Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Bottlenecks in granular flow: When does an obstacle increase the
  flowrate in an hourglass?   Bottlenecks occur in a wide range of applications from pedestrian and traffic
flow to mineral and food processing. We examine granular flow across a
bottleneck using particle-based simulations. Contrary to expectations we find
that the flowrate across a bottleneck actually increases if an opti- mized
obstacle is placed before it. The dependency of flowrate on obstacle diameter
is derived using a phenomenological velocity-density relationship that peaks at
a critical density. This relationship is in stark contrast to models of traffic
flow, as the mean velocity does not depend only on density but attains
hysteresis due to interaction of particles with the obstacle.

13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
10
The categories of this paper are: q-bio
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin
12
The categories of this paper are: nucl-th
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess
16
The categories of this paper are: stat
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
7
The categories of this paper are: hep-th
2
The categories of this paper are: math
20
The categories of this paper are: acc-phys
18
The categories of this paper are: econ