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Numerical computations of wind turbine wakes and wake interaction: Optimization and control
KTH, School of Engineering Sciences (SCI), Mechanics.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In the present thesis the wake flow behind wind turbines is analyzed numerically using large-eddy simulations. The wind turbine rotors are modeled by using either the actuator disc method or the actuator line method in which the blades are represented by body forces computed with airfoil data. Using these models, the boundary layers of the turbine blades are not resolved and most of the computational power is preserved to simulate the wake flow. The actuator disc method is used for the wake interaction studies of the Lillgrund wind farm. In this study the power production is simulated for two different wind directions and compared to measurements. A grid sensitivity study and a turbulence intensity study are also performed. As a last step the front row turbines are derated in an attempt to increase the total production of the farm. The results show that it is important to impose atmospheric conditions at the inlet in the simulations, otherwise production will be unrealistically low for some turbines in the farm. The agreement between the simulated and measured power is very good. The study based on derating the front row turbines does not show any positive increase on the farm production. The actuator line method is used for near wake analysis of the MEXICO rotor. In this study the near wake is simulated for five different flow cases and compared with particle image velocimetry (PIV) measurements. The analysis is performed by comparing size and circulation of the tip vortices, the radial and streamwise velocity distributions, the spatial expansion of the wake and the axial induction factor. The simulations and measurements generally are in agreement. In some cases, however, the measurements are affected by tunnel effects which are not captured in the simulations. In connection to the actuator disc method a power control strategy for operating conditions below rated power is implemented and tested. The strategy is first validated using an in-house developed blade element momentum code and then is implemented in the actuator disc method used in the EllipSys3D code. The initial tests show that the strategy responds as expected when changing the moment of inertia of the rotor and when varying the inlet conditions. Results from the implementation of the strategy in the actuator disc method in EllipSys3D show that the turbine adapts to the conditions it is operating in by changing its rotational velocity and power output when the inlet conditions are varied.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , vi, 56 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:18
Keyword [en]
Actuator disc method, Actuator line method, Blade element momentum method, Wind turbine wake, Wake interaction, CFD, LES, EllipSys3D
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-109522ISBN: 978-91-7501-624-5 (print)OAI: oai:DiVA.org:kth-109522DiVA: diva2:583107
Presentation
2013-01-25, D33, Lindstedtsvägen 5, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20130111

Available from: 2013-01-11 Created: 2013-01-07 Last updated: 2013-01-11Bibliographically 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. 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
3. Study of the effect of the presence of downstream turbines on upstream ones and use of a controller in CFD wind turbine simulation models
Open this publication in new window or tab >>Study of the effect of the presence of downstream turbines on upstream ones and use of a controller in CFD wind turbine simulation models
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2012 (English)Conference paper, Published paper (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-109521 (URN)
Conference
The Science of Making Torque from Wind
Funder
Swedish e‐Science Research CenterStandUp
Note

QC 20130111

Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2013-04-18Bibliographically approved

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