Spatial and temporal structures of turbulent bubble-driven flows in a rectangular water tank

Abstract

The spatial and temporal structures of turbulent water flows driven by air bubbles in a rectangular water tank were investigated. The time-resolved particle image velocimetry (PIV) technique was adopted for quantitative visualization. Flow rates of compressed air were changed from 2 to 4 l/min at 0.5 MPa, and the corresponding range of bubble-based Reynolds number ranged from 6,740 to 13,220. The dynamics of flow structures was further investigated by the time-resolved proper orthogonal decomposition (POD) analysis technique. When the flow rate was increased, the main vortex core moved to the side and bottom wall. Locations of peak turbulent kinetic energy regions depended on the bubble Reynolds number. Both spatial and temporal modes were quite different with respect to the flow rates. The first temporal mode was harmonized with the second temporal mode, with small oscillations in the case of the lowest Reynolds number. However, temporal modes oscillate with higher frequencies when the Reynolds number increases. Based on the result of the FFT analysis of each temporal mode, we conjectured that low-frequency oscillation was attributed to the recirculating flow, whereas a higher dominant frequency was related to the vibration of the free surface that interacts with the rising bubbles.