38 research outputs found
Development of high-order realizable finite-volume schemes for quadrature-based moment method
Kinetic equations containing terms for spatial transport, gravity, fluid drag and particle-particle collisions can be used to model dilute gas-particle flows. However, the enormity of independent variables makes direct numerical simulation of these equations almost impossible for practical problems. A viable alternative is to reformulate the problem in terms of moments of velocity distribution. Recently, a quadrature-based moment method was derived by Fox for approximating solutions to kinetic equation for arbitrary Knudsen number. Fox also described 1st- and 2nd-order finite-volume schemes for solving the equations. The success of the new method is based on a moment-inversion algorithm that is used to calculate non-negative weights and abscissas from moments. The moment-inversion algorithm does not work if the moments are non-realizable, meaning they do not correspond to a distribution function. Not all the finite-volume schemes lead to realizable moments. Desjardins et al. showed that realizability is guaranteed with the 1 st-order finite-volume scheme, but at the expense of excess numerical diffusion. In the present work, the nonrealizability of the standard 2 nd-order finite-volume scheme is demonstrated and a generalized idea for the development of high-order realizable finite-volume schemes for quadrature-based moment methods is presented. This marks a significant improvement in the accuracy of solutions using the quadrature-based moment method as the use of 1st-order scheme to guarantee realizability is no longer a limitation
Ultracold neutrons, quantum effects of gravity and the Weak Equivalence Principle
We consider an extension of the recent experiment with ultracold neutrons and
the quantization of its vertical motion in order to test the Weak Equivalence
Principle. We show that an improvement on the energy resolution of the
experiment may allow to establish a modest limit to the Weak Equivalence
Principle and on the gravitational screening constant. We also discuss the
influence of a possible new interaction of Nature.Comment: Revtex4, 4 pages. Discussion on the equivalence principle altered.
Bound is improve
Cosmological Hydrodynamics with Adaptive Mesh Refinement: a new high resolution code called RAMSES
A new N-body and hydrodynamical code, called RAMSES, is presented. It has
been designed to study structure formation in the universe with high spatial
resolution. The code is based on Adaptive Mesh Refinement (AMR) technique, with
a tree based data structure allowing recursive grid refinements on a
cell-by-cell basis. The N-body solver is very similar to the one developed for
the ART code (Kravtsov et al. 97), with minor differences in the exact
implementation. The hydrodynamical solver is based on a second-order Godunov
method, a modern shock-capturing scheme known to compute accurately the thermal
history of the fluid component. The accuracy of the code is carefully estimated
using various test cases, from pure gas dynamical tests to cosmological ones.
The specific refinement strategy used in cosmological simulations is described,
and potential spurious effects associated to shock waves propagation in the
resulting AMR grid are discussed and found to be negligible. Results obtained
in a large N-body and hydrodynamical simulation of structure formation in a low
density LCDM universe are finally reported, with 256^3 particles and 4.1 10^7
cells in the AMR grid, reaching a formal resolution of 8192^3. A convergence
analysis of different quantities, such as dark matter density power spectrum,
gas pressure power spectrum and individual haloes temperature profiles, shows
that numerical results are converging down to the actual resolution limit of
the code, and are well reproduced by recent analytical predictions in the
framework of the halo model.Comment: 21 pages and 13 low resolution JPEG images. Accepted for publication
in A&
Nonlinear r-modes in Rapidly Rotating Relativistic Stars
The r-mode instability in rotating relativistic stars has been shown recently
to have important astrophysical implications (including the emission of
detectable gravitational radiation, the explanation of the initial spins of
young neutron stars and the spin-distribution of millisecond pulsars and the
explanation of one type of gamma-ray bursts), provided that r-modes are not
saturated at low amplitudes by nonlinear effects or by dissipative mechanisms.
Here, we present the first study of nonlinear r-modes in isentropic, rapidly
rotating relativistic stars, via 3-D general-relativistic hydrodynamical
evolutions. Our numerical simulations show that (1) on dynamical timescales,
there is no strong nonlinear coupling of r-modes to other modes at amplitudes
of order one -- unless nonlinear saturation occurs on longer timescales, the
maximum r-mode amplitude is of order unity (i.e., the velocity perturbation is
of the same order as the rotational velocity at the equator). An absolute upper
limit on the amplitude (relevant, perhaps, for the most rapidly rotating stars)
is set by causality. (2) r-modes and inertial modes in isentropic stars are
predominantly discrete modes and possible associated continuous parts were not
identified in our simulations. (3) In addition, the kinematical drift
associated with r-modes, recently found by Rezzolla, Lamb and Shapiro (2000),
appears to be present in our simulations, but an unambiguous confirmation
requires more precise initial data. We discuss the implications of our findings
for the detectability of gravitational waves from the r-mode instability.Comment: 4 pages, 4 eps figures, accepted in Physical Review Letter
Neutrino oscillations in curved spacetime: an heuristic treatment
We discuss neutrino oscillations in curved spacetime. Our heuristic approach
can accomodate matter effects and gravitational contributions to neutrino spin
precession in the presence of a magnetic field. By way of illustration, we
perform explicit calculations in the Schwarzschild geometry. In this case,
gravitational effects on neutrino oscillations are intimately related to the
redshift. We discuss how spacetime curvature could affect the resonance
position and adiabaticity of matter-enhanced neutrino flavor conversion.Comment: 7 pages, REVTeX and 1 included style file. Submitted to Phys. Rev.
Three-dimensional general relativistic hydrodynamics II: long-term dynamics of single relativistic stars
This is the second in a series of papers on the construction and validation
of a three-dimensional code for the solution of the coupled system of the
Einstein equations and of the general relativistic hydrodynamic equations, and
on the application of this code to problems in general relativistic
astrophysics. In particular, we report on the accuracy of our code in the
long-term dynamical evolution of relativistic stars and on some new physics
results obtained in the process of code testing. The tests involve single
non-rotating stars in stable equilibrium, non-rotating stars undergoing radial
and quadrupolar oscillations, non-rotating stars on the unstable branch of the
equilibrium configurations migrating to the stable branch, non-rotating stars
undergoing gravitational collapse to a black hole, and rapidly rotating stars
in stable equilibrium and undergoing quasi-radial oscillations. The numerical
evolutions have been carried out in full general relativity using different
types of polytropic equations of state using either the rest-mass density only,
or the rest-mass density and the internal energy as independent variables. New
variants of the spacetime evolution and new high resolution shock capturing
(HRSC) treatments based on Riemann solvers and slope limiters have been
implemented and the results compared with those obtained from previous methods.
Finally, we have obtained the first eigenfrequencies of rotating stars in full
general relativity and rapid rotation. A long standing problem, such
frequencies have not been obtained by other methods. Overall, and to the best
of our knowledge, the results presented in this paper represent the most
accurate long-term three-dimensional evolutions of relativistic stars available
to date.Comment: 19 pages, 17 figure
Instabilities in the Ionization Zones Around the First Stars
We consider the evolution of the ionization zone around Population III stars
with in protogalaxies with at
redshifts , assuming that the dark matter profile is a modified
isothermal sphere. We study the conditions for the growth of instabilities in
the ionization zones. The Rayleigh-Taylor and thermal instabilities develop
efficiently in the ionization zones around 25-40 stars, while this
efficiency is lower for stars with . For more massive stars
(), the flux of ionizing photons is strong enough to
considerably reduce the gas density in the ionization zone, and the typical
lifetimes of stars ( Myr) are insufficient for the growth of
instabilities. The gas in a protogalaxy with with a 200
central star is completely ionized by the end of the star's lifetime;
in the case of a 120 central star, only one-third of the total mass
of gas is ionized. Thus, ionizing photons from stars with M_*\simlt 120
M_\odot cannot leave protogalaxies with M\simgt 10^7 M_\odot. If the masses
of the central stars are 25 and 40 , the gas in protogalaxies of this
mass remains essentially neutral. We discuss the consequences of the evolution
of the ionization zones for the propagation of the envelope after the supernova
explosions of the stars and the efficiency of enrichment of the intergalactic
medium in heavy elements.Comment: 11 pages, 4 figure
Determining Air Quality and Greenhouse Gas Impacts of Hydrogen Infrastructure and Fuel Cell Vehicles
The neutron and its role in cosmology and particle physics
Experiments with cold and ultracold neutrons have reached a level of
precision such that problems far beyond the scale of the present Standard Model
of particle physics become accessible to experimental investigation. Due to the
close links between particle physics and cosmology, these studies also permit a
deep look into the very first instances of our universe. First addressed in
this article, both in theory and experiment, is the problem of baryogenesis ...
The question how baryogenesis could have happened is open to experimental
tests, and it turns out that this problem can be curbed by the very stringent
limits on an electric dipole moment of the neutron, a quantity that also has
deep implications for particle physics. Then we discuss the recent spectacular
observation of neutron quantization in the earth's gravitational field and of
resonance transitions between such gravitational energy states. These
measurements, together with new evaluations of neutron scattering data, set new
constraints on deviations from Newton's gravitational law at the picometer
scale. Such deviations are predicted in modern theories with extra-dimensions
that propose unification of the Planck scale with the scale of the Standard
Model ... Another main topic is the weak-interaction parameters in various
fields of physics and astrophysics that must all be derived from measured
neutron decay data. Up to now, about 10 different neutron decay observables
have been measured, much more than needed in the electroweak Standard Model.
This allows various precise tests for new physics beyond the Standard Model,
competing with or surpassing similar tests at high-energy. The review ends with
a discussion of neutron and nuclear data required in the synthesis of the
elements during the "first three minutes" and later on in stellar
nucleosynthesis.Comment: 91 pages, 30 figures, accepted by Reviews of Modern Physic