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Determination of wind turbine near-wake length based on stability analysis
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Uppsala University.
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2014 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 524, no 1, 012155Article in journal (Refereed) Published
Abstract [en]

A numerical study on the wake behind a wind turbine is carried out focusing on determining the length of the near-wake based on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method. The wake is perturbed by applying stochastic or harmonic excitations in the neighborhood of the tips of the blades. The flow field is then analyzed to obtain the stability properties of the tip vortices in the wake of the wind turbine. As a main outcome of the study it is found that the amplification of specific waves (traveling structures) along the tip vortex spirals is responsible for triggering the instability leading to wake breakdown. The presence of unstable modes in the wake is related to the mutual inductance (vortex pairing) instability where there is an out-of-phase displacement of successive helix turns. Furthermore, using the non-dimensional growth rate, it is found that the pairing instability has a universal growth rate equal to π/2. Using this relationship, and the assumption that breakdown to turbulence occurs once a vortex has experienced sufficient growth, we provide an analytical relationship between the turbulence intensity and the stable wake length. The analysis leads to a simple expression for determining the length of the near wake. This expression shows that the near wake length is inversely proportional to thrust, tip speed ratio and the logarithmic of the turbulence intensity.

Place, publisher, year, edition, pages
2014. Vol. 524, no 1, 012155
Keyword [en]
Aerodynamics, Computational fluid dynamics, Flow control, Inductance, Large eddy simulation, Navier Stokes equations, Stability, Stochastic systems, Torque, Turbulence, Vortex flow, Wind tunnels, Wind turbines, A1. largeeddy simulations (LES), Harmonic excitation, Mutual inductance, Pairing instability, Simple expression, Stability analysis, Stability properties, Turbulence intensity, Wakes
National Category
Energy Engineering
URN: urn:nbn:se:kth:diva-167849DOI: 10.1088/1742-6596/524/1/012155ISI: 000344193600155ScopusID: 2-s2.0-84903719867OAI: diva2:820800
18 June 2014 through 20 June 2014, Copenhagen

QC 20150612

Available from: 2015-06-12 Created: 2015-05-22 Last updated: 2015-06-12Bibliographically approved

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Sarmast, SasanIvanell, StefanHenningson, Dan
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