Oscillations, Waves, and Interactions - GWDG

194 R. Mettin
Figure 17. Left: Experimental cone bubble structure (sonotrode of 12 cm diameter, visible
on top, driven at 20.5 kHz; from Ref. [59]). Right: Simulated bubble distribution (note the
different scales of the axes; from Ref. [39]).
With reasonable assumptions on pressure amplitudes and bubble sizes, one can
obtain a quite similar scenario and bubble structure in the particle simulation. An
example is shown on the right in Fig. 17. For the calculation presented an idealized
analytical pressure and phase distribution has been used, and cone extension and
bubble velocities are not matching closely. Nevertheless, the structure is reproduced
in principle.
7 Conclusion and perspectives
In this article, it has been outlined how macroscopic manifestations of acoustic cavitation
can be approached from a single bubble point of view. The formation of
cavitation fields out of individual bubbles was the motivation for a closer consideration
of the forces on and the life cycles of acoustically driven bubbles. Several
important factors have been discussed: the origin of bubbles, their volume oscillations
and shape stability, gas diffusion, acoustic forces and bubble translation. Some
experimental observations have been presented and theoretical concepts have been
reviewed, partly on basis of inevitable or useful idealizations and approximations.
The different aspects have been condensed into bubble life cycle diagrams that reveal
a first idea of the bubble populations and conditions expected in a given set-up.
Further information can be extracted from the trajectories of single bubbles in their
parameter space. The next step was the treatment of multibubble fields in space and
time as an N-body or interacting particle system. Although certain details of bubble
nucleation and narrow interaction are still not sufficiently known, the numerical
solution of the equations of bubble motion can reproduce various observed bubble