The role of preferential sweeping on particle settling velocities in turbulence in the presence of two-way coupling

In Tom \& Bragg (J. Fluid Mech., 871, pp. 244--270, 2019) we used theory and Direct Numerical Simulations (DNS) to explore how the preferential sweeping mechanism that generates enhanced particle settling velocities in turbulence operates at different scales of the flow. We showed that the scales that contribute to preferential sweeping depend on the particle Stokes number, settling parameter, and the flow Reynolds number. That analysis, however, assumed one-way coupling. Monchaux \& Dejoan (Phys. Rev. Fluids 2, 104302, 2017) showed using DNS that even when the particle mass loading is small, although the effect of the particles on the global flow properties is weak, the particles can still strongly modify the local flow in their vicinity by dragging the surrounding fluid down as they fall, significantly influencing their settling velocities. We show using DNS that this enhancement due to two-way coupling in the dilute regime disappears as the Reynolds number increases, and we explain this using our multiscale theory. We also consider denser regimes, and explain how the fluid-dragging effect competes with the preferential sweeping mechanism at different scales in the flow.