Flow in the closure region of partial attached cavitation.

Abstract

The flow near the closure region of partial attached cavitation was examined using qualitative and quantitative flow visualization techniques. The flows associated with closed and open attached cavitation were examined on nominally two- and three-dimensional test objects. The results of two-dimensional free-streamline theory were compared with the experimentally observed cavity flows. The phase-averaged and unsteady flow fields were examined using Particle Image Velocimetry (PIV). The inception and topology of attached cavitation were related to the non-cavitating flow. The cavities formed on three-dimensional geometries had both open and closed portions, whereas those formed on two-dimensional geometries were always open. The closed cavities on the three-dimensional geometries had laminar reattachment. Open cavities had either laminar or turbulent reattachment. The open cavities with laminar reattachment shed vortical cloud cavitation periodically, whereas open cavities with turbulent reattachment had irregular shedding of vortical structures. Cavity flows with laminar reattachment formed a re-entrant flow. A thin boundary layer on the cavity interface remained attached to the cavity interface. The cavity fared into the solid surface, and the flow was largely irrotational. The profiles of the closed cavities were qualitatively similar to those predicted by two-dimensional free-streamline theory with a re-entrant flow closure. In contrast, cavities that exhibited turbulent flow reattachment had recirculation within the cavity. The turbulent shear flow downstream of the open partial cavity had similar characteristics to those of a non-cavitating shear flow behind a rearward facing step. An adverse pressure gradient abruptly truncated the cavity in the case of the two-dimensional planar geometries. This resulted in the creation of a turbulent liquid shear flow in the wake of the cavity. The collapse of cloud cavitation was shown to introduce significant vorticity and turbulence into the liquid flow downstream of the cavity. The wall-bounded shear flow downstream of cavities with laminar reattachment was very thin with thickness on the order of the non-cavitating boundary layer. Open cavities with a turbulent reattachment produced a turbulent wake with a thickness on the order of the cavity thickness. This was substantially thicker than the non-cavitating boundary layer.