When is preferential sweeping important for particle settling if two-way coupling is present?

In Tom et al. (J. Fluid Mech. 947, A7, 2022) we explored how two-way coupling (2WC) affects particle settling in turbulence and whether preferential sweeping remains important when 2WC is present. Within the confines of the limited parameter regime explored, we observed that even when the particle mass loading is low, 2WC significantly enhances the particle settling compared with the one-way coupling (1WC) case. Moreover, contrary to previous claims, we showed that preferential sweeping remains the mechanism by which turbulence enhances particle settling. However, that study explored a limited portion of the parameter space. In this work, we explore the problem over a more extensive range of Stokes and Froude numbers, and especially mass-loadings. Our simulations explore regimes where the effect of 2WC on the global fluid statistics is negligible (as in Tom et al.) to strong. We see that inertial particles preferentially sample strain-dominated regions of the flow even at substantial mass loadings, and crucially, that the enhancement of the settling velocities due to turbulence arises principally in the vicinity of high-stain, low-vorticity regions of the flow. Thus, preferential sweeping remains active even in parameter regimes where it was previously argued to be unimportant.