Characterising the near-wake of a cross-flow turbine

Abstract

The performance and detailed near-wake characteristics of a vertical axis, cross-flow turbine (CFT) of aspect ratio 1 were measured in a large cross-section towing tank. The near-wake at one turbine diameter downstream was examined using acoustic Doppler velocimetry, where essential features regarding momentum, energy, and vorticity are highlighted. Dominant scales and their relative importance were investigated and compared at various locations in the measurement plane. Estimates for the terms in the mean streamwise momentum and mean kinetic energy equation were computed, showing that the unique mean vertical velocity field of this wake, characterised by counter-rotating swirling motion, contributes significantly more to recovery than the turbulent transport. This result sheds light on previous CFT studies showing relatively fast downstream wake recovery compared to axial-flow turbines. Finally, predictions from a Reynolds-averaged Navier-Stokes simulation with the commonly used actuator disk model were compared with the experimental results, evaluating its use as an engineering tool for studying flow in CFT arrays. Unsurprisingly, the model was not able to predict the near-wake structure accurately. This comparison highlights the need for improved parameterised engineering models to accurately predict the near-wake physics of CFTs.

A Large-Eddy Simulation Study of Vertical Axis Wind Turbine Wakes in the Atmospheric Boundary Layer

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A comprehensive review of the theoretical approaches for the airfoil design of lift-type vertical axis wind turbine

“…The performance of VWT was predicted accurately. The postprocedure is to analyze the output of the monitoring flow properties [95] or flow field [86]. The analyzed results offer guidance for the further optimization of the geometry.…”

Near‐wake flow simulation of a vertical axis turbine using an actuator line model

“…Moreover, VAWTs are inherently exposed to cyclic variation in the angle of attack, giving cyclic blade forces which can produce material fatigue damage. As the energy conversion process in VAWTs is based on the variation of the blade's circulation along its rotation, the produced wake differs significantly to the one created by HAWTs: The near wake structure is strongly dominated by the effects of the vortices produced on the blade tips (end effects) and the angle of attack variations, these create recovery levels because of the vertical flow transport which is larger than the one produced by the turbulent fluctuation . This characteristic is not present in HAWTs.…”

“…dominated by the effects of the vortices produced on the blade tips (end effects) and the angle of attack variations, these create recovery levels because of the vertical flow transport which is larger than the one produced by the turbulent fluctuation. 14 This characteristic is not present in HAWTs.…”