Analysis of the cavitation instabilities with time-resolved stereo and multiplane particle image velocimetry

Abstract

The present paper is devoted to the analysis of the various instabilities of cavitation attached to a two-dimensional (2D) profile. Time resolved stereo particle image velocimetry was conducted in a small-scale 2D Venturi type section, in different vertical planes in the streamwise direction, located at varying positions in the depth of the channel. These experiments enabled to obtain the time evolution of the three components of the velocity field in the cavitation area and to derive the time-averaged gradients in the spanwise direction. Test cases at various Reynolds numbers were conducted, maintaining either the pressure or the cavitation number constant, to discuss the impact of these parameters on the flow. Then, attention was focused on three distinct flow dynamics, namely, sheet cavitation, where no large-scale instability can be detected, single cloud cavitation, where a large cloud of vapor is shed periodically at the rear of the cavity, and multi-cloud cavitation, where the process is more complex, as more than one clouds are shed downstream. Data reveal that the structure and the structure of the re-entrant jet, which is one of the primary mechanisms of cloud cavitation, are more complex than reported in the previous studies. Although the jet can be detected as an intermittent low speed reverse flow in the streamwise direction, it is actually made of successive vortices about the channel depth, which are convected downstream while expanding in the vertical direction, causing the cavity lift and, thus, contributing to its final split and the cloud shedding.