The evolution of vertical-axis wind turbine (VAWT) wakes has a significant impact on the optimization of VAWT array layouts and performance parameters. Due to the complexity of their asymmetric vortex wakes, numerical simulations of two-dimensional wake planes require substantial computational resources, and experimental studies have primarily focused on the midspan horizontal plane. Notably, tip vortices exhibit distinct instability characteristics in the tip vortex-induced effect zone compared to other planes, being significantly influenced by shed vortices. This study conducts focused wind tunnel experiments to investigate the wake characteristics in the tip vortex-induced effect zone of a VAWT, with comparative analysis against the midspan horizontal plane wake. Wavelet transform was employed to analyze the multiscale time-frequency characteristics of wake evolution through wavelet energy spectra. By examining the spatial distribution of turbulent kinetic energy, the following conclusions were drawn: in the near-wake region of the tip vortex-induced effect zone, the central area is dominated by harmonic energy bands at three times the blade rotation frequency (3ƒR), while regions near both sides exhibit predominant energy bands at the fundamental blade rotation frequency (ƒR). In the downstream wake, the tip vortex-induced effect zone demonstrates a greater proportion of low-frequency, large-scale structures compared to the midspan horizontal plane. In the tip vortex-induced region, fluctuations of turbulent kinetic energy tend to be biased toward one side, whereas significant fluctuations occur on both sides in the midspan horizontal plane. Moreover, during downstream evolution, the dissipation of turbulent kinetic energy in the tip vortex-induced region is more pronounced than in the midspan horizontal plane. Simultaneously, turbulence intensity decreases in the central region but increases at both lateral edges.
Yu et al. (Mon,) studied this question.