.
    Korb, Henry
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Asmuth, Henrik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi. ForWind – Center for Wind Energy Research, Institute of Physics, University of Oldenburg, Oldenburg, Germany.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    The characteristics of helically deflected wind turbine wakes2023Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 965, artikkel-id A2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The helix approach is a new individual pitch control method to mitigate wake effects of wind turbines. Its name is derived from the helical shape of the wake caused by a rotating radial force exerted by the turbine. While its potential to increase power production has been shown in previous studies, the physics of the helical wake are not well understood to date. Open questions include whether the increased momentum in the wake stems from an enhanced wake mixing or from the wake deflection. Furthermore, its application to a row of more than two turbines has not been examined before. We study this approach in depth from both an analytical and numerical perspective. We examine large-eddy simulations (LES) of the wake of a single turbine and find that the helix approach exhibits both higher entrainment and notable deflection. As for the application to a row of turbines, we show that the phase difference between two helical wakes is independent of ambient turbulence. Examination of LES of a row of three turbines shows that power gains greatly depend on the phase difference between the helices. We find a maximum increase in the total power of approximately 10 % at a phase difference of 270°. However, we do not optimise the phase difference any further. In summary, we provide a set of analytical tools for the examination of helical wakes, show why the helix approach is able to increase power production, and provide a method to extend it to a wind farm.

    Fulltekst (pdf)
    fulltext
  • 45.
    Korb, Henry
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Asmuth, Henrik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi. Carl von Ossietzky Univ Oldenburg, Inst Phys, ForWind Ctr Wind Energy Res, D-26129 Oldenburg, Germany..
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Validation of a Lattice Boltzmann Solver Against Wind Turbine Response and Wake Measurements2023Inngår i: WAKE CONFERENCE 2023 / [ed] Bottasso, C Schepers, G Larsen, G Meyers, J Uzol, O Chatelain, P Aubrun, S Leweke, T, Institute of Physics Publishing (IOPP), 2023, Vol. 2505, artikkel-id 012008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Two of the major limitations facing the adoption of large-eddy simulation (LES) to the industry today are a lack of validation against full-scale measurements and the high computational cost. The lattice Boltzmann method is an approach to conduct LES that is suitable for parallelization on graphics processing units, leading to reduction in energy-tosolution by multiple orders of magnitude compared to Navier-Stokes solvers. We validate the lattice Boltzmann solver VirtualFluids against the measurements published in the SWiFT benchmark and the results obtained with LES by the participants in the benchmark. We compare inflow, turbine response and wake quantities and show that our method yields similar results. While the other LES methods vary in the required energy by one order of magnitude, our methodology is always about one to two orders of magnitude more efficient. The benchmark allows for a comparison to a large number of models, however, the scale of the turbine is not representative of modern turbines and therefore important challenges of modern turbines, such as blade deflection, could not be validated.

    Fulltekst (pdf)
    fulltext
  • 46.
    Korb, Henry
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Asmuth, Henrik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Stender, Merten
    Hamburg Univ Technol, Dynam Grp, Am Schwarzenberg Campus 1, D-21073 Hamburg, Germany..
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Exploring the application of reinforcement learning to wind farm control2021Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 1934, artikkel-id 012022Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Optimal control of wind farms to maximize power is a challenging task since the wake interaction between the turbines is a highly nonlinear phenomenon. In recent years the field of Reinforcement Learning has made great contributions to nonlinear control problems and has been successfully applied to control and optimization in 2D laminar flows. In this work, Reinforcement Learning is applied to wind farm control for the first time to the authors' best knowledge. To demonstrate the optimization abilities of the newly developed framework, parameters of an already existing control strategy, the helix approach, are tuned to optimize the total power production of a small wind farm. This also includes an extension of the helix approach to multiple turbines. Furthermore, it is attempted to develop novel control strategies based on the control of the generator torque. The results are analysed and difficulties in the setup in regards to Reinforcement Learning are discussed. The tuned helix approach yields a total power increase of 6.8 % on average for the investigated case, while the generator torque controller does not yield an increase in total power. Finally, an alternative setup is proposed to improve the design of the problem.

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    fulltext
  • 47.
    Li, Huidong
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Carlsson, Björn
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Wu, Lichuan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Hallgren, Christoffer
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Körnich, Heiner
    Swedish Meteorological and Hydrological Institute, Norrköping 60176, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    A sensitivity study of the WRF model in offshore wind modeling over the Baltic Sea2021Inngår i: Geoscience Frontiers, ISSN 1674-9871, Vol. 12, nr 6, artikkel-id 101229Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Accurate wind modeling is important for wind resources assessment and wind power forecasting. To improve the WRF model configuration for the offshore wind modeling over the Baltic Sea, this study performed a sensitivity study of the WRF model to multiple model configurations, including domain setup, grid resolution, sea surface temperature, land surface data, and atmosphere-wave coupling. The simulated offshore wind was evaluated against LiDAR observations under different wind directions, atmospheric stabilities, and sea status. Generally, the simulated wind profiles matched observations, despite systematic underestimations. Strengthening the forcing from the reanalysis data through reducing the number of nested domains played the largest role in improving wind modeling. Atmosphere-wave coupling further improved the simulated wind, especially under the growing and mature sea conditions. Increasing the vertical resolution, and updating the sea surface temperature and the land surface information only had a slight impact, mainly visible during very stable conditions. Increasing the horizontal resolution also only had a slight impact, most visible during unstable conditions. Our study can help to improve the wind resources assessment and wind power forecasting over the Baltic Sea.

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    fulltext
  • 48.
    Li, Zhongtian
    et al.
    KTH Royal Inst Technol, Sch Elect Engn & Comp Sci, Dept Electromagnet Engn, Stockholm, Sweden..
    Hilber, Patrik
    KTH Royal Inst Technol, Sch Elect Engn & Comp Sci, Dept Electromagnet Engn, Stockholm, Sweden..
    Laneryd, Tor
    Hitachi Energy Res, Västerås, Sweden..
    Navarro Diaz, Gonzalo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Impact of turbine availability and wake effect on the application of dynamic thermal rating of wind farm export transformers2024Inngår i: Energy Reports, E-ISSN 2352-4847, Vol. 11, s. 1399-1411Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dynamic thermal rating allows transformers to operate beyond the nameplate rating according to the actual weather and loading conditions. This paper proposes a methodology to improve the application of this technology in the design of new transformers or in the operation of existing transformers connected to wind farms by accurately predicting their load profiles, accounting for the influence of wake effect and turbine availability. Specifically, the variation of turbine availability due to the intermittent wind is considered in the load profile estimation. Additionally, a correction method, which can be incorporated into any wake model, is proposed to improve the accuracy of wake loss computation. A case study shows that the wake effect and the changing turbine availability shorten the time that the transformers maintain at full load, thereby reducing the aging rate of the wind farm export transformers. The findings suggest that considering these two factors in the DTR application can benefit the longevity and efficiency of wind farm exported transformers.

    Fulltekst (pdf)
    fulltext
  • 49.
    Li, Zhongtian
    et al.
    KTH Royal Inst Technol, Stockholm, Sweden.
    Morozovska, Kateryna
    KTH Royal Inst Technol, Stockholm, Sweden.
    Hilber, Patrik
    KTH Royal Inst Technol, Stockholm, Sweden..
    Laneryd, Tor
    Hitachi ABB Power Grids, Västerås, Sweden.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Vindenergi.
    Sizing Transformer Considering Transformer Thermal Limits and Wind Farm Wake Effect2021Inngår i: 2021 IEEE PES INNOVATIVE SMART GRID TECHNOLOGIES - ASIA (ISGT ASIA), IEEE, 2021Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Compared to the wind farm itself, wind farm transformers are often oversized with regard to their capacity and lifetime. One of the reasons is that power transformers are normally sized at planning stage according to their rated power limits instead of their thermal limits. The thermal limits are usually considered only in operation. In this paper, a new method is proposed to take thermal limits into account and size the wind farm transformer at planning stage based on the expected life of the transformer insulation. An analytical model of wind turbine wakes loss is combined with the transformer thermal model to calculate the expected lifetime of the transformer insulation more accurately. Different from the previous approaches, the proposed method considers both the wake effect and the time-varying ambient temperature. Results show that compared to using the constant temperature and predicted power output without wake loss consideration, the expected lifetime of transformer insulation evaluated after involving these two factors is closer to the result evaluated based on the measured wind power.

  • 50.
    Mann, J.
    et al.
    Tech Univ Denmark, Roskilde, Denmark..
    Angelou, N.
    Tech Univ Denmark, Roskilde, Denmark..
    Arnqvist, Johan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten- och landskapslära.
    Callies, D.
    Fraunhofer Inst Wind Energy & Energy Syst Tech IW, Kassel, Germany..
    Cantero, E.
    Natl Renewable Energy Ctr CENER, Sarriguren, Spain..
    Arroyo, R. Chavez
    Natl Renewable Energy Ctr CENER, Sarriguren, Spain..
    Courtney, M.
    Tech Univ Denmark, Roskilde, Denmark..
    Cuxart, J.
    Univ Les Illes Balears, Mallorca, Spain..
    Dellwik, E.
    Tech Univ Denmark, Roskilde, Denmark..
    Gottschall, J.
    Ivanell, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Kuehn, P.
    Fraunhofer Inst Wind Energy & Energy Syst Tech IW, Kassel, Germany..
    Lea, G.
    Tech Univ Denmark, Roskilde, Denmark..
    Matos, J. C.
    Inst Ciencia & Inovacao Engn Mecan & Gestao Ind I, Oporto, Portugal..
    Palma, J. M. L. M.
    Univ Porto, Fac Engn, Oporto, Portugal..
    Pauscher, L.
    Fraunhofer Inst Wind Energy & Energy Syst Tech IW, Kassel, Germany..
    Pena, A.
    Tech Univ Denmark, Roskilde, Denmark..
    Rodrigo, J. Sanz
    Natl Renewable Energy Ctr CENER, Sarriguren, Spain..
    Soederberg, S.
    WeatherTech Scandinavia AB, Uppsala, Sweden..
    Vasiljevic, N.
    Tech Univ Denmark, Roskilde, Denmark..
    Rodrigues, C. Veiga
    Univ Porto, Fac Engn, Oporto, Portugal..
    Complex terrain experiments in the New European Wind Atlas2017Inngår i: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, nr 2091, s. 1-23, artikkel-id 20160101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The New European Wind Atlas project will create a freely accessible wind atlas covering Europe and Turkey, develop the model chain to create the atlas and perform a series of experiments on flow in many different kinds of complex terrain to validate the models. This paper describes the experiments of which some are nearly completed while others are in the planning stage. All experiments focus on the flow properties that are relevant for wind turbines, so the main focus is the mean flow and the turbulence at heights between 40 and 300 m. Also extreme winds, wind shear and veer, and diurnal and seasonal variations of the wind are of interest. Common to all the experiments is the use of Doppler lidar systems to supplement and in some cases replace completely meteorological towers. Many of the lidars will be equipped with scan heads that will allow for arbitrary scan patterns by several synchronized systems. Two pilot experiments, one in Portugal and one in Germany, show the value of using multiple synchronized, scanning lidar, both in terms of the accuracy of the measurements and the atmospheric physical processes that can be studied. The experimental data will be used for validation of atmospheric flow models and will by the end of the project be freely available. This article is part of the themed issue 'Wind energy in complex terrains'.

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