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The interaction of small heavy solid particles with turbulence near the wall of a vertical downward channel flow is investigated by using direct numerical simulation (DNS) and Lagrangian particle tracking. The interest is focused on the effect of the particles on the near-wall coherent structures obtained by conditional sampling of DNS results of a particle-laden turbulent channel flow. The coherent structures are detected from instantaneous flow fields by using the vortex definition of Jeong and Hussain [J. Fluid Mech. 285, 69 (1995)]. The Reynolds number of the particle-free flow is Reτ≈180 based on the friction velocity and the wall half distance. The particle response time is 200 wall units and the average mass and volume fractions φm=0.5 and φv=6.8×10−5, respectively. The particle diameter is smaller than the Kolmogorov length scale and the grid spacing, the latter being small enough to adequately resolve the smaller fluid flow scales. The feedback effect of the particles on the carrier phase is taken into account by a point-force model. Purely elastic interparticle collisions are also considered. For both particle-free and particle-laden flows, the dominant coherent structures in the near-wall region are elongated quasistreamwise vortices. The addition of particles results in a weaker mean structure, with larger diameter and longer streamwise extent. The qualitative characteristics of the velocity distributions around the mean coherent structures are similar, independent of the particles. However, the coherent velocity fluctuations in the wall-normal and spanwise directions considerably decrease, and the low-speed streak is damped by the particles. The educed results show that the particles create a torque of opposite sign to the rotation of the mean vortex, which in turn reduces the streamwise vorticity of the structure. Consequently, the magnitude of fluid pressure decreases and the redistribution of turbulent kinetic energy from the streamwise to the other velocity components is significantly reduced.

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.

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