Turbulence modulation in dense liquid-solid channel flow

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 $1\%$ to $12\%$. We show that the mechanisms underpinning the flow modulation are twofold: (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 $12\%$.