Bubble clustering and trapping in large vortices. Part 1: Triggered bubbly jets investigated by phase-averaging

The characteristics of large coherent structures in turbulent, periodically-excited, as well as naturally-developing bubbly jets are investigated, and interactions between bubbles and large-scale vortices are studied quantitatively.As information on size and development of the large-eddy structures is very difficult to obtain in naturally-developing flows, periodical excitation of the jet with frequencies in the neighbourhood of natural instabilities is applied to systematically create large coherent structures. When bubbles enter such vortices, they can be trapped if certain conditions are fulfilled; a trapping criterion is developed.To study trapping phenomena, simultaneous two-phase PIV (particle image velocimetry), DOS (double optical sensors), LIF (laser induced fluorescence) and photographic techniques were applied for tracking the large vortices and bubble structures. In order to quantify the interaction between bubbles and the large vortices that are formed in the shear layer, characteristic phase-averaged quantities were determined by PIV and DOS.