Enhancement of inertial particle transport in rough-wall turbulence

Direct numerical simulation is employed to investigate the transport of inertial particles in turbulent channel flows over spanwise rod-roughened walls. The study examines the effects of roughness geometry by varying the streamwise spacing-to-height ratio

from 1 to 199. Results indicate that rod roughness elements significantly modulate near-wall turbulent structures and drag coefficients, as both form and total drag reach a maximum value at approximately

w divided by k equals 9 w / k = 9 $w/k=9$

. For the dispersed phase, the presence of rough walls suppresses the near-wall accumulation of inertial particles typically observed in smooth-wall turbulence. This reduction in preferential concentration is attributed to the modification of near-wall flow structures, which weakens the correlation between particle and fluid motions. Specifically, the roughness elements disrupt canonical turbulent coherent structures and generate additional roughness-induced flow structures. Unlike the direct collision rebounding effect observed in curved roughness geometries, this decorrelation diminishes the effectiveness of flow structures in trapping particles within the near-wall region and serves as the primary mechanism for reduced near-wall accumulation. Furthermore, a more uniform particle distribution in the wall-normal direction leads to increased streamwise bulk velocity and enhanced transport efficiency, which vary depending on particle inertia and the streamwise spacing-to-height ratio. These findings underscore the critical role of wall roughness geometry in modulating particle spatial distribution and streamwise transport in wall turbulence.