Air bubbles released from an underwater nozzle emit an acoustical pulse which is of interest both for the study of bubble detachment and for elucidating the mechanism of sound generation by a newly formed bubble. In this paper we calculate theoretically the sequence of bubble shapes from a given nozzle and show that there is for each nozzle a bubble of maximum volume vmax Assuming that the bubble becomes detached at its ‘neck’, and that the volume of the detached bubble equals the volume V* of the undetached bubble above its ’neck’, we determine for each nozzle diameter D an acoustic frequency f* corresponding to 'slow’ bubble release.Experiments show that the acoustic frequency, hence the bubble size, depends on the rate of air.flow to the bubble, but for slow rates of flow the frequency f is very close to the theoretical frequency f*.High-speed photographs suggest that when the bubble pinches off. the limiting form of the surface is almost a cone. This is accounted for by assuming a line sink along the axis of symmetry. Immediately following pinch-off there is evidence of the formation of an axial jet going upwards into the bubble. This may play a part in stimulating the emission of sound.

Air bubbles released from an underwater nozzle emit an acoustical pulse that is of interest both for the study of bubble detachment and for elucidating the mechanism of sound generation by a newly formed bubble. In this paper, the sequence of bubble shapes is calculated theoretically from a given nozzle, and it is shown that there is for each nozzle a bubble of maximum volume Vmax. Assuming that the bubble becomes detached at its “neck,” and that the volume of the detached bubble equals the volume V* of the undetached bubble above its “neck,” it is determined for each nozzle diameter D an acoustic frequency f* corresponding to “slow” bubble release. Experiments show that the acoustic frequency, hence the bubble size, depends on the rate of air flow to the bubble, but for slow rates of flow the frequency f is very close to the theoretical frequency f*. High-speed photographs suggest that when the bubble pinches off, the limiting form of the surface is almost a cone. This is accounted for by assuming a line sink along the axis of symmetry. Immediately following pinch off, there is evidence of the formation of an axial jet going upward into the bubble. This may play a part in stimulating the emission of sound.

Abstract. The paper details results from an experimental study on bubbles rising in still tap water.Shape and motion parameters of the bubbles were measured using a combination of high speed cinematography and digital image processing. The Reynolds numbers of the bubbles studied ranged from about 700 to 1300, with the bubbles exhibiting all the familiar shape and motion characteristics: oblate spheroids becoming "wobbly", and spiralling or zig-zagging motion becoming "rocking" as the bubble size increased. Time series of the bubble major axes revealed regular oscillations in the bubble shape. In most cases three frequencies could be readily identified, corresponding to those of vortex shedding from the bubble and two modes of ellipsoidal harmonics (modes 2,0 and 2,2). Comparison of time series of bubble shape and motion indicated a strong interaction between the shape oscillations of mode 2,0 and bubble motion. As the bubble size increased the frequency of both shape oscillation modes approached that of the vortex shedding, which remained constant at about 12 Hz for all of our experiments. The frequencies become equal for bubbles larger than in our study, at a Reynolds number of about 3000. Using data from the literature we found that the vortex shedding appears to become locked-in on the mode 2,0 shape oscillation.