SHAPE OSCILLATIONS OF RISING BUBBLES

The paper details results from an experimental study on bubbles rising in still tap water. Shape and motion parameters of the bubbles were m easured using a combination of high speed cinematography and digital i mage processing. The Reynolds numbers of the bubbles studied ranged fr om about 700 to 1300, with the bubbles exhibiting all the familiar sha pe and motion characteristics: oblate spheroids becoming ''wobbly'', a nd 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 cou ld be readily identified, corresponding to those of vortex shedding fr om 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 s trong interaction between the shape oscillations of mode 2,0 and bubbl e motion. As the bubble size increased the frequency of both shape osc illation modes approached that of the vortex shedding, which remained constant at about 12 Hz for all of our experiments. The frequencies be come 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 oscillatio n.