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

Particle image velocimetry (PIV) and high-speed photography are used to mea sure the flow structure at the closure region and downstream of sheet cavit ation. The experiments are performed in a water tunnel of cross section 6.3 5x5.08 cm(2) whose test area contains transparent nozzles with a prescribed pressure distribution. This study presents data on instantaneous and avera ged velocity, vorticity and turbulence when the ambient pressure is reduced slightly below the cavitation inception level. The results demonstrate tha t the collapse of the vapor cavities in the closure region is the primary m echanism of vorticity production. When the cavity is thin there is no rever se flow downstream and below the cavitation, i.e., a reentrant flow does no t 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 th e height of the stable cavity, dominate the flow downstream of the cavitati ng region. The averaged velocity distributions show that the unsteady colla pse of the cavities in the closure region involves substantial increase in turbulence, momentum, and displacement thickness. Two series of tests perfo rmed at the same velocity and pressure, i.e., at the same hydrodynamic cond itions, 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. s igma=4.41). This small decrease causes only a slight increase in the size o f the cavity, but has a significant impact on the turbulence level and mome ntum deficit in the boundary layer downstream. Ensemble averaging of the me asured instantaneous velocity distributions is used for estimating the liqu id 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 attach ed cavitation, one should account for the sharp (but still finite) gradient s in the liquid void fraction. The 2-D continuity equation can only be sati sfied when the gradients in void fraction are included in the analysis. Usi ng the momentum equation it is possible to estimate the magnitude of the "i nteraction term," i.e., the impact of the vapor phase on the liquid momentu m. It is demonstrated that, at least for the present test conditions, the i nteraction term can be estimated as the local pressure multiplied by the gr adient in void fraction. (C) 2000 American Institute of Physics. [S1070-663 1(00)00804-7].