Journal Article

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

An, Hongjie, Zeng, Qingyun, Ohl, Claus-Dieter, Zhang, A-Man, Tan, Beng Hau

2024

Overview

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, $\\lambda =R_{y,m}/R_{max}$ with $R_{y,m}$ 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.

• Publisher: Cambridge University Press
• Subject: Atmospheric pressure / Bubbles / Cameras / Cavitation / Distance / Experiments / Flow velocity / High speed photography / JFM Rapids / Lasers / Photography / Simulation / Velocity / Viscosity