Generation and degradation mechanisms of near-wall particle streaks

In wall-bounded turbulent flows, inertial particles can form distinct streaks near the wall. In this work, the theory of Johnson et al. ( J. Fluid Mech. , 2020, vol. 883, A27) for wall-normal distribution of particles is generalised to arbitrary directions to reveal the generation mechanism of particle streaks and the degradation mechanism by shear-induced lift force. The Lagrangian point-particle approach is coupled with direct numerical simulations of a turbulent channel flow to provide reliable data for analyses. Simulation results demonstrate that the shear-induced lift force can remarkably reduce the near-wall particle concentration and the degree of particle streaking. Through theoretical analysis, it is found that the generalised turbophoresis effect plays a prominent role in both near-wall accumulation and streaking of particles. More specifically, without the lift force, quasi-streamwise vortices sweep and tend to accumulate particles in regions with calm spanwise flow, which form particle streaks. In contrast, if the lift force is included, the combination of quasi-streamwise vortices and spanwise migration of particles within the viscous sublayer can produce an excessive localised upward lift that elevates the particles to a higher location, where the spanwise turbophoresis force is significantly weaker, leading to the degradation of particle streaking. The newly proposed mechanism is supported by the simulation data. Finally, the effect of the gravitational force is studied, which is found to counteract the lift force, resulting in an enhanced near-wall particle concentration and particle streaks at the bottom wall.