Instability of Ventilated Cavities

Cavitation number σ is defined by σ = (P_o - P_k)/q. It can be controlled by controlling the dynamic pressure q, the ambient pressure P_o, or the internal cavity pressure P_k. The present investigation was undertaken to study methods of controlling P_k by adding air to the wakes of fully submerged bodies. This process has been called ventilation. It was found that with P_o and q fixed, P_k increases, and hence σ decreases, nearly linearly with the rate of air supply. Numerical values are given in the paper for the normal flat plate, circular cylinder, and various hydrofoils, and these values may be extrapolated to other sizes of bodies than those tested. It was also found that once σ was decreased to a certain critical value, further increase in the air supply rate would not produce a continuing linear decrease in σ. Rather, σ remained nearly constant and the cavities began to vibrate. Vibration occurred with one, two, or more waves on the cavity surface. Only by going from a one-wave to a two-wave cavity could σ be reduced further, and the reduction was discontinuous . A detailed description of the vibrating cavities is given. Both two-dimensional and finite aspect-ratio bodies were tested. The tests were conducted in the two-dimensional, vertical, free-jet water tunnel at the St. Anthony Falls Hydraulic Laboratory. All bodies tested, regardless of aspect ratio and whether lifting or nonlifting, behaved quite similarly within both the vibrating and nonvibrating regimes. That the vibration was not peculiar to the vertical free-jet tunnel was demonstrated by comparison with results from a hydrofoil towed in a tank. In the case of lifting bodies of finite span operating near a free surface, air was found to enter, rather than leave, the cavities through the trailing vortexes.