Turbulent wall-bounded particle-laden flows exhibit a variety of interesting phenomena that can greatly impact the underlying carrier-phase turbulence. At sufficiently low particle concentrations and mass loadings, it is well established that inertial particles will accumulate in regions of high strain and avoid regions of high vorticity. At larger concentrations and mass loadings, intimate coupling between the phases may lead to flow instabilities, resulting in the spontaneous generation of dense clusters that can completely reorganize the structure of the underlying fluid turbulence. This work aims at investigating the effect of particle clustering on the carrier-phase turbulence in both dilute and moderately-dilute channel flows with a friction Reynolds number Re_τ=630 using highly-resolved Euler-Lagrange simulations. To study the effect of gravity on cluster dynamics, simulations are conducted with gravity aligned in the mean flow direction, as well as gravity opposing the mean flow direction (i.e., a riser configuration). Particle segregation and velocity statistics are compared for each case. It is shown that the fluid turbulence departs significantly from the initially fully-developed turbulent flow when subject to a moderately dilute suspension of particles. In the denser channel flows, the gas velocity retains a viscous sublayer, but displays a strongly reduced boundary layer thickness and a flatter velocity profile compared to the unladen and dilute flows, leading to larger friction velocity. The particle concentration profile along the channel height is not found to be modified greatly by the increased particle loading, but is found to depend strongly on the orientation of gravity.
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