Turbulence Modulation Induced by Bubble Swarm in Oscillating-Grid Turbulence

In the present study, liquid-phase turbulence modulation induced by a bubble swarm ascending in arbitrary turbulence was experimentally investigated. Liquid-phase homogeneous isotropic turbulence was formed using an oscillating grid in a cylindrical acrylic vessel of 149 mm in inner diameter. A bubble swarm consisting of 19 bubbles of 2.8 mm in equivalent diameter was examined; the bubble size and generating time were completely controlled using a bubble generating device through audio speakers. This bubble generating device was able to repeatedly control the bubble swarm arbitrarily and precisely. The bubble swarm was generated at a frequency of 4 Hz. The liquid phase motion was measured via two LDA (Laser Doppler Anemometer) probes. The turbulence intensity, spatial correlation and integral scale were calculated from LDA data obtained by the two spatially-separate-point measurement. When the bubble swarm was added, the turbulence intensity dramatically changed. The original isotropic turbulence was modulated to the anisotropic turbulence by the mutual interaction between the bubble swarm and the ambient isotropic turbulence. The integral scales were calculated from the spatial correlation function. The increase in turbulence intensity and the decrease in integral scale were observed by injecting the bubble swarm in oscillating-grid turbulence.