Laser-induced cavitation bubbles near a curved rigid boundary are observed experimentally using high-speed photography. An image theory is applied to obtain information on global bubble motion while a boundary integral method is employed to gain a more detailed understanding of the behaviour of a liquid jet that threads a collapsing bubble, creating a toroidal bubble. Comparisons between the theory and experiment show that when a comparable sized bubble is located near a rigid boundary the bubble motion is significantly influenced by the surface curvature of the boundary, which is characterized by a parameter ζ, giving convex walls for ζ < 1, concave walls for ζ > 1 and a flat wall when ζ = 1. If a boundary is slightly concave, the most pronounced migration occurs at the first bubble collapse. The velocity of a liquid jet impacting on the far side of the bubble surface tends to increase with decreasing parameter ζ. In the case of a convex boundary, the jet velocity is larger than that generated in the flat boundary case. Although the situation considered here is restricted to axisymmetric motion without mean flow, this result suggests that higher pressures can occur when cavitation bubbles collapse near a non-flat boundary. Bubble separation, including the pinch-off phenomenon, is observed in the final stage of the collapse of a bubble, with the oblate shape at its maximum volume attached to the surface of a convex boundary, followed by bubble splitting which is responsible for further bubble proliferation.
Vapour cavities in liquid flows have long been associated with cavitation damage to nearby solid surfaces and it is thought that the final stage of collapse, when a highspeed liquid jet threads the cavity, plays a vital role in this process. The present study investigates this aspect of the motion of laser-generated cavities in a quiescent liquid when the distance (or stand-off) of the point of inception from a rigid boundary is between 0.8 and 1.2 times the maximum radius of the cavity. Numerical simulations using a boundary integral method with an incompressible liquid impact model provide a framework for the interpretation of the experimental results. It is observed that, within the given interval of the stand-off parameter, the peak pressures measured on the boundary at the first collapse of a cavity attain a local minimum, while at the same time there is an increase in the duration of the pressure pulse. This contrasts with a monotonic increase in the peak pressures as the stand-off is reduced, when the cavity inception point is outside the stated interval. This phenomenon is shown to be due to a splash effect which follows the impact of the liquid jet. Three cases are chosen to typify the splash interaction with the free surface of the collapsing cavity: (i) surface reconnection around the liquid jet; (ii) splash impact at the base of the liquid jet; (iii) thin film splash. Hydrodynamic pressures generated following splash impact are found to be much greater than those produced by the jet impact. The combination of splash impact and the emission of shock waves, together with the subsequent re-expansion, drives the flow around the toroidal cavity producing a distinctive double pressure peak.
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 highspeed 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.
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