Experimental investigation of internal two-phase flow structures and dynamics of quasi-stable sheet cavitation by fast synchrotron x-ray imaging

Abstract

The quasi-stable sheet cavitation produced in a small Venturi channel is investigated using a fast synchrotron x-ray imaging technique aided with conventional high speed photography. The use of x rays instead of visible light solves cavitation opacity related issues, and x-ray phase contrast-based edge enhancement enables high-definition visualization of the internal two-phase morphology. The simultaneous acquisition of time-resolved velocity and void fraction fields through post-processing of the recorded x-ray images reveals, for the first time, the complex diphasic flow structures inside the sheet cavity, which is essentially divided into six characteristic parts. Distinct from the current mainstream view, the globally steady sheet cavitation is found to be characterized by a weak but constantly existing re-entrant flow that can penetrate the entire cavity. The turbulent velocity fluctuations inside the sheet cavity are also investigated. The turbulence level in the reverse flow region is observed to be as low as in the outer main flow, demonstrating the relatively steady status of the re-entrant flow. Unlike the streamwise and cross-stream fluctuations, the shear stress appears to be weakly correlated with the velocity gradient. The collapse of the vapor phase and the vaporization at the upstream cavity interface are found to be the primary causes of shear stress intensification.