The Motion of a Bubble Swarm in Decaying Oscillating-Grid Turbulence and the Modulation of the Liquid-Phase Motion

The modulation induced by interaction between bubble motion and liquid-phase motion is important to deeply understand the multi-scale structure of a bubbly flow. In order to quantitatively and systematically clarify the interaction between the bubble swarm and the ambient liquid-phase motion, ideally controlled turbulence and bubble swarm are required. In the present study, we employed the decaying turbulence formed by oscillating grid and a well-controlled bubble swarm launched by hypodermic needles and audio speakers. We formed homogenous isotropic turbulence by using an oscillating grid (oscillating frequency: 4 Hz, stroke: 40 mm) in a cylindrical acrylic pipe (diameter: 149 mm, height: 600 mm) filled with ion-exchanged and degassed water. The decaying turbulence was formed after stopping the oscillating grid. A bubble swarm (: the member bubble rose zigzagging in stagnant water) was launched into the decaying turbulence after two seconds from stop of the oscillating grid. We measured the bubble swarm motion by visualization and did the liquid-phase motion by PIV/LIF system with high-speed video cameras. The measurements were performed under three experimental conditions; the first one is only the decaying turbulence (Condition-O), the second one is the bubble swarm launched into the stagnant water (Condition-B), the third one is the bubble swarm launched into the decaying turbulence (Condition-OB). From the visualization results, it was found out that the positions of the bubbles were expanded in the horizontal direction compared with that under Condition-B. This indicates the transition of the bubble motion from two-dimensional motion to three-dimensional motion was enhanced by the ambient turbulence. We calculated standard deviations of the liquid-phase velocities from the PIV results. When the bubble swarm was launched into the decaying turbulence, the decay rates of the liquid-phase velocities were enhanced; i.e. the decay rates under Condition-OB were larger than those under Condition-O.