Force Characteristics of Restrained, Naturally Ventilated Hydrofoils in Regular Waves

Abstract

Earlier measurements of steady flow force characteristics of forced ventilated hydrofoils with trailing edge flaps have been supplemented with additional measurements using naturally ventilated hydrofoils. These measurements have been compared with a few theoretical points based on the non-linear theory of Wu and Wang for two-dimensional foils at non-zero cavitation number. The theory was also modified using Cumberbatch's method to include effects of finite span. Good agreement between the theory and experimental data Was obtained. Measurements were also made of the oscillatory lift and drag forces for restrained, naturally ventilated foils of finite span moving through a regular wave train. Flat plate foils with aspect ratios of 2 and 3 were used. The experimental data for the forces agreed fairly well with calculations based on the instantaneous angle of attack and cavitation number, and the reduced force coefficients indicated little variation with reduced frequency, νc/V, up to about 1.1. The phase angle between the maximum lift force and the wave crest increased with increasing reduced frequency. Computations for the phase angle based on quasi-steady assumptions were valid only at low reduced frequencies. Cavity wash-off was found to occur due to wave-induced changes in the angle of attack. The angle at which wash-off occurred was related to the wave characteristics for low aspect ratio foils. For a foil with an aspect ratio of 4, cavity wash-off generally occurred at the same angle of attack as that required to maintain a cavity in smooth water.