Acoustic cavitation: the fluid dynamics of non–spherical bubbles

In acoustic cavitation the spatial variation and time-dependent nature of the acoustic pressure field, whether it is a standing or propagating wave, together with the presence of other bubbles, particles and boundaries produces gradients and asymmetries in the flow field. This will inevitably lead to non-spherical bubble behaviour, often of short duration, before break-up into smaller bubbles which may act as nuclei for the generation of further bubbles. During the collapse phase, high temperatures and pressures will occur in the gaseous interior of the bubble. This paper concentrates on the non-spherical bubble extension to the earlier spherical-bubble studies for acoustic cavitation by exploiting the techniques that had previously been used to model incompressible hydraulic cavitation phenomena. Bubble behaviour near an oscillating boundary, jet impact and damage to boundaries, bubble interactions, bubble clouds and bubble behaviour near rough surfaces are considered. In many cases the key manifestation of the asymmetry is the development of a high-speed liquid jet that penetrates the interior of the bubble. Jetting behaviour can lead to high pressures, high strain rates (of importance to break-up of macromolecules) and toroidal bubbles, all of which can enhance mixing. In addition it may provide a mechanism for injecting the liquid into the hot bubble interior. Many practical applications such as cleaning, enhanced rates of chemical reactions, luminescence and novel metallurgical processes may be associated with this phenomenon.