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, thin jets from bubbles oscillating in very close proximity to a flat solid surface [2]. These jets result from self-impact of annular inflow at the axis of symmetry and can reach a speed of the order of 1000 m/s. The annular inflow and thereby fast jet formation, paradoxically, is viscosity induced, since the boundary layer at the solid surface plays a distinctive role. In this presentation, we describe details of the mechanism leading to fast jet formation and present numerical and experimental results on the phenomenon. The numerical model consists of a bubble filled with a small amount of non-condensable gas in a compressible liquid. We use the volume of fluid method to capture the interface between liquid and gas. The Navier Stokes equations are discretized with the finite volume method. The model is implemented in the open source software package OpenFOAM [1]. From the computations, one can derive the pressure load on the object from jet impact and bubble collapse shock wave. Capturing the phenomenon experimentally is a notoriously difficult task, since the short time window of ~100 ns and small spatial region of ~50 μm 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 raytracing of a fitting CFD simulation [3, 4].

1] Koch, M.; Lechner, C.; Reuter, F.; Köhler, K.; Mettin, R.; Lauterborn, W. (2016) "Numerical modeling of laser generated cavitation bubbles with the finite volume and volume of fluid method, using OpenFOAM". Comput. Fluids, 126, 71–90. doi:10.1016/j.compfluid.2015.11.008. [2] Lechner, C.; Lauterborn, W.; Koch, M.; Mettin, R. (2019) Fast, thin jets from bubbles expanding and collapsing in extreme vicinity to a solid boundary: A numerical study. Phys. Rev. Fluids, 4, 021601. doi:10.1103/PhysRevFluids.4.021601 [3] Koch, M. (2020) “Laser cavitation bubbles at objects: Merging numerical and experimental methods”. http://dx.doi.org/10.53846/goediss-8326. PhD thesis. Georg-August-Universität Göttingen, Third Physical Institute. [4] Koch, M.; Rosselló, J.M.; Lechner, C.; Lauterborn, W.; Eisener, J.; Mettin, R. (2021) Theory-assisted optical ray tracing to extract cavitation-bubble shapes from experiment. Exp. Fluids, 62, 60. doi:10.1007/s00348-020-03075-6