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Ch. Lechner, M. Koch, W. Lauterborn, R. Mettin:
"Fast Jets from Collapsing Cavitation Bubbles";
Talk: Cavitation meets Data Science, Göttingen, Deutschland (invited); 2022-06-10 - 2022-06-11.

We give an overview on our research of the past few years concerning the dynamics of cavitation bubbles. Cavitation
bubbles close to solid surfaces have been studied intensively in the last decades. Nevertheless, the problem 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 [1]. 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, are
viscosity induced.
In this presentation we describe the mechanism leading to fast jet formation and present (predominantly) numerical
results on jet formation. Furthermore, our first photographic evidence of this phenomenon is given, using high-speed
imaging of laser-generated bubbles under normal ambient conditions [2, 3].
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
[4].
For bubbles oscillating close to a flat solid boundary the influence of several dimensionless parameters on the jet
formation process is investigated: the non-dimensional initial distance, D∗ , from the solid for millimeter sized bubbles
in water [5], an initial eccentricity of spheroidal bubbles, and a bubble Reynolds number for bubbles oscillating right
at the solid in various liquids.
Fast jet formation is demonstrated to be a robust phenomenon. It is found for values D ∗ ≲ 0.2 independent of the
initial eccentricity of the bubble, while for D∗ ≳ 0.24 the well known "standard" jets form by axial flow focusing. The
transition scenario between "standard jet" and fast jet is involved.
For bubbles with D* = 0 fast jet formation persists for a large range of Reynolds numbers down to e.g. millimeter
sized bubbles collapsing in 50cSt silicone oil. A further reduction of the Reynolds number leads to a change in the jet
formation process with ever decreasing jet speeds: from the fast collapse of a spherical cap, over the formation of a
standard jet via involution of the bubble wall to the absence of any jets in very viscous liquids, as e.g. PAO40.
For fast jets and intriguing bubble shapes resulting from a variation in geometry [6] we refer to the (poster)
presentation of M. Koch.
[1] C. Lechner, W. Lauterborn, M. Koch, and R. Mettin, Fast, thin jets from bubbles expanding and collapsing in extreme
vicinity to a solid boundary: A numerical study, Phys. Rev. Fluids 4, 021601 (2019).
[2] M. Koch, Laser cavitation bubbles at objects: Merging numerical and experimental methods, PhD thesis, Georg-August-
Universität Göttingen, Third Physical Institute (2020), http://hdl.handle.net/21.11130/00-1735-0000-0005-1516-B.
[3] M. Koch, J. M. Rosselló, C. Lechner, W. Lauterborn, J. Eisener, and R. Mettin, Theory-assisted optical ray tracing to
extract cavitation-bubble shapes from experiment, Exp. Fluids 62, 60 (2021).
[4] M. Koch, C. Lechner, F. Reuter, K. Köhler, R. Mettin, and W. Lauterborn, Numerical modeling of laser generated cavitation
bubbles with the finite volume and volume of fluid method, using OpenFOAM, Comput. Fluids 126, 71 (2016).
[5] C. Lechner, W. Lauterborn, M. Koch, and R. Mettin, Jet formation from bubbles near a solid boundary in a compressible
liquid: Numerical study of distance dependence, Phys. Rev. Fluids 5, 093604 (2020).
[6] M. Koch, J. M. Rosselló, C. Lechner, W. Lauterborn, and R. Mettin, Dynamics of a laser-induced bubble above the flat
top of a solid cylinder-mushroom-shaped bubbles and the fast jet, Fluids 7, 2 (2022).

Bubble dynamics, cavitation, jets, erosion