Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: When is settling important for particle concentrations in wall-bounded
  turbulent flows?   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 $Sv\equiv v_s/u_\tau$ is
small ($v_s$ - Stokes settling velocity, $u_\tau$ - 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 $v_s$ compared with $u_\tau$ or any
other fluid velocity scale, but by the size of $v_s$ 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 $Sv\ll 1$, and
can reduce by an order of magnitude when $Sv$ is increased from $O(10^{-4})$
and $O(10^{-2})$. 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 is replaced by a preferential
sampling of sweep events due to the onset of the preferential sweeping
mechanism.

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The categories of this paper are: astro-ph
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The categories of this paper are: q-alg
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The categories of this paper are: alg-geom
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The categories of this paper are: cs
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The categories of this paper are: hep-ex
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The categories of this paper are: comp-gas
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The categories of this paper are: funct-an
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The categories of this paper are: q-bio
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The categories of this paper are: patt-sol
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The categories of this paper are: econ
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The categories of this paper are: math-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: gr-qc
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The categories of this paper are: chao-dyn
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The categories of this paper are: bayes-an
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The categories of this paper are: solv-int