Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of aerodynamic instability in the flow of a particle stream
  in a dilute gas   Forming macroscopic solid bodies in circumstellar discs requires local dust
concentration levels significantly higher than the mean. Interactions of the
dust particles with the gas must serve to augment local particle densities, and
facilitate growth past barriers in the metre size range. Amongst a number of
mechanisms that can amplify the local density of solids, aerodynamic streaming
instability (SI) is one of the most promising. This work tests the physical
assumptions of models that lead to SI in protoplanetary disks (PPDs). We
conduct laboratory experiments in which we track the three-dimensional motion
of spherical solid particles fluidized in a low-pressure, laminar,
incompressible, gas stream. The particle sizes span the Stokes-Epstein drag
regime transition and the overall dust-to-gas mass density ratio is close to
unity. Lambrechts et al. (2016) established the similarity of the laboratory
flow to a simplified PPD model flow. We obtain experimental results suggesting
an instability due to particle-gas interaction: i) there exist variations in
particle concentration in the direction of the mean drag forces; ii) the
particles have a tendency to 'catch up' to one another when they are in
proximity; iii) particle clumping occurs on very small scales, which implies
local enhancements above the background dust-to-gas mass density ratio by
factors of several tens; v) the presence of these density enhancements occurs
for a mean dust-to-gas mass density ratio approaching or greater than 1; v) we
find evidence for collective particle drag reduction when the local particle
number density becomes high and when the background gas pressure is high so
that the drag is in the continuum regime. The experiments presented here are
precedent-setting for observing SI under controlled conditions and may lead to
a deeper understanding of how it operates in nature.

2
The categories of this paper are: math
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
16
The categories of this paper are: stat
21
The categories of this paper are: adap-org
25
The categories of this paper are: patt-sol
24
The categories of this paper are: dg-ga
7
The categories of this paper are: hep-th
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: q-fin