Reynolds numbers effects on cavitation inception in secondary vortices in a turbulent shear layer

The unsteady pressure field and the distributions of nuclei in and around quasi-streamwise vortices (QSVs), in a shear layer behind a backward facing step, are experimentally investigated. The QSVs are the preferred sites of inception and the cavitation inception index, or the void fractions at the same index, increases with Reynolds number (Re). To explain these observations, tomographic particle tracking is used for calculating the velocity and pressure fields for non-cavitating flow. The pressure is lower, and its minima last longer within the QSVs compared to the surrounding flow. As a result of stretched vortex dynamics, the pressure minima are likely to be preceded by axial vorticity stretching and followed by contraction. Consequently, the regions of low pressure are localized and intermittent, consistent with the multi-point appearance of cavitation. With increasing Re, the pressure minima decay slower and the pressure correlations remain significant for longer. These trends can be explained as a result of viscous diffusion. The effects of nuclei availability are studied under controlled seeding of microbubbles using holography. For the current conditions, cavitation inception is dominated by pressure and relatively insensitive to nuclei distributions.