Effects of Cavitation on Periodic Wakes Behind Symmetric Wedges

Abstract

Measurements of the shedding frequency and spacing of cavitating vortex street wakes have been made for four wedge models having apex angles of 15, 30, 60, and 90 deg. The wake pattern made visible by the presence of cavitation bubbles enabled measurements of the shedding frequency and vortex street spacing to be taken. It was found that the cavitation produced negligible effects on the shedding frequency with decreasing cavitation number down to as low as half the incipient cavitation number. Further decrease in the cavitation number produced an increase in the shedding frequency to a maximum value followed by a decrease with a general decay to random shedding. The analysis of high-speed motion pictures demonstrated a general decrease in the cross-stream spacing of the two rows of vortices with decreasing cavitation number. The longitudinal spacing of vortices in the same row also decreased with decreasing cavitation number followed by a rapid increase at cavitation numbers lower than that producing the peak frequency. In a simplified analysis, it was hypothesized that the cavitation in the vortex cores caused a general sink effect in the shed wake. Modeling the cavitation with cylindrical cavitation bubbles, some agreement with the experimental results was obtained for the cross-stream spacing with decreasing cavitation number.