  For one-way coupled (1WC) flows, Tom & Bragg (J. Fluid Mech., 871, pp.
244-270, 2019) advanced the analysis of Maxey (J. Fluid Mech., 174, pp.
441-465, 1987), which applied to weakly inertial particles, to particles of
arbitrary inertia, and the new theoretical result revealed the role that
different scales play in the preferential sweeping mechanism that leads to
enhanced particle settling in turbulent flows. Monchaux & Dejoan (Phys. Rev.
Fluids, 2, 104302, 2017) showed using direct numerical simulations (DNS) that
while for low particle loading the effect of two-way coupling (2WC) on the
global flow statistics is weak, 2WC enables the particles to drag the fluid in
their vicinity down with them, significantly enhancing their settling, and they
argued that 2WC suppresses the preferential sweeping mechanism. We explore this
further by considering the impact of 2WC on the contribution made by eddies of
different sizes on the particle settling. In agreement with Monchaux & Dejoan,
we show that even for low loading, 2WC strongly enhances particle settling, and
we show how 2WC modifies the contribution from different flow scales. However,
contrary to their study, we show that preferential sweeping remains important
in 2WC flows. In particular, for both 1WC and 2WC flows, the settling
enhancement due to turbulence is dominated by contributions from particles in
straining regions of the flow, but for the 2WC case, the particles in these
regions also drag the fluid down with them, leading to an enhancement of their
settling compared to the 1WC case.
