Numerical study on the motion of microscopic oil droplets in high intensity isotropic turbulence

The rise of small oil droplets in water under isotropic turbulence conditions is analyzed computationally. The effort focuses on the puzzling behavior observed by Friedman and Katz [Phys. Fluids 14, 3059 (2002)], namely, that the rise velocity of droplets smaller than 800 μm in diameter is enhanced by turbulence, whereas the rise of larger droplets is suppressed. Specifically, the study explores whether these effects can be captured or explained using a simplified one-way coupling model that combines direct numerical simulation of the turbulent flow field with Lagrangian tracking of the droplets using a dynamical equation that accounts for buoyancy, virtual mass, pressure, drag, lift, and history forces. The computational method used is adapted from the model of Snyder [Phys. Fluids 19, 065108 (2007)], which showed excellent correlation between computational results and extensive experimental data for microbubbles in isotropic turbulence. The computed results indicate that, using the quasisteady, empirically determined drag and lift coefficients, one is unable to reproduce the experimentally observed droplet rise velocities. Numerical experiments on the effect of lift and history forces also indicate that, within a broad range of uncertainty, these forces do not account for the discrepancy between measured and computed trends. Guided by correlations obtained for the settling of heavy particles under high turbulence intensities, suppression of the drag and virtual mass coefficients for droplet diameters near ten times the Kolmogorov lengthscale was consequently postulated. Computed results indicate that, using this postulate, the simplified model is able to recover the observed enhancement of the mean rise of small droplets. These experiences underscore the difficulties in modeling the motion of small particles under high turbulence intensities, especially when the particle size is close to the turbulence microscale.