On the Evolution of Unequal Counter-Rotating Vortex Pairs

The influence of upstream inflow boundary perturbations on the evolution of counter-rotating vortex pairs with unequal strength and size is investigated using implicit large-eddy simulations. Lamb-Oseen vortex pairs are generated at a time-varying inflow boundary condition with parameters based on published experimental work. Previous research efforts indicate rapid, localized pressure drops in the weaker (secondary) vortex, motivating the present investigation into the dynamics and mechanisms leading to this phenomenon. Three perturbation amplitudes are applied to the vortices' time-dependent motion at the inflow boundary to determine their effect on the growth of long-wavelength instabilities and their subsequent dynamics. The results demonstrate a direct correlation between the initial perturbation amplitude and the streamwise location of the precipitous pressure drop in the secondary vortex. Specifically, larger amplitudes of the inflow vortex boundary accelerate the growth of the Crow instability, leading to more rapid and intense vortex interactions. This causes the secondary vortex to deform and stretch, culminating in the precipitous pressure reduction closer to the inflow boundary. These computational results provide a qualitative comparison to the experimental observations and offer a detailed analysis of the fluid dynamics that would lead to cavitation inception in vortex-dominated liquid flows.