  We use interface-resolved simulations to study near-wall turbulence
modulation by small inertial particles in dilute/semi-dilute conditions. We
considered three bulk solid mass fractions, $\Psi=0.34\%$, $3.37\%$, and
$33.7\%$, with only the latter two showing turbulence modulation. The increase
of the drag is strong at $\Psi=3.37\%$, but mild in the densest case. Actually,
two distinct regimes of turbulence modulation emerge: for smaller mass
fractions, the turbulence statistics are weakly affected and the near-wall
particle concentration increases the drag so the flow appears as a single phase
flow at slightly higher Reynolds number. Conversely, at higher mass fractions,
the particles modulate the turbulent dynamics over the entire flow, and the
interphase coupling becomes more complex. In this case, fluid Reynolds stresses
are attenuated, but the inertial particle dynamics increase the drag via
correlated velocity fluctuations leading to an overall drag increase. Hence, we
conclude that although particles at high mass fractions reduce the fluid
turbulent drag, the solid phase inertial dynamics may still increase the
overall drag.
