Jetting enhancement from wall-proximal cavitation bubbles by a distant wall

An additional distant wall is known to highly alter the jetting scenarios of wall-proximal bubbles. Here, we combine high-speed photography and axisymmetric volume of fluid (VoF) simulations to quantitatively describe its role in enhancing the micro-jet dynamics within the directed jet regime (Zeng et al., J. Fluid Mech., vol. 896, 2020, A28). Upon a favourable agreement on the bubble and micro-jet dynamics, both experimental and simulation results indicate that the micro-jet velocity increases dramatically as eta decreases, where eta = H/R-max is the distance between two walls H normalized by the maximum bubble radius R-max. The mechanism is related to the collapsing flow, which is constrained by the distant wall into a reverse stagnation-point flow that builds up pressure near the bubble's top surface and accelerates it into micro-jets. We further derive an equation expressing the micro-jet velocity U-jet = 87.94 gamma(0.5) (1 + (1/3) (eta - lambda(1.2))(-2)), where gamma = d/R-max is the stand-off distance to the proximal wall with d the distance between the initial bubble centre and the wall, lambda = R-y,R-m/R-max with R-y,R-m the distance between the top surface and the proximal wall at the bubble's maximum expansion. Viscosity has a minimal impact on the jet velocity for small gamma, where the pressure buildup is predominantly influenced by geometry.