Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Mechanisms of Dispersion in a Porous Medium   This paper studies the mechanisms of dispersion in the laminar flow through
the pore space of a $3$-dimensional porous medium. We focus on pre-asymptotic
transport prior to the asymptotic hydrodynamic dispersion regime, in which
solute motion may be described by the average flow velocity and a hydrodynamic
dispersion coefficient. High performance numerical flow and transport
simulations of solute breakthrough at the outlet of a sand-like porous medium
evidence marked deviations from the hydrodynamic dispersion paradigm and
identifies two distinct regimes. The first regime is characterized by a broad
distribution of advective residence times in single pores. The second regime is
characterized by diffusive mass transfer into low-velocity region in the wake
of solute grains. These mechanisms are quantified systematically in the
framework of a time-domain random walk for the motion of marked elements
(particles) of the transported material quantity. The model is parameterized
with the characteristics of the porous medium under consideration and captures
both pre-asymptotic regimes. Macroscale transport is described by an
integro-differential equation for solute concentration, whose memory kernels
are given in terms of the distribution of mean pore velocities and trapping
times. This approach quantifies the physical non-equilibrium caused by a broad
distribution of mass transfer time scales, both advective and diffusive, on the
representative elementary volume (REV). Thus, while the REV indicates the scale
at which medium properties like porosity can be uniquely defined, this does not
imply that transport can be characterized by hydrodynamic dispersion.

15
The categories of this paper are: nucl-ex
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The categories of this paper are: math
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
25
The categories of this paper are: patt-sol
5
The categories of this paper are: gr-qc
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The categories of this paper are: adap-org
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The categories of this paper are: alg-geom
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The categories of this paper are: atom-ph
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The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
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The categories of this paper are: eess
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The categories of this paper are: stat
0
The categories of this paper are: hep-ph
6
The categories of this paper are: astro-ph