The influence of Surface Cavitation on Hydrodynamic Forces

A cambered two-dimensional NACA 66 (MOD) wing section was tested in a high speed water tunnel. Two types of cavitation, leading edge sheet and midchord transient streak cavitation, were investigated. Lift, drag and pressure coefficients were measured concurrently with cavitation observations. A method for measuring surface pressure in transient cavities was successfully developed and is discussed in the text. In a partial cavity flow the pressure coefficients measured in the wetted region of the foil away from outside the cavity were found to coincide with the pressure measurements in a fully wetted flow condition. A significant increase in lift with increasing cavity length was measured. However, the lift started to fall off when the cavity length reached about 83 percent of the chord. The occurrence of midchord transient cavities was accompanied by a significant drop in lift and an increase in drag. The influence of surface cavitation on the two dimensional foil force was found to exhibit a strong similarity to propeller thrust breakdown and thrust enhancement as measured by Emerson and Sinclair.