Nucleation and cavitation inception in high Reynolds number shear layers

Abstract

The influence of nucleation on cavitation inception in a high Reynolds number shear layer in the wake of a backward-facing step was experimentally investigated in a water tunnel. The flow was investigated for two nuclei populations: the one naturally occurring in the water and for the water artificially seeded with monodisperse nuclei. Incipient events were observed to form in stretched quasi-streamwise vortices. The collapse of an incipient cavity resulted in a microbubble cloud dispersed into the shear layer and the step re-circulation zone. These microbubbles, generally larger than those naturally occurring in the water, act as preferential sites for re-nucleation, triggering the formation of developed cavitation. This phenomenon rendered statistical characterization of cavitation inception impractical for the natural nuclei population. The re-nucleation issue was addressed by seeding the flow with a population of large monodisperse nuclei, with a critical pressure higher than that of cavitation products. Spatial distribution of the nuclei within the seeded plume was characterized using a volumetric measurement based on Mie-scattering imaging. The ability to discern individual incipient events enabled examination of the effect of cavitation number and the nuclei injection rate on the inception event rate. The event rate was found to follow a power law with cavitation number and vary linearly with the injection rate. Mapping of spatial distribution of cavitation susceptibility was obtained by combining the spatial distributions of incipient events and nuclei concentration. The current work provides a valuable dataset for the development of computational tools for modeling of cavitation inception in nucleated flows.