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Large-eddy simulations of wind farm production and long distance wakes
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2015 (English)In: Wake Conference 2015, 2015, 012022- p., 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.

Place, publisher, year, edition, pages
2015. 012022- p., 012022
Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 625
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-261143DOI: 10.1088/1742-6596/625/1/012022ISI: 000358047700022OAI: oai:DiVA.org:uu-261143DiVA: diva2:849900
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: 2017-02-03Bibliographically approved
In thesis
1. Numerical Computations of Wakes Behind Wind Farms
Open this publication in new window or tab >>Numerical Computations of Wakes Behind Wind Farms
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

More and larger wind farms are planned offshore. As the most suitable build sites are limited 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 a long distance. 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.

The thesis contains numerical studies of these long distance wakes. In this study 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 farm to farm interaction.

Numerical studies are performed to get better knowledge about the use of this model for long distance wakes. The first study compares the simulation results with measurements behind an existing farm. Three parameter studies are thereafter setup to analyze how to best use the model. The first parameter study examines numerical and physical parameters in the model. The second one looks at the extension of the domain and turbulence as well as the characteristics of the flow far downstream. The third one gathers information on the downstream development of turbulence with different combinations of wind shear and turbulence level. The impact of placing the turbines at different distances from the turbulence plane is also studied. Finally a second study of an existing wind farm is performed and compared with a mesoscale model. The model is shown to be relevant also for studies of long distance wakes. Combining LES with a mesoscale model can be of interest.

Place, publisher, year, edition, pages
Uppsala: Institutionen för geovetenskaper, 2015. 57 p.
Series
Department of Earth Sciences Licentiate Thesis
Keyword
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
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-255859 (URN)
Presentation
, Visby (English)
Opponent
Supervisors
Available from: 2015-08-31 Created: 2015-06-18 Last updated: 2016-02-12Bibliographically approved

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Eriksson, OlaNilsson, KarlBreton, Simon-PhilippeIvanell, Stefan

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