Abstract

Dynamics of a Bimodal Bubble Cavitation Cluster in a Bipolar Acoustic Pulse

G.N.Sankin, D.V.Voronin, V.S.Teslenko (Lavrentyev Institute of Hydrodynamics of SB RAS, Novosibirsk, Russia); R.Mettin, R.Geisler, W.Lauterborn (Drittes Physikalisches Institut, Universitat Goettingen, Goettingen, Germany)

e-mail: georgiisankin@yahoo.com

Bubble dynamics and luminescence in a focusing shock wave have been investigated experimentally. An electromagnetic generator of focused bipolar acoustic pulses with a strong rarefaction component (positive peak of 75 MPa, negative peak of -42 MPa, peak-to-peak duration of 2.2 us) from a transducer as a spherical segment (focal distance of 55 mm) was used. The study of cavitation in the convergent acoustic pulse has revealed conditions where two groups of bubbles are formed (micro-bubbles and macro-bubbles which differ from their time of collapse). For the bimodal bubble cluster, it is found that the micro-bubbles, which have accumulated energy in the primary pulse, are responsible for light flashes with their collapse (Tc ~ 1 us) in an internal positive pressure. This has been identified and determined experimentally to be a secondary compression wave generated by an inertial expansion of macro-bubbles. For the pulse compression in the cluster, a strengthening of the collapse of the micro-bubble is observed compared with the collapse in a plane acoustic wave with the same parameters of pressure.

Section : 4