Effects of secondary flow on particle distribution and transport in rough-wall turbulent channel flows

Particle-laden turbulent flows over spanwise heterogeneous surfaces induce secondary flows of Prandtl’s second kind due to non-uniform transverse Reynolds stresses. However, the impact of these secondary flows on particle spatial distribution and transport remains insufficiently understood. In this study, we perform Eulerian–Lagrangian point-particle direct numerical simulations to investigate how secondary flows induced by streamwise-aligned rectangular riblets modulate particle dynamics within a turbulent channel flow. The results indicate that at a fixed spanwise periodicity, altering the riblet width causes a complex non-monotonic variation in the secondary flow. As the riblet width increases, the core intensity of the secondary flow initially decreases, subsequently increases, and ultimately decreases again. Concurrently, the wall-normal position of the secondary flow shifts outward, then moves closer to the wall, and finally shifts outward. The presence of these secondary flows creates a pronounced spanwise inhomogeneity in the particle distribution. Particles preferentially accumulate near the corners of the riblets, accompanied by higher concentrations within upwash regions and lower concentrations within downwash regions. Furthermore, increasing the riblet width leads to a non-monotonic particle distribution in the channel center, which consequently causes a non-monotonic variation in the streamwise transport velocity. Specifically, the particle transport velocity initially decreases, subsequently increases, and ultimately decreases again when the riblet width is increased. Notably, within an optimal range of riblet widths, the streamwise particle velocity exceeds the velocity observed over a smooth wall at an identical Reynolds number. Overall, when a specific riblet width forces the secondary flow closer to the wall and promotes its ejection capability, the flow structure more effectively entrains particles located near the wall into the bulk flow. This entrainment mechanism ultimately yields an increased particle concentration in the channel center and significantly enhances the overall streamwise transport. These findings provide a theoretical foundation for designing targeted surface topographies to passively manipulate particle dispersion in engineering and natural environments.