Numerical simulation of a confined cavitating gas bubble driven by ultrasound

This work investigates the flow disturbance generated by an ultrasonically driven gas bubble confined in a narrow gap over one acoustic cycle. Here, we provide a more accurate representation of ultrasonic cleaning by implementing a volume-of-fluid model in OpenFOAM that simulates the ultrasound as a sinusoidally time-varying pressure boundary condition. A modified Rayleigh-Plesset equation is solved to select an acoustic forcing that instigates bubble collapse. Simulations reveal the interaction between the inflow from the acoustic forcing and the flow deflected by the confining walls intensifies the strength of the self-piercing micro-jet(s), and consequently of the unsteady boundary layer flow, compared to the traditional collapse near a single rigid wall. Depending on the gap height and the position of bubble inception inside the gap, three distinct collapse regimes involving dual-jets or directed-jets are identified, each resulting in a different shear-stress footprint on the confining boundaries. Plots of the spatiotemporal evolution of the shear flow (that is difficult to measure experimentally) reveal peak shear-stress magnitudes at collapse that are double those reported for an undriven laser-induced bubble in similar geometric confinement. This twofold increase is attributed to the ultrasonic signal driving the collapse. Surprisingly, in our simulations we have not encountered a transferred-jet regime previously observed for an unforced bubble collapsing in a similar configuration. This unexpected finding highlights the different physics involved in modeling acoustically driven bubbles compared with the conventional laser-induced bubbles used in experiments.