Effects of wall roughness on particle dynamics in a spatially developing turbulent boundary layer

Direct numerical simulations were performed to investigate particle dynamics in a turbulent boundary layer over a rough wall using the two-way coupled Eulerian-Lagrangian point-particle approach. Wall roughness was represented by a series of staggered arranged hemispherical roughness elements. Particle sizes and roughness spacing were varied to analyze their effects on particle dynamics. The influence of the wall conditions on the particle velocity and distribution is significant in the near-wall region but negligible in the outer region. Wall roughness reduces the streamwise particle velocity and increases the fluctuating velocities. Moreover, wall roughness attenuates the particle trapping in the wall layer. Instead, particles mostly disperse above roughness elements. The reduction of the roughness spacing further attenuates the particle trapping. For smaller particles, the concentration is larger in the near-wall region, and the particle distribution along the wall-normal direction is more uniform. The attenuation of the particle trapping in the wall layer can be attributed to three physical mechanisms: (1) wall roughness increases the particle-wall collision frequency and enhances the wall-normal dispersion of particles; (2) the mature vortices and the coupling newly-born vortices, which are responsible for the particle trapping, are destroyed, and the shear layers produced at the top of roughness elements prevent particles from getting close to the wall and entrain them into the outer flow; (3) the particle back-reaction destroys the vortical structures and elongates the shear layers, which further prevents particles from nearing the wall. (C) 2018 Elsevier Ltd. All rights reserved.