|Title||Settling strongly modifies particle concentrations in wall-bounded turbulent flows even when the settling parameter is asymptotically small|
|Publication Type||Journal Article|
|Year of Publication||2021|
|Authors||AD Bragg, DH Richter, and G Wang|
|Journal||Physical Review Fluids|
We explore the role of gravitational settling on inertial particle concentrations in a wall-bounded turbulent flow. While it may be thought that settling can be ignored when the settling parameter Svvs/uτ is small (vs is Stokes settling velocity, uτ is fluid friction velocity), we show that even in this regime the settling may make a leading order contribution to the concentration profiles. This is because the importance of settling is determined, not by the size of vs compared with uτ or any other fluid velocity scale, but by the size of vs relative to the other mechanisms that control the vertical particle velocity and concentration profile. We explain this in the context of the particle mean-momentum equation, and show that in general, there always exists a region in the boundary layer where settling cannot be neglected, no matter how small Sv is (provided it is finite). Direct numerical simulations confirm the arguments and show that the near-wall concentration is highly dependent on Sv even when Sv≪1, and it can reduce by an order of magnitude when Sv is increased from O(10-4) to O(10-2). The results also show that the preferential sampling of ejection events in the boundary layer by inertial particles when Sv=0 is profoundly altered as Sv is increased, and it is replaced by a preferential sampling of sweep events due to the onset of the preferential sweeping mechanism.
|Short Title||Physical Review Fluids|