Cavitation inception during the interaction between a pair of counter-rotating vortices*

We perform Large-Eddy Simulation (LES) to study the interaction between a pair of unequal strength counter-rotating vortices in the wake of two hydrofoils at Re = 1.7 x 106 (same experimental conditions as reported in Knister et. al., 2020). Crow instability develops on the weaker vortex beyond one chord length (x/c ~ 1.0) downstream of the trailing edge, causing it to stretch and wrap around the stronger vortex. p < pv (p - pressure, pv - saturated vapor pressure) occurrences in the weaker vortex are spatially and temporarily intermittent, predominantly occurring between x/c: 1.1 - 1.5. The largest drop in its core pressure occurs in the regions where the cores are closest to each other. Probability distribution functions reveal that only a small part of the weaker vortex experiences p < pv. A dynamic process of increase in mean (spatially averaged) axial stretching followed by core pressure reduction is observed in the weaker vortex. The axial stretching is spatially non-uniform, resulting in a non-uniform distribution of pressure along its axis. The impact of axial stretching, initially localized, spreads along the vortex axis during the later stages of the Crow cycle resulting in more regions having lower pressure.