18,046 research outputs found
A simplified particulate model for coarse-grained hemodynamics simulations
Human blood flow is a multi-scale problem: in first approximation, blood is a
dense suspension of plasma and deformable red cells. Physiological vessel
diameters range from about one to thousands of cell radii. Current
computational models either involve a homogeneous fluid and cannot track
particulate effects or describe a relatively small number of cells with high
resolution, but are incapable to reach relevant time and length scales. Our
approach is to simplify much further than existing particulate models. We
combine well established methods from other areas of physics in order to find
the essential ingredients for a minimalist description that still recovers
hemorheology. These ingredients are a lattice Boltzmann method describing rigid
particle suspensions to account for hydrodynamic long range interactions
and---in order to describe the more complex short-range behavior of
cells---anisotropic model potentials known from molecular dynamics simulations.
Paying detailedness, we achieve an efficient and scalable implementation which
is crucial for our ultimate goal: establishing a link between the collective
behavior of millions of cells and the macroscopic properties of blood in
realistic flow situations. In this paper we present our model and demonstrate
its applicability to conditions typical for the microvasculature.Comment: 12 pages, 11 figure
Sound propagation and Mach cone in anisotropic hydrodynamics
This letter is based on a kinetic theory approach to anisotropic
hydrodynamics. We derive the sound wave equation in anisotropic hydrodynamics
and show that a corresponding wave front is ellipsoidal. The phenomenon of Mach
cone emission in anisotropic hydrodynamics is studied. It is shown that Mach
cone in anisotropic case becomes asymmetric, i. e. in this limit they're two
different angles, left and right with respect to the ultrasonic particle
direction, which are determined by the direction of ultrasonic particle
propagation and the asymmetry coefficient.Comment: 9 pages, 4 figure
Viscous Bianchi type I universes in brane cosmology
We consider the dynamics of a viscous cosmological fluid in the generalized
Randall-Sundrum model for an anisotropic, Bianchi type I brane. To describe the
dissipative effects we use the Israel-Hiscock-Stewart full causal thermodynamic
theory. By assuming that the matter on the brane obeys a linear barotropic
equation of state, and the bulk viscous pressure has a power law dependence on
the energy density, the general solution of the field equations can be obtained
in an exact parametric form. The obtained solutions describe generally a
non-inflationary brane world. In the large time limit the brane Universe
isotropizes, ending in an isotropic and homogeneous state. The evolution of the
temperature and of the comoving entropy of the Universe is also considered, and
it is shown that due to the viscous dissipative processes a large amount of
entropy is created in the early stages of evolution of the brane world.Comment: 13 pages, 5 figures, to appear in Class. Quantum Gra
Local orientational ordering in fluids of spherical molecules with dipolar-like anisotropic adhesion
We discuss some interesting physical features stemming from our previous
analytical study of a simple model of a fluid with dipolar-like interactions of
very short range in addition to the usual isotropic Baxter potential for
adhesive spheres. While the isotropic part is found to rule the global
structural and thermodynamical equilibrium properties of the fluid, the weaker
anisotropic part gives rise to an interesting short-range local ordering of
nearly spherical condensation clusters, containing short portions of chains
having nose-to-tail parallel alignment which runs antiparallel to adjacent
similar chains.Comment: 13 pages and 6 figure
The Stability of Weakly Collisional Plasmas with Thermal and Composition Gradients
Over the last decade, substantial efforts have been devoted to understanding
the stability properties, transport phenomena, and long-term evolution of
weakly-collisional, magnetized plasmas which are stratified in temperature.
These studies have improved our understanding of the physics governing the
intra-cluster medium (ICM), but assumed that ICM is a homogeneous. This,
however, might not be a good approximation if heavy elements sediment in the
inner region of the galaxy cluster. In this paper, we analyze the stability of
a weakly-collisional, magnetized plane-parallel atmosphere which is stratified
in both temperature and composition. This allows us to discuss for the first
time the dynamics of weakly-collisional environments where heat conduction,
momentum transport, and ion-diffusion are anisotropic with respect to the
direction of the magnetic field. We show that, depending on the relative signs
and magnitudes of the gradients in the temperature and the mean molecular
weight, the plasma can be subject to a wide variety of unstable modes which
include modifications to the magnetothermal instability (MTI), the
heat-flux-driven buoyancy instability (HBI), and overstable gravity modes
previously studied in homogeneous media. We discuss the astrophysical
implications of our findings for a representative galaxy cluster where helium
has sedimented. Our findings suggest that the core insulation that results from
the magnetic field configurations that arise as a natural consequence of the
HBI, which would be MTI stable in a homogeneous medium, could be alleviated if
the mean molecular weight gradient is steep enough, i.e., . This study constitutes a first step toward understanding the
interaction between magnetic turbulence and the diffusion of heavy elements in
the ICM. (abridged)Comment: 16 pages, 7 figures. This article supersedes arXiv:1111.3372 (5
pages, 3 figures). The present version of this article is published in Ap
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