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Exploring the effects of low-level-jets on the energy entrainment of vertical-axis wind turbines

Abstract:

We experimentally explored the effect of a series of low-level-jet (LLJ) velocity profiles on the energy entrainment of a single and a pair of counter-rotating Giromill-Darrieus vertical-axis wind turbines (VAWTs) models under two types of rotations. Planar particle image velocimetry (PIV) was used to obtain the mean flow and turbulence statistics in the wake of the model wind turbines. Incoming LLJ profiles had peaks coincident with the turbines midspan and top tip; complementary flow characterization with a canonic turbulent boundary layer is also included for comparison. Results show that the positive shear region of the LLJ velocity profiles increased the wake asymmetry due to higher vertical velocity gradients. The positive shear of the LLJ contributed to the enhancement of energy entrainment in the wake compared with a standard turbulent boundary layer profile by increasing the mean kinetic energy advection into the wake in the single and a pair of counter-rotating VAWTs. Comparatively, high vertical velocity gradients may be responsible for this phenomenon; it promoted stronger cross-flow and counter-rotating vortices in the wake.