Experiments with actively pitch-controlled and spring-loaded oscillating foils

Abstract

Experiments with a horizontal foil undergoing harmonic heave motion while traveling at a steady forward speed were performed. The objective was to study different pitch strategies for maximizing the forward thrust. An oscillating foil can replace the rotating propeller as ship propulsor, or the foil can be mounted to the ship in order to create auxiliary thrust in waves. The latter was the motivation behind the experiments presented in this paper. An actively pitch-controlled foil was tested, where two vanes measured the inflow angle to the foil and a motor pitched the foil based on a specified control algorithm. The effect of varying the parameters in the pitch-control algorithm was tested in practice and is discussed here. A spring-loaded foil was also tested. The pitch motion of the spring-loaded foil is shown to produce higher thrust than that of the actively pitch-controlled foil tested. It was also found that the efficiency with spring-loaded pitch motion was higher than the efficiency with forced harmonic pitch motion leading the heave motion with a 90° phase angle.