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Measurements on a wind turbine wake: 3D effects and bluff-body vortex shedding
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-1146-3241
2006 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 9, no 3, 219-236 p.Article in journal (Refereed) Published
Abstract [en]

The velocity held in the wake of a two-bladed wind turbine model (diameter 180 mm) has been studied under different conditions using a two-component hot wire. 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 freestream direction (the yaw angle was varied from 0 degrees to 20 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 woke by yawing the turbine. 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 the wake. This fluctuation was found both with and without freestream turbulence and also with a yawed turbine. The frequency expressed as a Strouhal number was shown to be independent of the freestream velocity or turbulence level, but the low frequency was only observed when the tip speed ratio (or equivalently the drag coefficient) was high. The shedding frequency changed also with the yaw angle. This is in agreement with the idea that the turbine sheds structures as a bluff body. The phenomenon, noticeable in all the velocity components, was further investigated using two-point cross-correlations of the velocity signals.

Place, publisher, year, edition, pages
2006. Vol. 9, no 3, 219-236 p.
Keyword [en]
wind turbine, wake, yaw, vortex shedding
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-9024DOI: 10.1002/we.156ISI: 000238377100003Scopus ID: 2-s2.0-33745166642OAI: oai:DiVA.org:kth-9024DiVA: diva2:14560
Note
Uppdaterad från "Accepted" till published: 20101018. QC 20101018Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2010-10-18Bibliographically 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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