83,506 research outputs found
On Shear-Free perturbations of FLRW Universes
A surprising exact result for the Einstein Field Equations is that if
pressure-free matter is moving in a shear-free way, then it must be either
expansion-free or rotation-free. It has been suggested this result is also true
for any barotropic perfect fluid, but a proof has remained elusive. We consider
the case of barotropic perfect fluid solutions linearized about a
Robertson-Walker geometry, and prove that the result remains true except for
the case of a specific highly non-linear equation of state. We argue that this
equation of state is non-physical, and hence the result is true in the
linearized case for all physically realistic barotropic perfect fluids. This
result, which is not true in Newtonian cosmology, demonstrates that the
linearized solutions, believed to result in standard local Newtonian theory, do
not always give the usual behaviour of Newtonian solutions
Motion of a sphere in the presence of a plane interface. Part 2. An exact solution in bipolar co-ordinates
A general solution for Stokesâ equation in bipolar co-ordinates is derived, and then applied to the arbitrary motion of a sphere in the presence of a plane fluid/fluid interface. The drag force and hydrodynamic torque on the sphere are then calculated for four specific motions of the sphere; namely, translation perpendicular and parallel to the interface and rotation about an axis which is perpendicular and parallel, respectively, to the interface. The most significant result of the present work is the comparison between these numerically exact solutions and the approximate solutions from part 1. The latter can be generalized to a variety of particle shapes, and it is thus important to assess their accuracy for this case of spherical particles where an exact solution can be obtained. In addition to comparisons with the approximate solutions, we also examine the predicted changes in the velocity, pressure and vorticity fields due to the presence of the plane interface. One particularly interesting feature of the solutions is the fact that the direction of rotation of a freely suspended sphere moving parallel to the interface can either be the same as for a sphere rolling along the interface (as might be intuitively expected), or opposite depending upon the location of the sphere centre and the ratio of viscosities for the two fluids
A new family of exact and rotating solutions of fireball hydrodynamics
A new class of analytic, exact, rotating, self-similar and surprisingly
simple solutions of non-relativistic hydrodynamics are presented for a
three-dimensionally expanding, spheroidally symmetric fireball. These results
generalize earlier, non-rotating solutions for ellipsoidally symmetric
fireballs with directional, three-dimensional Hubble flows. The solutions are
presented for a general class of equations of state that includes the lattice
QCD equations of state and may feature inhomogeneous temperature and
corresponding density profiles.Comment: Dedicated to T. Kodama on the occasion of his 70th birthday. 15
pages, no figures. Accepted for publication at Phys. Rev. C. Minor rewritings
from previous versio
Large Scale Features of Rotating Forced Turbulence
Large scale features of a randomly isotropically forced incompressible and
unbounded rotating fluid are examined in perturbation theory. At first order in
both the random force amplitude and the angular velocity we find two types of
modifications to the fluid equation of motion. The first correction transforms
the molecular shear viscosity into a (rotation independent) effective
viscosity. The second perturbative correction leads to a new large scale
non-dissipative force proportional to the fluid angular velocity in the slow
rotation regime. This effective force does no net work and alters the
dispersion relation of inertial waves propagating in the fluid. Both
dynamically generated corrections can be identified with certain components of
the most general axisymmetric ``viscosity tensor'' for a Newtonian fluid.Comment: 12 pages, 2 figures, RevTeX, and accepted for publication in Phys.
Rev.
Rotation and pseudo-rotation
Eigenvectors of stress-energy tensor (the source in Einstein's equations)
form privileged bases in description of the corresponding space-times. When one
or more of these vector fields are rotating (the property well determined in
differential geometry), one says that the space-time executes this rotation.
Though the rotation in its proper sense is understood as that of a timelike
congruence (vector field), the rotation of a spacelike congruence is not a less
objective property if it corresponds to a canonical proper basis built of the
just mentioned eigenvectors. In this last case, we propose to speak on
pseudo-rotation. Both properties of metric, its material sources, and
space-time symmetries are considered in this paper.Comment: 13 pages, no figures, contains parts of the PhD Thesis of H. Vargas
Rodr\'igue
Weak turbulence theory for rotating magnetohydrodynamics and planetary dynamos
A weak turbulence theory is derived for magnetohydrodynamics under rapid
rotation and in the presence of a large-scale magnetic field. The angular
velocity is assumed to be uniform and parallel to the constant
Alfv\'en speed . Such a system exhibits left and right circularly
polarized waves which can be obtained by introducing the magneto-inertial
length . In the large-scale limit (; being
the wave number), the left- and right-handed waves tend respectively to the
inertial and magnetostrophic waves whereas in the small-scale limit () pure Alfv\'en waves are recovered. By using a complex helicity
decomposition, the asymptotic weak turbulence equations are derived which
describe the long-time behavior of weakly dispersive interacting waves {\it
via} three-wave interaction processes. It is shown that the nonlinear dynamics
is mainly anisotropic with a stronger transfer perpendicular () than
parallel () to the rotating axis. The general theory may converge to
pure weak inertial/magnetostrophic or Alfv\'en wave turbulence when the large
or small-scales limits are taken respectively. Inertial wave turbulence is
asymptotically dominated by the kinetic energy/helicity whereas the
magnetostrophic wave turbulence is dominated by the magnetic energy/helicity.
For both regimes a family of exact solutions are found for the spectra which do
not correspond necessarily to a maximal helicity state. It is shown that the
hybrid helicity exhibits a cascade whose direction may vary according to the
scale at which the helicity flux is injected with an inverse cascade if
and a direct cascade otherwise. The theory is relevant for the
magnetostrophic dynamo whose main applications are the Earth and giant planets
for which a small () Rossby number is expected.Comment: 4 figures, 33 page
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