Particle transfer in the wall region of turbulent boundary layers is dominated by the coherent structures which control the turbulence regeneration cycle. Coherent structures bring particles toward the wall and away from the wall and favour particle segregation in the viscous region giving rise to nonuniform particle distribution profiles which peak close to the wall. In this work, we focus on the transfer mechanism of different size particles and on the influence of gravity on particles deposition. By tracking O(105) particles in Direct Numerical Simulation (DNS) of a turbulent channel flow at Reτ = 150, we find that particles may reach the wall directly or may accumulate in the wall region, under the low-speed streaks. Even though low-speed streaks are ejection-like environments, particles are not re-entrained into the outer region. Particles segregated very near the wall by the trapping mechanisms we investigated in a previous work  are slowly driven to the wall. We find that gravity plays a role on particle distribution but, for small particles (τp+ < 3), the controlling transfer mechanism is related to near-wall turbulence structure.
Transfer, Segregation and Deposition of Heavy Particles in Horizontal and Vertical Turbulent Channel Flows
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Marchioli, C, Sbrizzai, F, & Soldati, A. "Transfer, Segregation and Deposition of Heavy Particles in Horizontal and Vertical Turbulent Channel Flows." Proceedings of the ASME/JSME 2003 4th Joint Fluids Summer Engineering Conference. Volume 1: Fora, Parts A, B, C, and D. Honolulu, Hawaii, USA. July 6–10, 2003. pp. 501-508. ASME. https://doi.org/10.1115/FEDSM2003-45737
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