  We investigate the mechanisms by which inertial solid particles modulate
turbulence and alter the fluid mass transport in dense turbulent liquid-solid
flows. To this end, we perform Euler-Lagrange simulations at friction Reynolds
number 180, particle friction Stokes number 7.9, particle-to-fluid density
ratio 8.9, and particle volume fraction ranging from 1% up to 12%. We show that
the mechanisms underpinning the flow modulation are two fold: (I) the increase
of the suspension's apparent kinematic viscosity with increasing solid volume
fraction and (II) turbulence modulation through the particle feedback force.
For solid volume fraction below 3%, the increase of the suspension's apparent
kinematic viscosity by the disperse particles accounts for most of the flow
modification, namely, the reduction of turbulent fluctuations, reduction of the
bulk fluid velocity, and increase of friction coefficient. In denser channels,
the particle feedback force leads to greater reduction of bulk fluid velocity
and increase of friction coefficient than can be accounted for solely based on
the increased apparent kinematic viscosity. In these cases, particle stress
significantly alters the stress balance, to a point where it exceeds the
Reynolds stress at solid volume fraction 12%.
