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The stability and development of tip and root vortices behind a model wind turbine
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-3251-8328
2013 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 9, 1591- p.Article in journal (Refereed) Published
Abstract [en]

When designing new wind farms, one has to rely on models describing the flow field around and inside the farm, since direct numerical simulation is far too computationally expensive. In order to develop better models for power prediction of wind farms, knowledge about the flow field around turbines, the stability of the wakes and the interaction between them is essential. Since the conditions during field measurements are difficult to control, wind tunnel measurements play an important role when studying wakes behind wind turbines. Within the present work, an experimental methodology has been developed to study the evolution and stability of the tip vortices shed from the rotor blades of a small-scale turbine model. The stability of the tip vortices was studied by introducing a periodic disturbance to the flow, which is shown to have a clear effect on the development of the vortices. Prior to the vortex breakdown, clear signs of vortex pairing were also observed. A parameter study was performed by varying the amplitude and frequency of the forced disturbance, and the effect on the tip vortices was evaluated. This experiment is one of the first where the influence of a periodic disturbance on a wind turbine wake is studied, something that previously has been performed in a number of numerical studies.

Place, publisher, year, edition, pages
2013. Vol. 54, no 9, 1591- p.
Keyword [en]
Horizontal-Axis, Boundary-Layer, Wake
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-133974DOI: 10.1007/s00348-013-1591-6ISI: 000326043500006Scopus ID: 2-s2.0-84882495356OAI: oai:DiVA.org:kth-133974DiVA: diva2:664397
Funder
Swedish Energy Agency
Note

QC 20131115

Available from: 2013-11-15 Created: 2013-11-14 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Wind-turbine wake flows - Effects of boundary layers and periodic disturbances
Open this publication in new window or tab >>Wind-turbine wake flows - Effects of boundary layers and periodic disturbances
2014 (English)Doctoral 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 and a subject of intensive research. These problems are caused by the velocity gradients and high turbulence levels present in the wake of a turbine. In order to better estimate the total power output and life time of a wind-power farm, knowledge about the development and stability of wind-turbine wakes is crucial.

In the present thesis, the flow field around small-scale model wind turbines has been investigated experimentally in two wind tunnels. The flow velocity was measured with both hot-wire anemometry and particle image velocimetry. To monitor the turbine performance, the rotational frequency, the power output and the total drag force on the turbine were also measured. The power and thrust coefficients for different tip-speed ratios were calculated and compared to the blade element momentum method, with a reasonable agreement. The same method was also used to design and manufacture new turbine blades, which gave an estimate of the distribution of the lift and drag forces along the blades.

The influence of the inlet conditions on the turbine and the wake properties was studied by subjecting the turbine to both uniform in flow and different types of boundary layer in flows. In order to study the stability and development of the tip vortices shed from the turbine blades, a new experimental setup for phase-locked measurements was constructed. The setup made it possible to introduce perturbations of different frequencies and amplitudes, located in the rear part of the nacelle. With a newly developed method, it was possible to characterize the vortices and follow their development downstream, using only the streamwise velocity component.

Measurements were also performed on porous discs placed in different configurations. The results highlighted the importance of turbine spacings. Both the measurements on the turbine and the discs were also used to compare with large eddy simulations using the actuator disc method. The simulations managed to predict the mean velocity fairly well in both cases, while larger discrepancies were seen in the turbulence intensity.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. viii, 35 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:12
Keyword
wind power, wind-turbine model, wind tunnel, porous disc, hot-wire anemometry, particle image velocimetry, blade element momentum method, large eddy simulations, actuator disc method
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-144475 (URN)978-91-7595-112-6 (ISBN)
Public defence
2014-05-14, D2, Lindstedtsvägen 5, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20140424

Available from: 2014-04-24 Created: 2014-04-23 Last updated: 2014-04-24Bibliographically approved

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Fransson, Jens H. M.

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