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Numerical Computations of Wakes Behind Wind Farms: A tool to study Farm to Farm interaction
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. (Wind Energy Campus Gotland)
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

More and larger wind farms are planned offshore. As the most suitable sites to build are limited these new wind farms will be constructed near to each other in so called wind farm clusters. Behind the wind turbines in these farms there is a disrupted flow of air called a wake that is characterized by reduced wind speed and increased turbulence. These individual turbine wakes combine to form a farm wake that can travel long distances. In wind farm clusters farm to farm interaction will occur, i.e. the long distance wake from one wind farm will impact the wind conditions for other farms in the surrounding area.

This thesis contains numerical studies of these long distance wakes. In these studies Large Eddy Simulations (LES) using an Actuator Disc method (ACD) are used. A prescribed boundary layer is used where the wind shear is introduced using body forces. The turbulence, based on the Mann model, is introduced as fluctuating body forces upstream of the farm. A neutral atmosphere is assumed. The applied method has earlier been used for studies of wake effects inside farms but not for the longer distances needed for the study of farm to farm interaction. Parameter studies are setup to analyze how to best use the model for the study of long distance wakes with regards to 1) numerical and physical parameters in the model, 2) the extension of the domain and turbulence as well as the characteristics of the flow far downstream and 3) the downstream development of turbulence with different combinations of wind shear and turbulence level.

Using an initial simulation setup a wind farm was studied and preliminary results were obtained. These results were subsequently improved upon by applying the simulation setup adjustments indicated in the parameter studies. A comparison with a mesoscale model was also done. The mesoscale model was shown to be relevant for studies of long distance wakes in another study comparing LES and WRF. Finally an idealized farm to farm interaction case was studied with focus on the impact of including the Coriolis force in the simulations.

Combining LES with a mesoscale model is of interest to study further.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 73
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1789
Keywords [en]
Wind turbine, Wind power, Wind farm, Wakes, Long distance wakes, Farm-Farm, Farm to farm interaction, Wind farm cluster, Large Eddy simulations, LES, Actuator disc method, ACD, CFD, Ellipsys3D, Prescribed boundary layer, PBL, Forced boundary layer, FBL
National Category
Earth and Related Environmental Sciences
Research subject
Meteorology
Identifiers
URN: urn:nbn:se:uu:diva-379125ISBN: 978-91-513-0616-2 (print)OAI: oai:DiVA.org:uu-379125DiVA, id: diva2:1296827
Public defence
2019-06-13, E22, Uppsala universitet - Campus Gotland, Cramérgatan 3, Visby, 10:15 (English)
Opponent
Supervisors
Available from: 2019-04-26 Created: 2019-03-18 Last updated: 2019-06-17
List of papers
1. Large-eddy simulations of the evolution of imposed turbulence in forced 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 forced boundary layers in a very long domain
(English)In: Article in journal (Refereed) Submitted
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-379122 (URN)
Available from: 2019-03-12 Created: 2019-03-12 Last updated: 2019-03-18
2. Large-eddy simulations of wind farm production and long distance wakes
Open this publication in new window or tab >>Large-eddy simulations of wind farm production and long distance wakes
2015 (English)In: Wake Conference 2015, 2015, p. 012022-, article id 012022Conference paper, Published paper (Refereed)
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.

Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 625
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-261143 (URN)10.1088/1742-6596/625/1/012022 (DOI)000358047700022 ()
Conference
Wake Conference, JUN 09-11, 2015, Visby, SWEDEN
Funder
Swedish National Infrastructure for Computing (SNIC)StandUp for Wind
Available from: 2015-08-31 Created: 2015-08-31 Last updated: 2019-03-18Bibliographically approved
3. Analysis of long distance wakes behind a row of turbines – a parameter study
Open this publication in new window or tab >>Analysis of long distance wakes behind a row of turbines – a parameter study
2014 (English)In: Science of Making Torque from Wind, 2014, p. 012152-Conference paper, Published paper (Refereed)
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.

Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 524
National Category
Physical Sciences Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-237704 (URN)10.1088/1742-6596/524/1/012152 (DOI)000344193600152 ()
Conference
5th Science of Making Torque from Wind Conference, JUN 18-20, 2014, Tech Univ Denmark, Copenhagen, DENMARK
Funder
Swedish National Infrastructure for Computing (SNIC)StandUp for Wind
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2019-03-18Bibliographically approved
4. Analysis of long distance wakes of Horns Rev 1 using actuator disc approach
Open this publication in new window or tab >>Analysis of long distance wakes of Horns Rev 1 using actuator disc approach
Show others...
2014 (English)In: Journal of Physics: Conference Series, ISSN 1742-6588, Vol. Vol. 555, no nr 1, article id 012032Article in journal (Refereed) Published
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.

National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-238304 (URN)
Conference
The Science of making Torque from Wind, Oldenburg
Funder
Swedish National Infrastructure for Computing (SNIC), SNIC 001/11-99StandUp for Wind
Available from: 2014-12-11 Created: 2014-12-11 Last updated: 2019-03-18
5. Wake downstream of the Lillgrund wind farm - A Comparison between LES using the actuator disc method and a Wind farm Parametrization in WRF
Open this publication in new window or tab >>Wake downstream of the Lillgrund wind farm - A Comparison between LES using the actuator disc method and a Wind farm Parametrization in WRF
2015 (English)In: WAKE CONFERENCE 2015, 2015, p. 012028-, article id 012028Conference paper, Published paper (Refereed)
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.

Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 625
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-261142 (URN)10.1088/1742-6596/625/1/012028 (DOI)000358047700028 ()
Conference
Wake Conference, JUN 09-11, 2015, Visby, SWEDEN
Funder
Swedish National Infrastructure for Computing (SNIC)StandUp for Wind
Available from: 2015-08-31 Created: 2015-08-31 Last updated: 2019-03-18Bibliographically approved
6. Impact of Wind Veer and the Coriolis Force for an Idealized Farm to Farm Interaction Case
Open this publication in new window or tab >>Impact of Wind Veer and the Coriolis Force for an Idealized Farm to Farm Interaction Case
2019 (English)In: Applied Sciences, E-ISSN 2076-3417, Vol. 9, no 5, article id 922Article in journal (Refereed) Published
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.

Keywords
long distance wake, farm to farm interaction, wind farm cluster, large Eddy simulations, LES, actuator disc, ACD, forced boundary layer, FBL, coriolis, wind veer
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-379116 (URN)10.3390/app9050922 (DOI)000462504400109 ()
Funder
Swedish National Infrastructure for Computing (SNIC)
Available from: 2019-03-12 Created: 2019-03-12 Last updated: 2019-04-25Bibliographically approved
7. The Long distance wake behind Horns Rev I studied using large eddy simulations and a wind turbine parameterization in WRF
Open this publication in new window or tab >>The Long distance wake behind Horns Rev I studied using large eddy simulations and a wind turbine parameterization in WRF
Show others...
2017 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 854, article id 012012Article in journal (Refereed) Published
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.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-362564 (URN)10.1088/1742-6596/854/1/012012 (DOI)000435276400012 ()
Conference
Wake Conference 2017, 30 May to 1 June 2017, Visby, Sweden
Available from: 2018-10-07 Created: 2018-10-07 Last updated: 2019-03-18Bibliographically approved

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