Unsteady Force and Cavity Characteristics for Ventilated Hydrofoils

Abstract

Experimental measurements were made of the unsteady cavity and force characteristics for both forced and naturally ventilated hydrofoils of finite span submerged below a free surface. Unsteady cavity characteristics were studied for a force-ventilated wedge subjected to a sudden change in either the .air flow rate to the cavity or angle of attack. Differences were observed in air entrainment rates measured for the unsteady case as compared to the corresponding instantaneous steady case. The magnitude of the differences depended on the kind of unsteadyness introduced.

Unsteady force characteristics were determined for naturally ventilated foils undergoing either a sinusoidal heaving motion or a harmonic oscillation of a trailing edge flap. The amplitude of the oscillatory lift was found to increase with increasing reduced frequency for a heaving foil, whereas the oscillatory lift was essentially constant up to reduced frequencies of 1.2 for the foil with an oscillating flap. There is limited agreement of the data with available theory.