On the Kinematics of Sheet and Cloud Cavitation and Related Erosion

Abstract

The influence of flow parameters such as cavitation number and Reynolds number on the cavitating cloud behavior and aggressiveness is analysed in an experimental work. The focused geometry is a convergent-divergent nozzle with a given radius of curvature at the minimum cross section. By means of a high-speed camera the kinematics of cloud cavitation is visualized. The shape of the cloud is a horse shoe (U-shaped) with two legs ending at the material surface which is in agreement with the Helmholtz vortex theorem. Indeed it is worthwhile to look at the cavitation cloud as a ring vortex whose second half is a mirror vortex within the material. Due to the convection flow, the legs of the vortex are elongated and hence the rotational speed of the vortex core will increase. Thus cavitation bubbles will concentrate within the legs of the vortex and that behavior is observed in the cavitation experiments. The aggressiveness of the cloud is quantified by using soft metal inserts adapted on the nozzle geometry. The interpretation of the plastic deformation, called pits, is done with a 2-dimensional optical measurement system, which is developed to scan large and curved surfaces. In this way damage maps are obtained. Consequently dimensional analysis is used to analyse and generalize the experimental results. Thus a critical Reynolds number is found for the transition from sheet to cloud cavitation. Further an upper limit for the Strouhal number exists for the given geometry. A physical model for the critical Reynolds number is given elsewhere [1]. Also a model for the dynamics of the observed stretched cloud with circulation is published by Buttenbender and Pelz [2].