Ground effects on the propulsion of an undulating pectoral fin with various aspect ratios

The propulsive performance of an undulating pectoral fin with various aspect ratios is numerically investigated with the consideration of the ground effect. The kinematics of the fin is prescribed as a sinusoidal wave and the flow field is calculated by solving the Unsteady Reynolds-Averaged Navier-Stokes equations. It is found that for higher aspect ratios, the mean thrust coefficient is linear with the square of the normalized relative velocity and the inverse square of the wavelength ratio whereas for lower aspect ratios, the relations with the velocity and wavelength become cubic and fourth power respectively. The Strouhal number is found to be a scaling parameter for longer wavelength cases. The ground effect reduces the thrust force in most cases examined in this paper while the propulsive efficiency remains relatively unchanged. Compared with the fin with longer wavelengths, the mean thrust created by the fin with a short wavelength is remarkably less influenced by the ground effect. It is believed that there is a switch from the lift-based mechanism to the added-mass mechanism as the wavelength decreases. The lift-based mechanism is the main thrust production mechanism at a longer wavelength whereas the fin with a short wavelength primarily utilizes the added-mass mechanism, i.e. is less sensitive to the change of the pressure distribution over the surface of the fin due to the ground effect.