8,188 research outputs found
Structure formation in the presence of relativistic heat conduction: corrections to the Jeans wave number with a stable first order in the gradients formalism
The problem of structure formation in relativistic dissipative fluids was
analyzed in a previous work within Eckart's framework, in which the heat flux
is coupled to the hydrodynamic acceleration, additional to the usual
temperature gradient term. It was shown that in such case, the pathological
behavior of fluctuations leads to the disapperance of the gravitational
instability responsible for structure formation. In the present work the
problem is revisited now using a constitutive equation derived from
relativistic kinetic theory. The new relation, in which the heat flux is not
coupled to the hydrodynamic acceleration, leads to a consistent first order in
the gradients formalism. In this case the gravitational instability remains,
and only relativistic corrections to the Jeans wave number are obtained. In the
calculation here shown the non-relativistc limit is recovered, opposite to what
happens in Eckart's case.Comment: 10 pages, no figure
Non-Hamiltonian features of a classical pilot-wave dynamics
A bouncing droplet on a vibrated bath can couple to the waves it generates,
so that it becomes a propagative walker. Its propulsion at constant velocity
means that a balance exists between the permanent input of energy provided by
the vibration and the dissipation. Here we seek a simple theoretical
description of the resulting non-Hamiltonian dynamics with a walker immersed in
a harmonic potential well. We demonstrate that the interaction with the
recently emitted waves can be modeled by a Rayleigh-type friction. The Rayleigh
oscillator has well defined attractors. The convergence toward them and their
stability is investigated through an energetic approach and a linear stability
analysis. These theoretical results provide a description of the dynamics in
excellent agreement with the experimental data. It is thus a basic framework
for further investigations of wave-particle interactions when memory effects
are included.Comment: 10 pages, 6 figure
Shock propagation and stability in causal dissipative hydrodynamics
We studied the shock propagation and its stability with the causal
dissipative hydrodynamics in 1+1 dimensional systems. We show that the presence
of the usual viscosity is not enough to stabilize the solution. This problem is
solved by introducing an additional viscosity which is related to the
coarse-graining scale of the theory.Comment: 14 pages, 16 figure
Surface wave excitations and backflow effect over dense polymer brushes
Polymer brushes are increasingly used to tailor surface physicochemistry for
various applications such as wetting, adhesion of biological objects,
implantable devices, etc. We perform Dissipative Particle Dynamics simulations
to study the behavior of dense polymer brushes under flow in a slit-pore
channel. We discover that the system displays flow inversion at the brush
interface for several disconnected ranges of the imposed flow. We associate
such phenomenon to collective polymer dynamics: a wave propagating on the brush
surface. The relation between the wavelength, the amplitude and the propagation
speed of the flow-generated wave is consistent with the solution of the Stokes
equations when an imposed traveling wave is assumed as boundary condition (the
famous Taylor's swimmer).Comment: 5 pages, 6 figures, supplemental material (5 pages, 4 figures)
attached below the letter and before bibliography, videos available on
request to the corresponding author, submitted to SciRe
The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts
The recent Fermi observation of GRB 080916C shows that the bright photosphere
emission associated with a putative fireball is missing, which suggests a
Poynting-flux-dominated outflow. We propose a model of gamma-ray burst (GRB)
prompt emission in the Poynting-flux-dominated regime, namely, the
Internal-Collision-induced MAgnetic Reconnection and Turbulence (ICMART) model.
It is envisaged that the GRB central engine launches an intermittent,
magnetically-dominated wind, and that in the GRB emission region, the ejecta is
still moderately magnetized. Similar to the internal shock (IS) model, the
mini-shells interact internally at the traditional internal shock radius. Most
of these early collision have little energy dissipation, but serve to distort
the ordered magnetic field lines. At a certain point, the distortion of
magnetic field configuration reaches the critical condition to allow fast
reconnection seeds to occur, which induce relativistic MHD turbulence in the
interaction regions. The turbulence further distorts field lines easing
additional magnetic reconnections, resulting in a runway release of the stored
magnetic field energy (an ICMART event). Particles accelerated in the ICMART
region radiate synchrotron photons that power the observed gamma-rays. Each
ICMART event corresponds to a broad pulse in the GRB lightcurve, and a GRB is
composed of multiple ICMART events. This model retains the merits of the IS and
other models, but may overcome several difficulties/issues faced by the IS
model (e.g. low efficiency, fast cooling, electron number excess,
Amati/Yonetoku relation inconsistency, and missing bright photosphere). It
predicts two-component variability time scales, and a decreasing Ep and
polarization degree during each ICMART event. The model may be applied to most
Fermi LAT GRBs that have time-resolved, featureless Band-function spectra
(abridged).Comment: ApJ, in press (submitted on May 6, 2010). 27 emulateapj pages, 4
figures. Minor changes to match the published versio
Synchrotron brightness distribution of turbulent radio jets
Radio jets are considered as turbulent mixing regions and it is proposed that the essential small scale viscous dissipation in these jets is by emission of MHD waves and by their subsequent strong damping due, at least partly, to gyro-resonant acceleration of supra-thermal particles. A formula relating the synchrotron surface brightness of a radio jet to the turbulent power input is deduced from physical postulates, and is tested against the data for NGC315 and 3C31 (NGC383). The predicted brightness depends essentially on the collimation behavior of the jet, and, to a lesser extent, on the CH picture of a 'high' nozzle with accelerating flow. The conditions for forming a large scale jet at a high nozzle from a much smaller scale jet are discussed. The effect of entrainment on the prediction is discussed with the use of similarity solutions. Although entrainment is inevitably associated with the turbulent jet, it may or may not be a dominant factor depending on the ambient density profile
Hydro-micromechanical modeling of wave propagation in saturated granular media
Biot's theory predicts the wave velocities of a saturated poroelastic
granular medium from the elastic properties, density and geometry of its dry
solid matrix and the pore fluid, neglecting the interaction between constituent
particles and local flow. However, when the frequencies become high and the
wavelengths comparable with particle size, the details of the microstructure
start to play an important role. Here, a novel hydro-micromechanical numerical
model is proposed by coupling the lattice Boltzmann method (LBM) with the
discrete element method (DEM. The model allows to investigate the details of
the particle-fluid interaction during propagation of elastic waves While the
DEM is tracking the translational and rotational motion of each solid particle,
the LBM can resolve the pore-scale hydrodynamics. Solid and fluid phases are
two-way coupled through momentum exchange. The coupling scheme is benchmarked
with the terminal velocity of a single sphere settling in a fluid. To mimic a
pressure wave entering a saturated granular medium, an oscillating pressure
boundary condition on the fluid is implemented and benchmarked with
one-dimensional wave equations. Using a face centered cubic structure, the
effects of input waveforms and frequencies on the dispersion relations are
investigated. Finally, the wave velocities at various effective confining
pressures predicted by the numerical model are compared with with Biot's
analytical solution, and a very good agreement is found. In addition to the
pressure and shear waves, slow compressional waves are observed in the
simulations, as predicted by Biot's theory.Comment: Manuscript submitted to International Journal for Numerical and
Analytical Methods in Geomechanic
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