Characterization of interactions between bubbles and fluid turbulence utilizing PTV-Based SGS

The mechanisms of turbulence modification by dispersed bubbles in an upward bubbly pipe flow were investigated. The liquid-phase velocity was measured by particle tracking velocimetry with Sub-Kolmogorov time and spatial resolution. Sub-Kolmogorov resolution is high enough to obtain small scale structures of the modified turbulence by bubbles. Gas-phase behaviors were measured by a shape projection imaging technique. Two different bubble diameters at the void fraction up to 1.5% were examined. The profiles of mean streamwise velocity of water were flattened in the pipe middle region, because the bubbles accumulated near the pipe wall accelerated the fluid. The flattened mean flow profiles suppressed the shear-induced turbulence intensities. In the present study, we introduced a scaling method by expanding the spatial filtering techniques applied for measured velocity fields to characterize length scales that govern the energy transfer between bubbles and turbulence. Large energy transport from bubbles to the fluids was clearly observed at the scale of from 2 to 3 times the bubble diameter. The findings above give guidance on the physics and modeling of multiphase turbulence.