uu.seUppsala University Publications
Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Numerical Computations of Wakes Behind Wind Farms
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
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 [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
National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:uu:diva-255859OAI: oai:DiVA.org:uu-255859DiVA: diva2:847316
Presentation
, Visby (English)
Opponent
Supervisors
Available from: 2015-08-31 Created: 2015-06-18 Last updated: 2016-02-12Bibliographically approved
List of papers
1. Analysis of long distance wakes of Horns Rev I using actuator disc approach
Open this publication in new window or tab >>Analysis of long distance wakes of Horns Rev I using actuator disc approach
Show others...
2014 (English)In: Science of Making Torque from Wind 2012, 2014Conference paper, Published paper (Refereed)
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.

Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 555
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-246873 (URN)10.1088/1742-6596/555/1/012032 (DOI)000347871200032 ()
Conference
4th Scientific Conference on Science of Making Torque from Wind, OCT 09-11, 2012, Oldenburg, GERMANY
Funder
StandUp for Wind
Available from: 2015-03-11 Created: 2015-03-11 Last updated: 2017-02-03Bibliographically approved
2. 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, 012152- p.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: 2017-02-03Bibliographically approved
3. 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, 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.

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: 2017-02-03Bibliographically approved
4. 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
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-260221 (URN)
Funder
Swedish National Infrastructure for Computing (SNIC), SNIC 2014/8-5
Available from: 2015-08-17 Created: 2015-08-17 Last updated: 2016-04-25
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, 012028- p., 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: 2017-02-03Bibliographically approved

Open Access in DiVA

fulltext(1337 kB)160 downloads
File information
File name FULLTEXT01.pdfFile size 1337 kBChecksum SHA-512
a9ebe5c6111aee5f7c0289ffcc0d9abc7c7d2d1a0dc0c31222562b1f06132fcb224f5ae916089005f39f3e271bdf3a38317b2ab094f46bc9cf7ae9d3f19e697d
Type fulltextMimetype application/pdf

Authority records BETA

Eriksson, Ola

Search in DiVA

By author/editor
Eriksson, Ola
By organisation
LUVAL
Meteorology and Atmospheric Sciences

Search outside of DiVA

GoogleGoogle Scholar
Total: 160 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

urn-nbn

Altmetric score

urn-nbn
Total: 775 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf