Energy Spectral Properties Due to Bubble-Liquid Interaction in Oscillating-Grid Turbulence

In the present study, liquid-phase turbulence modulation induced by bubble swarms ascending in arbitrary turbulence is experimentally investigated. Liquid-phase homogeneous isotropic turbulence is formed using an oscillating grid in a cylindrical acrylic vessel of 149 mm in inner diameter. A bubble swarm of 19 bubbles is examined. The bubble size and generating time are completely controlled applying a bubble generating device using audio speakers. The bubble swarms are generated at a frequency of 4 Hz. The motion of individual bubbles of the bubble swarm is captured by a high-speed video camera. The liquid phase motion is also measured by PIV. The turbulence intensity, spatial correlation, integral scale and energy spectrum are calculated from the PIV data. It is confirmed that the oscillating-grid turbulence is an isotropic turbulence from the PIV result. When the bubble swarm is added, the original isotropic turbulence is modulated to an anisotropic turbulence by the mutual interaction between the bubble swarm and ambient isotropic turbulence. Moreover the bubble motion is notably changed. The bubble swarm contributes an increase of the energy spectrum to the ambient oscillating-grid turbulence. Moreover the suppression of energy spectrum occurred by interaction between bubble swarm and oscillating-grid turbulence compared with the energy spectrum in the flow induced by bubble swarms.