4,094 research outputs found
Aggregation-diffusion equations: dynamics, asymptotics, and singular limits
Given a large ensemble of interacting particles, driven by nonlocal
interactions and localized repulsion, the mean-field limit leads to a class of
nonlocal, nonlinear partial differential equations known as
aggregation-diffusion equations. Over the past fifteen years,
aggregation-diffusion equations have become widespread in biological
applications and have also attracted significant mathematical interest, due to
their competing forces at different length scales. These competing forces lead
to rich dynamics, including symmetrization, stabilization, and metastability,
as well as sharp dichotomies separating well-posedness from finite time blowup.
In the present work, we review known analytical results for
aggregation-diffusion equations and consider singular limits of these
equations, including the slow diffusion limit, which leads to the constrained
aggregation equation, as well as localized aggregation and vanishing diffusion
limits, which lead to metastability behavior. We also review the range of
numerical methods available for simulating solutions, with special attention
devoted to recent advances in deterministic particle methods. We close by
applying such a method -- the blob method for diffusion -- to showcase key
properties of the dynamics of aggregation-diffusion equations and related
singular limits
Spikes and diffusion waves in one-dimensional model of chemotaxis
We consider the one-dimensional initial value problem for the viscous
transport equation with nonlocal velocity with a given kernel . We show the existence
of global-in-time nonnegative solutions and we study their large time
asymptotics. Depending on , we obtain either linear diffusion waves ({\it
i.e.}~the fundamental solution of the heat equation) or nonlinear diffusion
waves (the fundamental solution of the viscous Burgers equation) in asymptotic
expansions of solutions as . Moreover, for certain aggregation
kernels, we show a concentration of solution on an initial time interval, which
resemble a phenomenon of the spike creation, typical in chemotaxis models
The AU Microscopii Debris Disk: Multiwavelength Imaging and Modeling
(abridged) Debris disks around main sequence stars are produced by the
erosion and evaporation of unseen parent bodies. AU Microscopii (GJ 803) is a
compelling object to study in the context of disk evolution across different
spectral types, as it is an M dwarf whose near edge-on disk may be directly
compared to that of its A5V sibling beta Pic. We resolve the disk from 8-60 AU
in the near-IR JHK' bands at high resolution with the Keck II telescope and
adaptive optics, and develop a novel data reduction technique for the removal
of the stellar point spread function. The point source detection sensitivity in
the disk midplane is more than a magnitude less sensitive than regions away
from the disk for some radii. We measure a blue color across the near-IR bands,
and confirm the presence of substructure in the inner disk. Some of the
structural features exhibit wavelength-dependent positions. The disk
architecture and characteristics of grain composition are inferred through
modeling. We approach the modeling of the dust distribution in a manner that
complements previous work. Using a Monte Carlo radiative transfer code, we
compare a relatively simple model of the distribution of porous grains to a
broad data set, simultaneously fitting to midplane surface brightness profiles
and the spectral energy distribution. Our model confirms that the large-scale
architecture of the disk is consistent with detailed models of steady-state
grain dynamics. Here, a belt of parent bodies from 35-40 AU is responsible for
producing dust that is then swept outward by the stellar wind and radiation
pressures. We infer the presence of very small grains in the outer region, down
to sizes of ~0.05 micron. These sizes are consistent with stellar mass-loss
rates Mdot_* << 10^2 Mdot_sun.Comment: ApJ accepted, 56 pages, preprint style. Version in emulateapj with
high-resolution figures available at http://tinyurl.com/y6ent
Group properties and invariant solutions of a sixth-order thin film equation in viscous fluid
Using group theoretical methods, we analyze the generalization of a
one-dimensional sixth-order thin film equation which arises in considering the
motion of a thin film of viscous fluid driven by an overlying elastic plate.
The most general Lie group classification of point symmetries, its Lie algebra,
and the equivalence group are obtained. Similar reductions are performed and
invariant solutions are constructed. It is found that some similarity solutions
are of great physical interest such as sink and source solutions,
travelling-wave solutions, waiting-time solutions, and blow-up solutions.Comment: 8 page
Asymptotic dynamics of attractive-repulsive swarms
We classify and predict the asymptotic dynamics of a class of swarming
models. The model consists of a conservation equation in one dimension
describing the movement of a population density field. The velocity is found by
convolving the density with a kernel describing attractive-repulsive social
interactions. The kernel's first moment and its limiting behavior at the origin
determine whether the population asymptotically spreads, contracts, or reaches
steady-state. For the spreading case, the dynamics approach those of the porous
medium equation. The widening, compactly-supported population has edges that
behave like traveling waves whose speed, density and slope we calculate. For
the contracting case, the dynamics of the cumulative density approach those of
Burgers' equation. We derive an analytical upper bound for the finite blow-up
time after which the solution forms one or more -functions.Comment: 23 pages, 10 figures; revised version updates the analysis in sec.
2.1 and 2.2, and contains enhanced discussion of the admissible class of
social interaction force
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