Oscillations, Waves, and Interactions - GWDG

192 R. Mettin
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Figure 15. Comparison of experimental and simulated bubble tracks in a filamentary
streamer (from Ref. [54]).
to any disturbance. Nevertheless, the simulated bubble tracks can capture essential
features of the observed structures, like the gross arrangement of filaments, bubble
translation velocities, or bubble collision frequencies.
6.2 Double layer structure
A bubble structure that is frequently found in intense, well developed standing waves
is a double layer of dendritic filaments (“jellyfish” structure [53,55,56]). Experimental
observations at 25 kHz are shown in Fig. 16(a) and (b). The two layers are arranged
symmetrically to a pressure nodal plane, and each layer is situated between pressure
node and antinode region. Bubbles occur from the zone in-between the layers and
travel on dendrites to the layer centers. From time to time, bigger bubble clusters
form in the layers and move back towards the nodal plane, dissolving on their way.
Figures 16(c) and (d) present a simulation by the particle approach. The sound
field has been set to a standing wave of 25 kHz with an antinode pressure of 200 kPa,
and bubble creation has been assumed to happen in ring regions close to the pressure
nodal plane. It can be seen that the main aspect of the structure is captured by the
model. Detailed bubble tracks have not been recorded experimentally in this case,
and the validation of the simulation approach is substantially by eye. Nevertheless,
the model is helpful to explain the occurence and the shape of the structure. Important
aspects, though, concerning the bubble sources and the dynamics of the larger
bubble clusters, still have to be clarified. Possibly the liquid flow between the layers
is a further factor to be taken into account, as recent results from sonochemiluminescence
measurements suggest [57].