CFD modelling of an acoustically driven gas bubble in confinement

In ultrasonic cleaning, acoustically-driven bubbles remove micro- or nano- scale contamination from solid surfaces. However, violent acoustic forcing may cause unwanted surface damage. While this dual nature of cavitation bubbles has been recognized for decades, the fundamental physics of bubble-boundary interaction is still not fully understood. This thesis elucidates some of the underlying bubble-cleaning mechanisms through compressible volume-of-fluid simulations with consideration of viscosity and surface tension conducted in OpenFOAM. The inclusion of viscosity enables the capturing of bubbleinduced shear-forces on nearby surfaces that are difficult to measure experimentally and widely overlooked in acoustic cavitation literature. Theoretical descriptions of bubble dynamics are used to verify the proposed numerical model, while experimental validation with high-speed images and data from the literature provides additional confidence in the results. A narrow-gap configuration reflective of cleaning in highly-confined geometries like microchannels is examined. Simulations reveal the interaction between the inflow from the acoustic forcing and the ow deflected by the confining walls intensifies the strength of the self-piercing micro-jet(s), and consequently of the unsteady boundary flow, compared to a bubble collapsing near a single rigid wall. Depending on the gap height and the position of bubble inception, three different collapse regimes are identified, each exhibiting a different shear-stress footprint and delay in collapse time. It is shown that the number of self-piercing jets formed during collapse is typically equal to the number of confining boundaries, but not always. An increased liquid viscosity is shown to inhibit bubble splitting and dual-jet formation. These findings have implications in other acoustic cavitation applications involving confinement, since, through careful selection of control parameters the desired dynamic behaviour to suit the application can be achieved. The implemented code is a versatile tool that is readily parallelisable, putting the modelling of more complicated geometries and extension to the fully-3D case within reach