Modification of particle laden near-wall turbulence in a vertical channel bounded by rough walls

The paper deals with the mutual interactions between micro-particles and turbulence in vertical channel bounded by rough walls. Particles are momentum coupled with the fluid and are treated as pointwise spheres subject to Stokes drag, buoyancy and gravity forces. Two-way coupling is investigated using direct numerical simulation (DNS) of fluid flow with lagrangian particle tracking (LPT) and point-force approximation for small, heavy particles with a diameter smaller than the Kolmogorov length scale of the fluid. The particle back-reaction on the fluid flow is modelled by means of a modified version of the Particle-Source-In-Cell (PSIC) model. The friction Reynolds number of the unladen flow is Reτ=180, based on the friction velocity, uτ, of the free particle flow and the half wall distance, δ. The Stokes number, based on the wall units considered in the study, is St+=20 while the average mass and volume fractions ψm=0.141 and ϕv=1.7×10−4, respectively. The interest is focused on the effects of particles on the near-wall coherent structures. The issue has been widely analysed in turbulent flow bounded by smooth walls, whereas the effect of the roughness on momentum coupled two-phase flows has been much less investigated. The study of turbulence modulation is coupled to the analysis of slip velocity distribution, since both drag and particles back-reaction are closely linked to differences in fluid velocity seen by each particle and its own velocity. The effect of the roughness of the wall is investigated taking into account elastic rebounds of particles onto the wavy wall, instead of introducing a virtual rebound model. Results show that the main features of turbulence are generally maintained but the streamwise turbulence intensity increases whereas the velocity fluctuations in the wall-normal directions are damped, along with substantial attenuation of the Reynolds shear stress.