Parametric study on hydro-elasticity characteristics of auto-pitch wing-in-ground effect oscillating foil propulsors

A parametric study on the propulsive performance of auto-pitch wing-in-ground effect oscillating foil propulsors (APWIGs) was conducted using an unsteady Reynolds Averaged Navier-Stokes solver. The harmonic sinusoidal function is actively imposed to the heave motion of such a configuration, which has a passively flow-induced pitch motion restrained by torsional springs. Comparative investigation between APWIGs and fully prescribed system was performed, indicating that APWIGs is capable of producing a satisfactory propulsive efficiency within a wide range of advance speed. The jet-like profiles implying the time-averaged momentum surplus were obtained in the wake of APWIGs, which is attributed to the produced reverse Kármán vortex street behind each oscillating foil. Obvious vortex shedding along both leading edge and trailing edge can be observed at a high reduced frequency, while the low oscillating frequency corresponding to a small reduced frequency tends to produce the high efficiency due to the highly attached flow around foil surface. Furthermore, comprehensive computations for the influence of multiple parameters on the hydro-elasticity responses and propulsive characteristics of APWIGs were conducted in the current study. We noted that heaving amplitude and equilibrium distance only affect the maximum flow-induced pitching angle. Nonetheless, the position of elastic pitching axis shows a significant effect on both pitch-leading phase difference and the maximum pitching angle. The maximum increase of propulsive efficiency around 12% due to the wing-in-ground effect was discovered within the considered parametric space. It was found that the APWIGs can achieve a high efficiency of over 70%, with the appropriate combination of geometry and motion parameters.