Local flow topology in droplet-laden homogeneous isotropic turbulence*

A recent DNS study [J. Fluid Mech. 806 (2016) 356-412] showed that the introduction of droplets into homogeneous isotropic turbulence (HIT) increased the decay rate of turbulence kinetic energy due to enhanced dissipation near the droplet interface. Further analysis of this DNS dataset has shown that, over a range of droplet Weber numbers, density ratios, and viscosity ratios, the dissipation rate is about four times larger in a sublayer containing the droplet interface than it is in the carrier flow. To better understand the physical mechanisms leading to this increase, we characterize the local flow topology near the interface by computing the invariants of the velocity-gradient, rate-of-strain, and rate-of-rotation tensors in the carrier and droplet fluid. By conditioning their joint probability density functions on distance from the interface, the DNS results show that away from the interface the local flow topology resembles canonical HIT, but as the droplet interface is approached, the flow features increasingly deviate from HIT.