Vibration of Cavitating Hydrofoils

Abstract

A primarily experimental research program has been carried out using a free-jet water tunnel for the purpose of studying force and moment fluctuations on cavitating two-dimensional hydrofoils. Both a symmetrical wedge and a non-symmetrical wedge were tested for a wide range of cavity lengths and several different elastic conditions. Fluctuations in lift and moment were of primary concern in the experiments.

It was revealed that the force and moment were quite steady if the cavity was longer than two chords unless an excessive amount of ventilation caused cavity pulsations. For a shorter cavity, however, the flow was generally very unstable, and severe vibrations were noted. A cavity of any length was found to be basically unstable and to oscillate at a characteristic frequency which was primarily a function of the cavity length. The vibrating cavity may cause an elastically supported foil to vibrate severely when the cavity is short. The largest-amplitude vibration often occurred when the cavity length was approximately equal to one chord.

Flutter-like vibrations were noted in the first and second natural modes of the two-degree-of-freedom system. The frequency of these vibrations was found to be practically independent of the cavity length. The severest vibration in the first natural mode usually occurred when the average cavity length was approximately equal to one chord, whereas vibration in the second natural mode was found more likely to occur when the cavity was very short.