Relating Trailing Vortex Cavitation Inception To Single Phase Flow Measurements

The single phase and cavitating tip vortex shed by a NACA .66-209 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 0.2Uoo Core pressure fluctuations greater than the freestream dynamic pressure have been measured. These fluctuations are thought to accompany local axial velocity fluctuations. With saturated water, inception occurs at mean core pressures above the vapour pressure. However, the pressure fluctuations are certainly substantial enough to account for these elevated inception indices. The fluctuations also explain why inception occurs immediately downstream of the hydrofoil. 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 decreases dramatically with decreasing dissolved air content. The cause of this decrease is not known.