Measurements of the Unsteady Force on Cavitating Hydrofoils in a Free Jet

Abstract

Experimental data concerning force and other related quantities associated with unsteady super cavitating flow about bodies tested in a free-jet water tunnel are reported herein. Three types of unsteady flows were studied -- those due to pitching oscillation, sudden ventilation, and sudden angle of attack change. All test bodies were of such geometrical configuration that separation could occur only at two or three fixed points. The first type of unsteady flows was subdivided into three categories, each having different characteristics. When a flat plate was oscillated about a large mean angle of attack at small cavitation number and without ventilation, the cavity pressure remained unchanged. When air was supplied to the cavity for ventilation, the cavity pressure oscillated with the same frequency as that of the body oscillation. When the plate was oscillated about a small mean angle of attack, there was a change in the cavity configuration even without ventilation and the resulting flow was quite irregular. The plate was oscillated at the maximum reduced frequency of 0.03. The second phase of the experiment involved measurements of cavity pressure, cavity length, and the force on the body following a sudden ventilation of an otherwise steady cavity. It was found that the change in cavity length and the change in the force lagged behind the cavity pressure change. Furthermore, the rate of cavity increase never exceeded the free-stream speed. An attempt was also made to measure the response of the flow to a sudden angle of attack change. It was concluded that, due to the oscillatory nature of the cavity, a faster angle of attack change than was attained in the experiment is needed to obtain a useful unit function response curve.