How sensitive is the near-wall accumulation of inertial particles to settling in a vertical turbulent boundary layer?

For horizontal turbulent boundary layers, it was shown in Bragg, Richter and Wang [Physical Review Fluids, 2021], that even when the settling number Sv is small, settling can strongly modify the near-wall accumulation of inertial particles. In applications, particles can feel a gravitational force either normal to the wall (horizontal flows) or parallel to the wall (vertical flows). For vertical flows, settling does not explicitly impact the transport of the particles in the direction of the walls. However, it does affect it implicitly since settling impacts how the particles interact with the turbulent flow, and the timescales of the flow measured along the particle trajectories. To explore whether the sensitivity to small Sv that was observed for horizontal boundary layers also occurs for vertical boundary layers, we conduct direct numerical simulations (DNS) of particles settling in a vertical turbulent channel flow for a range of Stokes numbers (St) and Sv. Our results also provide insights into how the preferential sweeping of the flow by the particles depends on distance from the wall, which is different from the case of a horizontal flow where other mechanisms also impact the particle settling velocity.