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Experimental studies of wind turbine wakes: power optimisation and meandering
KTH, School of Engineering Sciences (SCI), Mechanics.
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 [en]
wind energy, power optimisation, active control, yaw, vortex shedding, wake meandering
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-598OAI: oai:DiVA.org:kth-598DiVA: diva2:14563
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
List of papers
1. The application of PIV to the wake of a wind turbine in yaw
Open this publication in new window or tab >>The application of PIV to the wake of a wind turbine in yaw
2001 (English)In: DLR-Mitteilung / [ed] Kompenhans, J., 2001, 155-162 p.Conference paper, Published paper (Refereed)
Abstract [en]

PIV has been used in a wind tunnel study of the wake of a 0.25m diameter two bladed model horizontal axis wind turbine (HAWT). Velocity fields of the wake from one to five rotor diameters downstream of the wind turbine model are shown, both with the turbine aligned in the flow and yawed. Data analysis is mainly based on time averaged velocity profiles of the wake for a range of yaw angles [0 ±10, ±20, ±30] in a plane parallel to the flow. Results show the size and persistence of the velocity deficit and tip vortices in the wake, and the wake deflection in yaw. It is shown that active control of turbine yaw angles could be an advantage for overall maximization of power output from wind farms.

Series
DLR-Mitteilung, ISSN 14348462 ; 3
Keyword
Charge coupled devices, Data processing, Elementary particles, Flow of fluids, Image analysis, Photointerpretation, Power generation, Rotors, Stream flow, Velocity measurement, Wakes, Wind tunnels
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9020 (URN)
Conference
Particle Image Velocimetry; Gottingen; 17 September 2001 through 19 September 2001
Note
QC 20101018Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2010-10-18Bibliographically approved
2. Potential improvement of wind turbine array efficiency by active wake control (AWC)
Open this publication in new window or tab >>Potential improvement of wind turbine array efficiency by active wake control (AWC)
2003 (English)Conference paper, Published paper (Refereed)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9021 (URN)
Note
QC 20101018Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2010-10-18Bibliographically approved
3. Influence of the number of blades on the wake of a wind turbine model
Open this publication in new window or tab >>Influence of the number of blades on the wake of a wind turbine model
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9022 (URN)
Note
QC 20101018Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2010-10-18Bibliographically approved
4. The upstream flow of a 3-bladed wind turbine in yaw
Open this publication in new window or tab >>The upstream flow of a 3-bladed wind turbine in yaw
2005 (English)In: International Energy Agency Annex XI/XX Symposium, 2005, 113-116 p.Conference paper, Published paper (Refereed)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9023 (URN)
Conference
International Energy Agency Annex XI/XX Symposium 2005, CENER, Pamplona, Spain
Note

QC 20101018

Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2012-09-24Bibliographically approved
5. Measurements on a wind turbine wake: 3D effects and bluff-body vortex shedding
Open this publication in new window or tab >>Measurements on a wind turbine wake: 3D effects and bluff-body vortex shedding
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.

Keyword
wind turbine, wake, yaw, vortex shedding
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9024 (URN)10.1002/we.156 (DOI)000238377100003 ()2-s2.0-33745166642 (Scopus ID)
Note
Uppdaterad från "Accepted" till published: 20101018. QC 20101018Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2010-10-18Bibliographically approved
6. Wind turbine near wakes and comparisons to the wake behind a disc
Open this publication in new window or tab >>Wind turbine near wakes and comparisons to the wake behind a disc
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.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9025 (URN)2-s2.0-30744452472 (Scopus ID)
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
7. A note on the frequency of wind turbine wake meandering
Open this publication in new window or tab >>A note on the frequency of wind turbine wake meandering
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9026 (URN)
Note
QC 20101018Available from: 2006-01-26 Created: 2006-01-26 Last updated: 2010-10-18Bibliographically approved

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