  We explore the role of gravitational settling on inertial particle
concentrations in a wall-bounded turbulent flow. While it may be thought that
settling can be ignored when the Stokes settling velocity $v_s$ is small
compared with the fluid friction velocity $u_\tau$, we show that even in this
regime the settling may make a leading order contribution to the concentration
profiles. This is because the importance of settling is determined, not by the
size of $v_s$ compared with $u_\tau$ or any other fluid velocity scale, but by
its size relative to the turbophoretic drift velocity. We explain this in the
context of the particle mean-momentum equation, and show that in general, there
always exists a region in the boundary layer where settling cannot be
neglected, no matter how small $v_s/u_\tau$ is (provided it is finite). Direct
numerical simulations confirm the arguments, and show that even for
$v_s/u_\tau=O(10^{-2})$, the near wall particle concentration can be orders of
magnitude larger than for the case where $v_s/u_\tau=0$, and for
$v_s/u_\tau=O(10^{-3})$ enhancements of up to a factor of two are observed. For
weakly inertial particles, it is found that even for $v_s/u_\tau=O(10^{-2})$,
the preferential sampling of ejection events in the boundary layer by the
inertial particles is profoundly modified compared with the $v_s/u_\tau=0$
case.
