Experiments to determine the velocity fields around the skirt bubbles

When a large bubble rises in a viscous Newtonian fluid, for a certain range of conditions, a “skirt” develops behind the bubble (thin layer of gas). Early experimental studies identified a single recirculation attached to the tip of the skirt film. However, a recent numerical study by Legendre (PRFluids, 2022) identified a second toroidal recirculation within the skirt bubble. In the present study, we experimentally investigate the flow fields within the skirt bubble using particle image velocimetry (PIV). The experimental setup to study the skirt bubble consists of a 10 cm × 10 cm × 1 m tank with a hemispherical “dumping cup” used to make bubbles of the required diameter. Viscous Newtonian fluids prepared from water-corn syrup mixtures of varying viscosities were used. Our preliminary results agree with the relation for terminal velocity and drag force provided for all bubble shapes by Legendre (PRFluids, 2022). To identify the vortex within the skirt bubble, the Q-criterion was used. The results compare well with the predictions but some discrepancies were found which will be discussed in the presentation.