Dynamic structures of bubble-driven liquid flows in a cylindrical tank

The spatial and temporal structures of turbulent water flows driven by air bubbles in a cylindrical tank were investigated. The time-resolved particle image velocimetry technique was adopted for quantitative visualization. Flow rates of compressed air were changed from 1 to 5 L/min at 0.5 MPa, and the corresponding range of bubble-based Reynolds number (Re) ranged from 8,300 to 21,100. The dynamics of flow structures was further investigated by the time-resolved proper orthogonal decomposition analysis technique. With increasing Re, mean velocity fields driven by the rising bubbles are almost same, but turbulence is dramatically enhanced. Both spatial and temporal modes were quite different with respect to the air flow rates. Three most dominant spatial structures are recirculating flow, bubble-induced motion, and sloshing of free surface, the bigger the latter the higher Re. We found the frequency of sloshing motion from flow visualization and the FFT analysis of temporal modes.