Ventilated Cavities on Submerged Three-Dimensional Hydrofoils

Abstract

Experimental studies have been made to determine the air-entrainment rates of ventilated cavities for hydrofoils of finite span submerged below a free surface. The study was divided naturally into two parts; reentrant-jet cavities and pulsating or trailing-vortex cavities. A correlation parameter has been derived to permit determination of the air-entrainment rates for cavities in the reentrant-jet regime assuming that all the air is lost from the cavity through the reentrant jet. As air was also lost through the tip vortices, some dependence was noted with aspect ratio. Air entrainment was a function of velocity for a given cavitation number and ambient pressure. Cavity instability or pulsation was observed at high air-injection rates for thin cavities. Trailing vortices were observed for thick cavities at the higher injection rates. The frequency of the pulsations tended to increase as the foil approached the free surface. Measurements of lift and drag forces indicated little effect of the free surface for submergence ratios of 1 to 3 chords. Data extrapolated to σ = 0 agreed favorably with theory-by Johnson, and data at the higher σ correlated with theory by Cumberbatch.