Unsteady forces on oscillating hydrofoils

Abstract

An experimental apparatus is described which has proved to be well suited to the determination of the unsteady lift forces on a hydrofoil oscillating in heave under a free surface. Results are presented for tests covering a wide range of reduced frequency with several foil models in fully wetted, base-ventilated, and supercavitating flow. Tests were performed with aspect ratio one foils and also in two-dimensional flow. The effects of submergence depth below the free surface, angle of attack, and oscillation amplitude were investigated. The experimental findings for the fully wetted foils generally agree fairly well with the available theoretical results. In supercavitating flow, the cavities were established by forced ventilation. The characteristics of the ventilated cavities under oscillation are discussed. The variation in the unsteady lift coefficients with cavity length, and the attendant unsteady cavity pressure, are also presented. The average values of the unsteady lift coefficients are found to differ appreciable from theoretical calculations, and some factors which may contribute to this difference are considered. The present work represents the first extensive set of unsteady force measurements on oscillating hydrofoils, and several new phenomena are revealed by the results. The implications of these findings for practical problems are discussed, and some suggestions are offered for further investigations.