  We use theory and Direct Numerical Simulations (DNS) to explore the average
vertical velocities and spatial distributions of inertial particles settling in
a wall-bounded turbulent flow. The theory is based on the exact phase-space
equation for the Probability Density Function describing particle positions and
velocities. This allowed us to identify the distinct physical mechanisms
governing the particle transport. We then examined the asymptotic behavior of
the particle motion near the wall, revealing the fundamental differences to the
near wall behavior that is produced when incorporating gravitational settling.
When the average vertical particle mass flux is zero, the averaged vertical
particle velocity is zero away from the wall due to the particles
preferentially sampling regions where the fluid velocity is positive, which
balances with the downward Stokes settling velocity. When the average mass flux
is negative, the combined effects of turbulence and particle inertia lead to
average vertical particle velocities that can significantly exceed the Stokes
settling velocity, by as much as ten times. Sufficiently far from the wall, the
enhanced vertical velocities are due to the preferential sweeping mechanism.
However, as the particles approach the wall, the contribution from the
preferential sweeping mechanism becomes small, and a downward contribution from
the turbophoretic velocity dominates the behavior. Close to the wall, the
particle concentration grows as a power-law, but the nature of this power law
depends on the particle Stokes number. Finally, our results highlight how the
Rouse model of particle concentration is to be modified for particles with
finite inertia.
