32,642 research outputs found
Dusty gas with one fluid
In this paper, we show how the two-fluid equations describing the evolution
of a dust and gas mixture can be reformulated to describe a single fluid moving
with the barycentric velocity of the mixture. This leads to evolution equations
for the total density, momentum, the differential velocity between the dust and
the gas phases and either the dust-to-gas ratio or the dust fraction. The
equations are similar to the usual equations of gas dynamics, providing a
convenient way to extend existing codes to simulate two-fluid mixtures without
modifying the code architecture. Our approach avoids the inherent difficulties
related to the standard approach where the two phases are separate and coupled
via a drag term. In particular, the requirements of infinite spatial and
temporal resolution as the stopping time tends to zero are no longer necessary.
This means that both small and large grains can be straightforwardly treated
with the same method, with no need for complicated implicit schemes. Since
there is only one resolution scale the method also avoids the problem of
unphysical trapping of one fluid (e.g. dust) below the resolution of the other.
We also derive a simplified set of equations applicable to the case of strong
drag/small grains, consisting of the standard fluid equations with a modified
sound speed, plus an advection-diffusion equation for the dust-to-gas ratio.
This provides a simple and fast way to evolve the mixture when the stopping
time is smaller than the Courant timestep. We present a Smoothed Particle
Hydrodynamics implementation in a companion paper.Comment: Accepted for publication in MNRAS (very minor revisions included
Nonequilibrium fluid-dynamics in the early stage of ultrarelativistic heavy-ion collisions
To describe ultrarelativistic heavy-ion collisions we construct a three-fluid
hydrodynamical model. In contrast to one-fluid hydrodynamics, it accounts for
the finite stopping power of nuclear matter, i.e. for nonequilibrium effects in
the early stage of the reaction. Within this model, we study baryon dynamics in
the BNL-AGS energy range. For the system Au+Au we find that kinetic equilibrium
between projectile and target nucleons is established only after a time
. Observables which are
sensitive to the early stage of the collision (like e.g. nucleon flow)
therefore differ considerably from those calculated in the one-fluid model.Comment: 36 pages, Late
Motion by Stopping: Rectifying Brownian Motion of Non-spherical Particles
We show that Brownian motion is spatially not symmetric for mesoscopic
particles embedded in a fluid if the particle is not in thermal equilibrium and
its shape is not spherical. In view of applications on molecular motors in
biological cells, we sustain non-equilibrium by stopping a non-spherical
particle at periodic sites along a filament. Molecular dynamics simulations in
a Lennard-Jones fluid demonstrate that directed motion is possible without a
ratchet potential or temperature gradients if the asymmetric non-equilibrium
relaxation process is hindered by external stopping. Analytic calculations in
the ideal gas limit show that motion even against a fluid drift is possible and
that the direction of motion can be controlled by the shape of the particle,
which is completely characterized by tensorial Minkowski functionals.Comment: 11 pages, 5 figure
Accelerator Based Fusion Reactor
A feasibility study of fusion reactors based on accelerators is carried out.
We consider a novel scheme where a beam from the accelerator hits the target
plasma on the resonance of the fusion reaction and establish characteristic
criteria for a workable reactor. We consider the reactions , and in this study. The critical temperature of the plasma is determined
from overcoming the stopping power of the beam with the fusion energy gain. The
needed plasma lifetime is determined from the width of the resonance, the beam
velocity and the plasma density. We estimate the critical beam flux by
balancing the energy of fusion production against the plasma thermo-energy and
the loss due to stopping power for the case of an inert plasma. The product of
critical flux and plasma lifetime is independent of plasma density and has a
weak dependence on temperature. Even though the critical temperatures for these
reactions are lower than those for the thermonuclear reactors, the critical
flux is in the range of for the plasma density
in the case of an inert plasma. Several
approaches to control the growth of the two-stream instability are discussed.
We have also considered several scenarios for practical implementation which
will require further studies. Finally, we consider the case where the injected
beam at the resonance energy maintains the plasma temperature and prolongs its
lifetime to reach a steady state. The equations for power balance and particle
number conservation are given for this case.Comment: To be published in Nuclear Fusion as a letter, 7 pages, 2 figure
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