Cavitation in complex separated flows

Abstract

An investigation of two classes of separated cavitating flows has been performed. The first investigation is related to cavitating vortices above delta shaped vortex generators, in particular cavitating vortex breakdown. Visual observations of the cavities are presented together with noise signatures and simulations of the cavity flow fields. A number of previously unreported features of cavitating vortex breakdown flow fields are presented. In particular, it was found that re-entrant jets are frequently present in the vortex upstream the vortex generator apex when breakdown appeared above the model. Furthermore, it has been shown that the appearance of the transition from vortex to bubble cavitation is determined by the position of breakdown. Cavities were also observed in secondary vortices when breakdown occured above the vortex generator, or close to the trailing edge. The experiments were accompanied by numerical simulations that showed reasonable agreement with the experiments, both concerning cavity appearance and vortex breakdown positions. A method for classifying the flow fields based on characteristic properties normal to the leading edge is finally presented.

The second topic of the report is the flow field around cavitation inducers in a rotating disk apparatus (RDA). An inducer with a circular cross section proved to be outstanding as far as erosion rate on a downstream material specimen is concerned. Based on numerical simulations of the flow field around four different inducers, it is proposed that the supremacy of the circular inducer is due to its efficient transport of bubbles from where they are generated to where they collapse. However, this conclusion only seem to be valid for low inducer aspect ratios. Based on the observations, a new approach to the design of cavitation inducers is proposed.