Modification of Wall-Bounded Vorticity and Surface Loading by Successive Vortex Ring Impacts on Concave Cavities

Successive vortex ring interactions with concave geometries play a fundamental role in unsteady flow processes relevant to both biological and engineering systems, including artificial voice rehabilitation and heat transfer enhancement. This study presents an experimental investigation of the successive impact of two co-axial vortex rings on a concave hemicylindrical cavity, with a focus on how the wall-bounded vorticity and surface loading evolve as a function of the leapfrogging phase between two vortex rings at the time of cavity surface impact. Vortex rings with a circulation-based Reynolds number of Re_Γ = 1500 were generated using a piston-cylinder mechanism. Two different cavity-to-ring size ratios, γ = R_cyl / R_V = 3, & 2, were used to explore the geometric effect of the interaction. The phase of leapfrogging at the time of impact was controlled by adjusting the initial separation distance between the rings. Flow development was quantified using particle image velocimetry. Results demonstrate that the intensity of the secondary vorticity generated on the cavity surface, and its subsequent ejection is strongly dependent on both the cavity size and the leapfrogging phase between the two primary vortex rings at the time of impact. The wall pressure loading is also enhanced as the phase of leapfrogging progresses.