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Wakes behind wind turbines - Studies on tip vortex evolution and stability
KTH, School of Engineering Sciences (SCI), Mechanics.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The increased fatigue loads and decreased power output of a wind turbine placed in the wake of another turbine is a well-known problem when building new wind power farms. In order to better estimate the total power output of a wind power farm, knowledge about the development and stability of wind turbine wakes is crucial. In the present thesis, the wake behind a small-scale model turbine was investigated experimentally in a wind tunnel. The velocity in the wake was measured with hot-wire anemometry, for different free stream velocities and tip speed ratios. To characterize the behaviour of the model turbine, the power output, thrust force and rotational frequency of the model were also measured. These results were then compared to calculations using the Blade Element Momentum (BEM) method. New turbine blades for the model was constructed using the same method, in order to get an estimate of the distribution of the lift and drag forces along the blades. This information is needed for comparisons with certain numerical simulations, which however remains to be performed.By placing the turbine at different heights in a turbulent boundary layer, the effects of forest turbulence on wind turbine outputs (power and thrust) could also be investigated.The evolution of the tip vortices shed from the turbine blades was studied by performing velocity measurements around the location of the tip vortex breakdown. The vortices' receptivity to disturbances was then studied by introducing a disturbance in the form of two pulsed jets, located in the rear part of the nacelle. In order to introduce a well-defined disturbance and perform phase-locked measurements, a new experimental setup was constructed and successfully tested for two different disturbance frequencies. The mean streamwise velocity and the streamwise turbulence intensity was found to scale well with the free stream velocity and the spreading of the wake was found to be proportional to the square root of the downstream distance.  The comparison for power and thrust between measurements and BEM calculations showed good agreement in some cases but worse agreement when the pitch angle of the blades was small.The velocity measurements showed that the tip vortices can be susceptible to disturbances and an earlier breakdown could be detected. However, more measurements need to be made to fully investigate the dependance on disturbance frequency and amplitude.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , vi, 78 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:06
Keyword [en]
wind power, aerodynamics, hot-wire anemometry, vortex dynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-93806OAI: oai:DiVA.org:kth-93806DiVA: diva2:524104
Presentation
2012-05-22, E33, Lindstedtsvägen 3, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20120504Available from: 2012-05-04 Created: 2012-04-27 Last updated: 2012-05-04Bibliographically approved
List of papers
1. Phase-locked hot-wire measurements on the breakdown of wind turbine tip vortices
Open this publication in new window or tab >>Phase-locked hot-wire measurements on the breakdown of wind turbine tip vortices
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-94012 (URN)
Note

QS 2012

Available from: 2012-05-04 Created: 2012-05-04 Last updated: 2014-04-24Bibliographically approved
2. Effect of forest turbulence on wind turbine outputs
Open this publication in new window or tab >>Effect of forest turbulence on wind turbine outputs
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-94013 (URN)
Funder
StandUp for Wind
Note

QC 20170119

Available from: 2012-05-04 Created: 2012-05-04 Last updated: 2017-01-19Bibliographically approved

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CiteExportLink to record
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Citation style
  • apa
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Output format
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