Transport of inertial ellipsoidal particles in turbulent flow over rough walls

The dispersion of rigid elongated particles in turbulent channel flow bounded by rough walls is examined. In particular, the interplay between particle inertia, particle length, and wall roughness characteristics in determining particle spatial distribution, preferential orientation, and alignment within the flow is analyzed. The dispersion process is investi- gated by performing direct numerical simulations of the turbulent flow coupled with the Lagrangian tracking of the particles, modeled as prolate ellipsoids with varying aspect ratio and inertia, under dilute flow conditions. Simulations are carried out at friction Reynolds number Reτ = 180 based on the channel half height in a domain confined by walls with different two-dimensional roughness characteristics. Particle elongation is varied consider- ing different aspect ratios (ranging from λ = 1 for the reference case of spherical particles to λ = 10 for the longest particle set) while particle inertia is varied considering different Stokes numbers (ranging from St+ = 1 for the least inertial particles to St+ = 100 for the most inertial ones), for a total of 12 different particle sets. Our results show that particles are affected by the turbulent structures that form near the rough walls in a way that is biased both by elongation, namely length, and inertia. This is not observed with smooth walls: In this case, increasing particle inertia always leads to a stronger turbophoretic particle drift to the walls. Elongation is observed to have a quantitative effect on the slip velocity statistics, particularly for particles with small Stokes number. Differences induced by a change of the aspect ratio tend to vanish as particle inertia increases, since the relative translational motion between the particles and the surrounding fluid appears to be closely connected to preferential concentration phenomena. Wall roughness modifies the overall dynamics of the particles, as well as their distribution, orientation, and alignment with flow direction, especially in the near-wall region. Roughness hinders long-term wall deposition of particles and determines a more uniform spatial distribution across the channel. In the near-wall region, elongated particles tend to align with the streamwise direction even in the presence of large-scale roughness, albeit to a lesser extent compared to the case of smooth walls. A stronger effect of roughness is noticed on the alignment along the spanwise and wall-normal directions. Such alignment, as well as preferential orientation, is hampered by the strong velocity fluctuations induced by roughness and the behavior of the ellipsoidal particles resembles again that of spherical particles.