Effect of roughness on elongated particles in turbulent channel flow

In this paper, we examine the deposition and resuspension of rigid elongated particles transported by turbulence in a channel bounded by two-dimensional roughness. To analyze these processes, we use an Euler- Lagrange approach based on Direct Numerical Simulation of the carrier phase and Lagrangian Particle Tracking of the dispersed phase. Four different channel configurations are considered: One is the classical channel flow bounded by smooth flat walls, whereas the other three correspond to a channel with walls of increasing roughness. The roughness shape is obtained by superimposition of sinusoidal functions with different amplitude k and is characterized by the mean absolute value of the amplitude, k = 0.012, 0.024 and 0.050 (k = 0 for a smooth wall). The friction Reynolds number is Re = 150 for all cases. Particles are modeled as prolate ellipsoids and classified according to their aspect ratio lambda. Three different particles sets are considered: lambda= 1, corresponding to the reference case of spheres, lambda = 3, corresponding to slightly elongated particles, and lambda= 10, corresponding to long fiber-like particles. The particle response time is St(+) = 5 for all sets. In turbulent flow bounded by smooth walls, particles are known to accumulate preferentially in the near-wall region, leaving the central region of the channel scarcely populated. Wall roughness produces a completely different scenario: Particles exhibit a more homogeneous distribution along the wall-normal direction. We show that the aspect ratio does not affect the preferential distribution and the velocity statistics of the particles. The effect of elongation, however, becomes important for their preferential orientation, which is much weaker than in the smooth-walls case, in the near-wall region, while recovering the smooth-walls case in the outer region of the channel. This finding supports the validity of Townsend's similarity hypothesis, namely that the bulk flow dynamics are unaffected by the roughening of the bounding walls.