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Wind turbine near wakes and comparisons to the wake behind a disc
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-1146-3241
2005 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The wake development behind wind turbines affects power losses in turbine parks aswell as the flow induced rotor loads due to wake turbulence. In the near wake region tipvortices are a distinct feature of the flow. Such vortices do not exist behind a circulardisc but instead the flow is characterized by a large scale vortex shedding. In our windtunnel experiments we have observed similar large scale vortex shedding in the wake of atwo-bladed wind turbine model. The frequency of the shedding in terms of the Strouhalnumber decreases with increasing tip speed ratio and levels out at a value which is close tothat observed for a solid disc. For low tip speed ratios no large scale vortex shedding wasobserved. When the turbine was yawed the shedding frequency increased in a similar wayas for an inclined disc. In order to further investigate this phenomenon we have studiedthe vortex shedding behind solid and porous discs. All our results indicate that a turbineat high tip speed ratio sheds large scale structures in a similar way as a disc. We alsohypothesize that the vortex shedding is the cause for the observed meandering of wakeswhich has been observed behind full scale turbines.

Place, publisher, year, edition, pages
2005. 15593-15604 p.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-9025Scopus ID: 2-s2.0-30744452472OAI: oai:DiVA.org:kth-9025DiVA: diva2:14561
Conference
43rd AIAA Aerospace Sciences Meeting and Exhibit, 10-13 January 2005, Reno, Nevada, USA
Note

QC 20141204

Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2014-12-04Bibliographically approved
In thesis
1. Experimental studies of wind turbine wakes: power optimisation and meandering
Open this publication in new window or tab >>Experimental studies of wind turbine wakes: power optimisation and meandering
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Wind tunnel studies of the wake behind model wind turbines with one, two and three blades have been made in order to get a better understanding of wake development as well as the possibility to predict the power output from downstream turbines working in the wake of an upstream one. Both two-component hot-wire anemometry and particle image velocimetry (PIV) have been used to map the flow field downstream as well as upstream the turbine. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the free stream direction (the yaw angle was varied from 0 to 30 degrees). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side showing the potential of controlling the wake position by yawing the turbine. The power output of a yawed turbine was found to depend strongly on the rotor. The possibility to use active wake control by yawing an upstream turbine was evaluated and was shown to have a potential to increase the power output significantly for certain configurations. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low frequency fluctuation both in the wake and in the flow outside. This fluctuation was found both with and without free stream turbulence and also with a yawed turbine. The non-dimensional frequency (Strouhal number) was independent of the freestream velocity and turbulence level but increases with the yaw angle. However the low frequency fluctuations were only observed when the tip speed ratio was high. Porous discs have been used to compare the meandering frequencies and the cause in wind turbines seems to be related to the blade rotational frequency. It is hypothesized that the observed meandering of wakes in field measurements is due to this shedding.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. ix, 46 p.
Series
Trita-MEK, ISSN 0348-467X ; 2005:19
Keyword
wind energy, power optimisation, active control, yaw, vortex shedding, wake meandering
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-598 (URN)
Public defence
2006-02-10, Sal D3, Lindstedtsvägen 5, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20101018Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2010-10-18Bibliographically approved

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Alfredsson, Henrik P.

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