2002, Vol.5, No.1, pp.71-77
Cavitation bubbles in acoustic resonators, when the
forcing is strong enough, are observed to arrange in branched
filamentary structures resembling very much electrical discharge
patterns. The filamentary cavitation structures, named acoustic
streamers, consist of oscillating bubbles of small size that move
towards the central pressure antinode, being free from
coalescence. No satisfactory theory exists yet to explain the
formation of such structures. The present paper seeks to
establish a mechanism responsible for the acoustic streamer
formation, based on the approximation of pairwise interactions
between bubbles. To this end, using the Lagrangian formalism,
equations of radial and translational motions of two spherical
gas bubbles are derived, allowing for both coupling between the
volume and translational bubble oscillations and mutual radiation
interaction between the bubbles. The equations are then used in
numerical investigation of translational motion of two small,
driven well below resonance, bubbles in strong acoustic fields
with pressure amplitudes exceeding 1 bar. It is shown that, if
the forcing exceeds a threshold, the bubbles form a bound pair
with a dynamical equilibrium spacing rather than collide and
coalesce as they do in a weak field. This result, hopefully,
seems to provide an explanation for the cavitation bubble
patterns.
Key words:
acoustic cavitation, acoustic streamer, bubble
dynamics
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