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Large-eddy simulations of the evolution of imposed turbulence in prescribed boundary layers in a very long domain
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
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(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-166655OAI: diva2:811741

QS 2015

Available from: 2015-05-13 Created: 2015-05-13 Last updated: 2015-05-19Bibliographically approved
In thesis
1. Numerical computations of wind turbine wakes and wake interaction
Open this publication in new window or tab >>Numerical computations of wind turbine wakes and wake interaction
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.
TRITA-MEK, ISSN 0348-467X ; 2015:04
National Category
Engineering and Technology
urn:nbn:se:kth:diva-166658 (URN)978-91-7595-558-2 (ISBN)
Public defence
2015-06-04, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)

QC 20150519

Available from: 2015-05-19 Created: 2015-05-13 Last updated: 2015-05-19Bibliographically approved

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Nilsson, KarlIvanell, Stefan
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