Wake physics of two-dimensional flapping-hydrofoil turbines

We present a numerical study on the wake of two-dimensional flapping-hydrofoil turbines using Reynolds averaged Navier-Stokes method with shear stress transport k-omega model. The adaptive mesh refinement was applied for vortex simulations. The pitching amplitude ranges from 50 degrees to 90 degrees, and the reduced frequency ranges from 0.10 to 0.20. By varying the reduced frequency and pitching amplitude, we visualized three different types of wakes, and they are the von Karman wake, the mixed wake, and the chaotic wake. We found that there is a critical value of the reduced frequency to determine whether the wake will eventually develop into a standard von Karman wake. When the vortices leave the hydrofoil, they first form a classical staggered arrangement and then develop into a stable double-row configuration. The regular motions of vortices along specific trajectories are explained by analyzing the resultant velocity using velocity polygon, taking into account the effects of vortex interactions and environmental factors. The main component of vortex induced velocities at specific locations is always opposite to the freestream velocity, which is the cause of velocity attenuation in the wake. With the increase in the reduced frequency and pitching amplitude, the velocity attenuation is getting worse. The maximum velocity attenuation usually occurs farther downstream from the hydrofoil for large reduced frequencies and large pitching amplitudes. The wake of a flapping-hydrofoil turbine is divided into four feature zones by studying the time-varying characteristics of velocities, which deepens the understanding of the wake and can provide important references in selecting the optimal location for downstream turbines.