Flow physics behind the wake of a flapping hydrofoil turbine near a wall

Abstract

By solving Reynolds-Averaged Navier–Stokes equations with the shear stress transport k–ω turbulence model, the two-dimensional wake of a flapping hydrofoil turbine near a wall was studied, including evolution of vortices, spatial distribution of velocity, time-averaged and time-varying characteristics of the flow field. A custom adaptive mesh refinement approach was used for vortex simulations. Unlike the double-row configuration wake behind a flapping hydrofoil turbine in no near-wall situation, the wake of a flapping hydrofoil turbine near a wall shows an approximate single-row vortex pattern, which makes the wake energy more concentrated and beneficial for recovery by downstream turbines. As the near-wall spacing decreases, the intensity of vortices gradually increases, but the change in the degree of vortex diffusion toward the side is non-monotonic. Especially, due to the continuous diffusion of vortices to the side, several equally spaced velocity recovery zones are formed near the centerline, which improves the conditions for energy recovering. As the pitching amplitude increases, the degree of vortex diffusion to the side monotonically decreases; the intensity of vortices increases, which exacerbates the unevenness of velocity field. As the motion frequency increases, the degree of vortex diffusion monotonically increases, the velocity attenuation is more severe, and the trajectories of all vortices become increasingly consistent. Anyway, the velocity intensification stabilization and fluctuation zones in the wake are the top two choices for installing downstream turbines. The study expands our understanding of the wake of a flapping hydrofoil turbine and can provide reference for improving the power of downstream turbines.