Analysis of the slip velocity between the two phases in a high-speed cavitating flow

The liquid / vapor mixture in a cavitating flow is usually considered as a homogeneous medium in the current models, but it is shown in this study that the two phases have very different dynamics. Two separate experiments have been performed to measure simultaneously the liquid and vapor velocity fields in two-dimensional sheet cavities on a small-scale Venturi type section. One is based on optical Particle Image Velocimetry (PIV), using a laser light sheet illumination, and the second one is based on fast X-ray imaging. In the first case two components of the velocity are obtained at mid-span of the test section, while in the latter case, the velocity fields are integrated in the width of the test section. One other difference is that optical PIV enables to get the entire velocity field in the whole sheet cavity, while X-ray imaging provides higher resolution results in portions of the cavitation area only.  It is shown first that the two techniques measure similar time-averaged velocities both in the liquid and the vapor phases. The resulting slip velocities between the two phased obtained in both cases are thus in good agreement and reach about 50% of the liquid velocity in a large part of the cavitation area. In general, the vapor velocity is significantly lower than the liquid one, including in the re-entrant jet close to the bottom wall. It is also shown that the non-dimensional slip velocity is almost constant in the sheet cavity and does also not depend on the Reynolds number or the cavitation number. Eventually, the dynamics of the two phases in the area of the re-entrant jet is specifically discussed, based on the data obtained from X-ray imaging: the instantaneous results indicate that the mechanism is based on a liquid reverse flow that subsequently entrains some bubbles in the upstream direction.