Role of boundary conditions and wall orientation on the transport of settling inertial particles in wall-bounded flows

TL;DR

This study investigates how boundary conditions and wall orientation influence the transport of settling inertial particles in wall-bounded turbulent flows, revealing that particle-wall interactions and flow orientation significantly affect particle settling behavior.

Contribution

It demonstrates how particle boundary conditions and flow orientation alter the impact of settling velocity on particle transport in wall-bounded flows, using direct numerical simulations.

Findings

Settling effects are negligible in vertical channels for small $Sv$.

Particle boundary conditions significantly influence settling sensitivity.

Low-speed streaks can suppress particle settling near walls.

Abstract

Gravitational settling affects particle transport in turbulent flows in two ways; explicitly, by introducing a finite settling velocity, and implicitly, by modifying how the particles interact with the flow field. For wall-bounded flows, when the wall is horizontal (gravity perpendicular to the wall) both the explicit and implicit effects of settling impact the particle transport towards the wall, whereas when the wall is vertical (gravity parallel to the wall) only the implicit effect plays a role. Surprisingly, it was recently demonstrated that even when the settling parameter $Sv$ is very small, settling can play a very significant role in controlling the near-wall transport in a horizontal channel. In this paper, we use direct numerical simulations to explore how this finding is affected by the particle boundary conditions and whether it also occurs in vertical channels where only the implicit effect of settling plays a role. We show that the sensitivity of the particle transport to $Sv$ depends upon the particle boundary conditions, with elastic-collisions (that generate a zero mean particle flux) and absorbing-wall conditions (that generate a negative mean particle flux) exhibiting qualitatively and quantitatively different sensitivities to $Sv$. It is found that for vertical channels the impact of settling on the particle transport is in fact negligible if $Sv$ is small, and only becomes significant when $Sv\geq O(1)$. Finally, we examine the physical mechanisms governing the settling velocity of particles in vertical channel flows, and find that low-speed streaks can play a significant role in suppressing the settling velocity of the particles near the walls, in contrast to the core region where the preferential sweeping mechanism leads to an enhancement of their settling velocity.