Bubble collapse directly at an object: fast jet and shock wave

Abstract

Cavitation bubbles close to solid surfaces have been studied intensively in the last decades. Nevertheless, the issue is still good for surprises. A striking example is the formation of very fast jets from bubbles oscillating in close proximity to a flat solid surface. These jets result from self-impact of annular inflow at the axis of symmetry and can reach a speed of the order of 1000m/s. The annular inflow and thereby fast jet formation, paradoxically, are viscosity induced. The mechanism leading to fast jet formation is described, and numerical and experimental results are shown. The model consists of a bubble filled with a small amount of non-condensable gas in a compressible liquid. The Navier-Stokes equations are discretized with the finite volume method, and the volume of fluid method is applied to capture the interface between liquid and gas. Capturing the phenomenon experimentally is a notoriously difficult task, since the short time window of ~100ns and small spatial region of ~50μm where the jet occurs are demanding. Our first photographic evidence of this phenomenon is given, using high-speed imaging of laser-generated bubbles under normal ambient conditions, enhanced to subpixel resolution via ray tracing of a fitting CFD simulation.