Instability of Ventilated Cavities

Cavitation number σ is defined by σ = (p0pk)/q. It can be controlled by controlling the dynamic pressure q, the ambient pressure p0, or, in the case of quasi-steady cavities, the internal cavity pressure pk. The present investigation was undertaken to study methods of controlling pk by adding air to the wakes of fully submerged bodies. This process has been called ventilation. It was found that for small air-flow rates, pk increased and σ decreased nearly linearly with the rate of air supply. It was also found that onceσ had 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 σbe reduced further, and the reduction was discontinuous. 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.