Modulation of homogeneous turbulence in buoyancy driven bubbly flows

Turbulent multiphase flows are ubiquitous in nature and engineering applications. Examples include air bubbles in the ocean, dispersed pollutants in atmosphere, waterfall mists and bubble columns in process technology. In all these flows,there exists a complex interaction between the dispersed particle phase and the carrier phase triggering modification of the underlying turbulence. In this work, we investigate the modulation of homogeneous turbulence in buoyancy driven bubbly flows. To this end, we use Direct Numerical Simulation (DNS) to study rising bubbles in a two-fluid system (5 % volume fraction of the dispersed phase) with density and viscosity ratio of the carrier phase to the dispersed phase equal to 100. In our study, spherical bubbles are initially randomly distributed in the turbulent flow, the Volume of Fluid method is exploited to capture the complex feature of the liquid-gas interface, and the energy is injected at large scales using the Arnold-Beltrami-Childress (ABC) forcing to sustain the background turbulence. In the present work, we examine the effect of gravity on turbulence modulation in terms of global quantities, energy spectra, fluctuating velocity correlation and scale by scale (SBS) energy budgets. The results show that by increasing Galilei number (ratio of buoyancy to viscous force) larger bubbles rise faster due to the buoyancy force, resulting in anisotropic flow behavior, an increase of the energy content at small scales, and also augmentation of bubble break ups and consequently total interface area when increasing the ratio between bubble rising velocity and turbulence fluctuations.