Numerical investigation of auto-pitch wing-in-ground effect oscillating foil propulsor

The propulsive characteristics of auto-pitch wing-in-ground effect oscillating foil propulsors (APWIGs) were numerically investigated through an unsteady Reynolds Averaged Navier-Stokes solver. The kinematics of such a biplane configuration is characterized by the prescribed heave motion and flow-induced pitch motion restrained by a torsional spring for each foil. Based on the validated numerical model, the comparison of propulsive performance between APWIGs and single auto-pitch oscillating foil, as well as dual-foil heave-only configuration, was conducted at different advance speeds. Results show that APWIGs is advantageous in both thrust production and efficiency enhancement over other two configurations due to the resulting wing-in-ground effect and substantial reduction of flow separation by the flow-regulated pitch motion. Furthermore, the effect of torsional spring stiffness on the propulsion of APWIGs was studied under different loaded conditions. It was found that both the maximum pitching angle and phase difference of pitch with heave are dramatically affected by the spring stiffness, which has major contribution to the hydrodynamic behaviours of the foils. Under a certain operating speed, an optimal torsional spring stiffness that produces the best propulsive performance can be found. With respect to the parametric space in the current study, the APWIGs can achieve a constant high efficiency over 70% by employing an appropriate spring stiffness.