Numerical investigation of momentum exchange between particles and coherent structures in low Re turbulent channel flow

The interaction between particles and coherent structures is studied by using discrete particle simulation combined with direct numerical simulation of gaseous flow in a vertical channel. A conditional sampling scheme is used to examine the modifications of the near-wall quasistreamwise vortices by the momentum exchange between the phases. The particle effect on the fluid flow is modeled by a point-force approximation. The particle diameters are smaller than both the smallest flow length scales and the computational grid spacing. Results are obtained for particle ensembles with four response times ranging from 10 to 200 wall units in numerical simulations with and without gravitational settling in the streamwise direction and interparticle collisions. It is found that the size of the quasistreamwise vortices is increased up to 25% in the presence of particles. The increase is larger for the smallest inertia particles studied, which is partly due to their locally nonuniform spatial distribution. The underlying organized fluid motions induced by the structures are substantially attenuated due to the momentum coupling of the phases. A reduction of 5%–55% is observed in the coherent fluid velocities and vorticities. The size of the coherent structures is additionally augmented by streamwise gravitational settling and interparticle collisions, accompanied by more obvious modifications in the surrounding fluid flow. The latter findings are explained by the stronger direct particle effect in these cases, which is also reflected on the energy redistribution between the fluid velocity components, affecting further the fluid turbulence.