Correlating Single Phase Flow Measurements With Observations of Trailing Vortex Cavitation

Abstract

The single phase and cavitating tip vortex shed by a NACA 66-209 rectangular plan-form, rounded tip hydrofoil has been studied. Single-phase measurements of instantaneous flow velocity were made by taking double-pulsed holograms of microbubbles moving in and around the vortex core. The tailored air bubble technique of Ooi and Acosta (1983) was employed to measure both the mean and fluctuating single phase vortex core static pressure. Cavitation inception was determined visually. The flow in the vortex core is highly unsteady; the r.m.s. axial velocity fluctuation can be as high as 0.2U∞. Core pressure fluctuations greater than the freestream dynamic pressure have been observed. These fluctuations are commensurate with the axial velocity unsteadiness. Cavitation inception in saturated water occurs at mean core pressures above the vapor pressure. However, the pressure fluctuations are certainly substantial enough to account for these elevated inception indices. The rapid establishment of fully developed trailing vortex cavitation when the cavitation number is reduced below inception is attributable to the small variation in mean core pressure with downstream distance. The inception index increases substantially with dissolved air content. The cause of this dependence is not presently known.