Flow structure and modeling issues in the closure region of attached cavitation

Particle image velocimetry (PIV) and high-speed photography are used to measure the flow structure at the closure region and downstream of sheet cavitation. The experiments are performed in a water tunnel of cross section 6.35x5.08 cm(2) whose test area contains transparent nozzles with a prescribed pressure distribution. This study presents data on instantaneous and averaged velocity, vorticity and turbulence when the ambient pressure is reduced slightly below the cavitation inception level. The results demonstrate that the collapse of the vapor cavities in the closure region is the primary mechanism of vorticity production. When the cavity is thin there is no reverse flow downstream and below the cavitation, i.e., a reentrant flow does not occur. Instead, the cavities collapse as the vapor condenses, creating in the process hairpin-like vortices with microscopic bubbles in their cores. These hairpin vortices, some of which have sizes as much as three times the height of the stable cavity, dominate the flow downstream of the cavitating region. The averaged velocity distributions show that the unsteady collapse of the cavities in the closure region involves substantial increase in turbulence, momentum, and displacement thickness. Two series of tests performed at the same velocity and pressure, i.e., at the same hydrodynamic conditions, but at different water temperatures, 35 degrees C and 45 degrees C, show the effect of small changes in the cavitation index (sigma=4.69 vs. sigma=4.41). This small decrease causes only a slight increase in the size of the cavity, but has a significant impact on the turbulence level and momentum deficit in the boundary layer downstream. Ensemble averaging of the measured instantaneous velocity distributions is used for estimating the liquid void fraction, average velocities, Reynolds stresses, turbulent kinetic energy and pressure distributions. The results are used to examine the mass and momentum balance downstream of the cavitating region. It is shown that in dealing with the ensemble-averaged flow in the closure region of attached cavitation, one should account for the sharp (but still finite) gradients in the liquid void fraction. The 2-D continuity equation can only be satisfied when the gradients in void fraction are included in the analysis. Using the momentum equation it is possible to estimate the magnitude of the interaction term, i.e., the impact of the vapor phase on the liquid momentum. It is demonstrated that, at least for the present test conditions, the interaction term can be estimated as the local pressure multiplied by the gradient in void fraction. (C) 2000 American Institute of Physics. [S1070-6631(00)00804-7].