Ch. Lechner, M. Koch, W. Lauterborn, R. Mettin:

"Collapse of laser-generated cavitation bubbles close to a solid boundary - results from a numerical model";

Talk: The 12th European Fluid Mechanics Conference, Wien; 2018-09-09 - 2018-09-13.

Cavitation bubbles collapsing close to solid boundaries have been subject to

intense investigations for several decades.

They show a rich variety of phenomena that qualitatively and quantitatively

depend on the normalized distance $D^*$ from the boundary.

Due to their potential role in eroding the solid surface

the most prominent of these phenomena are the formation of a high-speed

axial liquid jet that pierces the bubble and the emission of shock waves

possibly forming very complex patterns. During collapse and subsequent

rebound the bubble moves towards the solid boundary such that the second

collapse happens directly at the wall for a certain range of initial $D^*$.

The experimental method to generate cavitation bubbles by focusing a short

laser pulse into a transparent liquid, while being minimal invasive,

allows to produce bubbles at well defined positions that expand to

some maximum volume in a very reproducible way. Amongst others, this method

has been used to study cavitation bubbles close to a solid boundary

in a systematic way\footnote{Philipp, Lauterborn, \emph{J.~Fluid Mech.}

\textbf{361} (1998)}${}^{,}$\footnote{Lindau, Lauterborn,

\emph{J.~Fluid Mech.} \textbf{479}, 327 (2003)}.

Numerical simulations can reveal details on the collapse

that are not, or not easily, accessible in the experiment. Recent

numerical investigations showed the formation of a secondary sheet-like

annular jet\footnote{Lechner, Koch et al.,

\emph{J.~Acoust.~Soc.~Am.~}\textbf{142},

3649 (2017)}${}^{,}$\footnote{Lauterborn, Lechner et al.~\emph{IMA J.~Appl.~Math.},

accepted} and demonstrated the propagation of a

strong tension wave in the funnel of the torus bubble

after jet impact$^4$, the latter supporting the view of the counter-jet

observed in experiments as secondary cavitation.

In this talk we present results from numerical simulations modeling the

collapse and rebound of laser-generated cavitation bubbles with an initial

normalized distance $D^* \in [1.02,2.6]$ from a rigid wall.

The numerical model consists of a bubble filled with a small amount of

non-condensable gas in a compressible liquid obeying the Tait equation of

state. 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. We have adapted a compressible two-phase solver from

the open source software package OpenFOAM\footnote{The OpenFOAM(R) Extend

Project. http://sourceforge.net/projects/openfoam-extend} for

our purpose\footnote{Koch, Lechner et al.,

\emph{Comput.~Fluids} \textbf{126}, 71 (2016)}.

We describe the dynamics of the bubble (jet formation and impact,

splitting of the torus bubble, first collapse, rebound, second collapse),

the pattern of shock waves as well as

the pressure signal at the solid boundary as a function of $D^*$.

The numerical results concerning bubble shape parameters, jet velocities

and pressure signals are compared to experimental data.

Created from the Publication Database of the Vienna University of Technology.