Effects of the vertical axis wind turbine on the velocity field of the wake flow

Abstract

This paper deals with the wake flow behind a vertical axis wind turbine, focusing on the variation in the wake structure relative to the tip-speed ratio λ. The wind turbine model was placed in the test section, 2m wide, 2m height and 5.3m long, of the low-speed wind tunnel. Measurements were made of the U and V components of the instantaneous velocity in the wake of the model for various tip-speed ratio λ of the wind turbine using an X type hot-wire. The instantaneous data obtained were used to determine time mean and phase-locked mean velocities and turbulence intensities. The momentum loss and fluctuation energy in the wake were also estimated as characteristic quantities. The results obtained showed that in a low tip-speed ratio range of λ/λ_opt<0.67 (λ_opt was an optimum tip-speed ratio to extract maximum power from the wind), the periodic fluctuation energy (PE) accounted for 40% of the total fluctuation energy (TE) because periodic and low-frequency velocity fluctuations were generated by the large-scale vortices induced by the dynamic stall of the turbine blades. The fluctuations decayed quickly and PE also became zero where λ/λ_opt exceeded approximately 0.7. In high tip-speed ratio λ/λ_opt>1.07, TE was equal to turbulent energy because high-frequency velocity fluctuations were generated by the instabilities of the separated shear-layer from the blades. The results showed that a medium tip-speed ratio range of 0.67<λ/λ_opt<1.07 was optimum for not only minimum momentum loss and fluctuation energy but also maximum wind turbine power.