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Numerical computations of wind turbine wakes and wake interaction
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When wind turbines are placed in farms, wake effects reduce the overall power production. Also, turbine loads are significantly increased since turbulence levels are high within the wake flow. Therefore, when planning for a wind farm, it is imperative to have an understanding of the flow conditions in the farm in order to estimate the power losses and to optimize the durability of the turbines to be selected for the farm. The possibilities granted by numerical modeling and the development of computational capabilities give an opportunity to study these flow conditions in detail, which has been the key focus of this doctoral work.

The actuator disc method is used for predicting the power production of the Lillgrund wind farm. The results of the simulations are compared to measurements from the actual wind farm, which are found to be in very good agreement. However, some uncertainties are identified in the modeling of the turbine. One of the uncertainties is that a generic rotor is used in the Lillgrund case. In order to quantify the errors resulting from this generalization three different rotor configurations are simulated in various flow conditions. Generally, it can be stated that the choice of rotor configuration is not crucial if the intention of the simulations is to compute the mean wake characteristics subject to turbulent inflow. Another uncertainty is that the turbines in the Lillgrund case were simulated without a power controller. Therefore, a power controller is implemented and used in simulations. Generally, the controller reduces the thrust of the turbines, reduces turbulence intensity and increases velocity levels in the wake flow. However, the use of a controller was observed to have a small impact on the power production.

The effects of using the technique of imposing pregenerated turbulence and a prescribed boundary layer in the simulation are analyzed in order to verify its applicability in very long domains. It is observed that close to the plane of imposed turbulence, the conditions are mainly dependent on the imposed turbulence while far downstream the turbulence, regardless of its initial characteristics, is in near equilibrium with the prescribed wind shear.

The actuator line method is validated using measurements of the near wake behind the MEXICO rotor. The analysis is performed by comparing position, size and circulation of the tip vortices, as well as velocity distributions in the wake flow. The simulations and measurements are generally found to be in good agreement apart from the tip vortex size, which is greatly overestimated in the simulations.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , ix, 40 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2015:04
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-166658ISBN: 978-91-7595-558-2 (print)OAI: oai:DiVA.org:kth-166658DiVA: diva2:811755
Public defence
2015-06-04, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20150519

Available from: 2015-05-19 Created: 2015-05-13 Last updated: 2015-05-19Bibliographically approved
List of papers
1. Large-eddy simulations of the Lillgrund wind farm
Open this publication in new window or tab >>Large-eddy simulations of the Lillgrund wind farm
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2015 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 18, no 3, 449-467 p.Article in journal (Refereed) Published
Abstract [en]

The power production of the Lillgrund wind farm is determined numerically using large-eddy simulations and compared with measurements. In order to simulate realistic atmospheric conditions, pre-generated turbulence and wind shear are imposed in the computational domain. The atmospheric conditions are determined from data extracted from a met mast, which was erected prior to the establishment of the farm. In order to allocate most of the computational power to the simulations of the wake flow, the turbines are modeled using an actuator disc method where the discs are imposed in the computational domain as body forces which for every time step are calculated from tabulated airfoil data. A study of the influence of imposed upstream ambient turbulence is performed and shows that higher levels of turbulence results in slightly increased total power production and that it is of great importance to include ambient turbulence in the simulations. By introducing ambient atmospheric turbulence, the simulations compare very well with measurements at the studied inflow angles. A final study aiming at increasing the farm production by curtailing the power output of the front row turbines and thus letting more kinetic energy pass downstream is performed. The results, however, show that manipulating only the front row turbines has no positive effect on the farm production, and therefore, more complex curtailment strategies are needed to be tested.

Place, publisher, year, edition, pages
John Wiley & Sons, 2015
Keyword
Actuator disc, Large-eddy simulation, Power estimation, Wakes, Wind farms
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-109519 (URN)10.1002/we.1707 (DOI)000348899700005 ()2-s2.0-84921862637 (Scopus ID)
Note

QC 20150318. Updated from submitted to published.

Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06Bibliographically approved
2. Airfoil data sensitivity analysis for actuator disc simulations used in wind turbine applications
Open this publication in new window or tab >>Airfoil data sensitivity analysis for actuator disc simulations used in wind turbine applications
2014 (English)In: SCIENCE OF MAKING TORQUE FROM WIND 2014 (TORQUE 2014), 2014, Vol. 524, 012135- p.Conference paper, Published paper (Refereed)
Abstract [en]

To analyse the sensitivity of blade geometry and airfoil characteristics on the prediction of performance characteristics of wind farms, large-eddy simulations using an actuator disc (ACD) method are performed for three different blade/airfoil configurations. The aim of the study is to determine how the mean characteristics of wake flow, mean power production and thrust depend on the choice of airfoil data and blade geometry. In order to simulate realistic conditions, pre-generated turbulence and wind shear are imposed in the computational domain. Using three different turbulence intensities and varying the spacing between the turbines, the flow around 4-8 aligned turbines is simulated. The analysis is based on normalized mean streamwise velocity, turbulence intensity, relative mean power production and thrust. From the computations it can be concluded that the actual airfoil characteristics and blade geometry only are of importance at very low inflow turbulence. At realistic turbulence conditions for an atmospheric boundary layer the specific blade characteristics play an minor role on power performance and the resulting wake characteristics. The results therefore give a hint that the choice of airfoil data in ACD simulations is not crucial if the intention of the simulations is to compute mean wake characteristics using a turbulent inflow.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-166657 (URN)10.1088/1742-6596/524/1/012135 (DOI)000344193600135 ()2-s2.0-84903748209 (Scopus ID)
Conference
5th Science of Making Torque from Wind Conference
Note

QC 20150519

Available from: 2015-05-13 Created: 2015-05-13 Last updated: 2015-06-04Bibliographically approved
3. Evaluation of the effects of using a power controller in LES/ACD simulations
Open this publication in new window or tab >>Evaluation of the effects of using a power controller in LES/ACD simulations
2015 (English)Report (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-166656 (URN)
Note

QC 20150519

Available from: 2015-05-13 Created: 2015-05-13 Last updated: 2015-05-19Bibliographically approved
4. Study of the influence of imposed turbulence on the asymptotic wake deficit in a very long line of wind turbines
Open this publication in new window or tab >>Study of the influence of imposed turbulence on the asymptotic wake deficit in a very long line of wind turbines
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2014 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 70, 153-163 p.Article in journal (Refereed) Published
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. (C) 2014 Elsevier Ltd. All rights reserved.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-166653 (URN)10.1016/j.renene.2014.05.009 (DOI)000339131600015 ()2-s2.0-84902265810 (Scopus ID)
Note

QC 20150519

Available from: 2015-05-13 Created: 2015-05-13 Last updated: 2017-12-04Bibliographically approved
5. Large-eddy simulations of the evolution of imposed turbulence in prescribed boundary layers in a very long domain
Open this publication in new window or tab >>Large-eddy simulations of the evolution of imposed turbulence in prescribed boundary layers in a very long domain
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-166655 (URN)
Note

QS 2015

Available from: 2015-05-13 Created: 2015-05-13 Last updated: 2015-05-19Bibliographically approved
6. Validation of the actuator line method using near wake measurements of the MEXICO rotor
Open this publication in new window or tab >>Validation of the actuator line method using near wake measurements of the MEXICO rotor
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2015 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 18, no 3, 499-514 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of the present work is to validate the capability of the actuator line method to compute vortex structures in the near wake behind the MEXICO experimental wind turbine rotor. In the MEXICO project/MexNext Annex, particle image velocimetry measurements have made it possible to determine the exact position of each tip vortex core in a plane parallel to the flow direction. Determining center positions of the vortex cores makes it possible to determine the trajectory of the tip vortices, and thus the wake expansion in space, for the analyzed tip speed ratios. The corresponding cases, in terms of tip speed ratios, have been simulated by large-eddy simulations using a Navier - Stokes code combined with the actuator line method. The flow field is analyzed in terms of wake expansion, vortex core radius, circulation and axial and radial velocity distributions. Generally, the actuator line method generates significantly larger vortex cores than in the experimental cases, but predicts the expansion, the circulation and the velocity distributions with satisfying results. Additionally, the simulation and experimental data are used to test three different techniques to compute the average axial induction in the wake flow. These techniques are based on the helical pitch of the tip vortex structure, 1D momentum theory and wake expansion combined with mass conservation. The results from the different methods vary quite much, especially at high values of λ.

Place, publisher, year, edition, pages
John Wiley & Sons, 2015
Keyword
ACL method, CFD, EllipSys3D, MEXICO rotor, MexNext
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-109520 (URN)10.1002/we.1714 (DOI)000348899700008 ()2-s2.0-84921881756 (Scopus ID)
Note

QC 20150318. Updated from submitted to published.

Correction in:Wind Energ. 2015; 18:1683 DOI: 10.1002/we.1864, WOS:000358730800012

Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06Bibliographically approved

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Output format
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