2,571 research outputs found
Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows
A shear-improved Smagorinsky model is introduced based on recent results
concerning shear effects in wall-bounded turbulence by Toschi et al. (2000).
The Smagorinsky eddy-viscosity is modified subtracting the magnitude of the
mean shear from the magnitude of the instantaneous resolved strain-rate tensor.
This subgrid-scale model is tested in large-eddy simulations of plane-channel
flows at two different Reynolds numbers. First comparisons with the dynamic
Smagorinsky model and direct numerical simulations, including mean velocity,
turbulent kinetic energy and Reynolds stress profiles, are shown to be
extremely satisfactory. The proposed model, in addition of being physically
sound, has a low computational cost and possesses a high potentiality of
generalization to more complex non-homogeneous turbulent flows.Comment: 10 pages, 6 figures, added some reference
A boundary integral formalism for stochastic ray tracing in billiards
Determining the flow of rays or non-interacting particles driven by a force or velocity field is fundamental to modelling many physical processes. These include particle flows arising in fluid mechanics and ray flows arising in the geometrical optics limit of linear wave equations. In many practical applications, the driving field is not known exactly and the dynamics are determined only up to a degree of uncertainty. This paper presents a boundary integral framework for propagating flows including uncertainties, which is shown to systematically interpolate between a deterministic and a completely random description of the trajectory propagation. A simple but efficient discretisation approach is applied to model uncertain billiard dynamics in an integrable rectangular domain
Numerical Simulation of the Hydrodynamical Combustion to Strange Quark Matter
We present results from a numerical solution to the burning of neutron matter
inside a cold neutron star into stable (u,d,s) quark matter. Our method solves
hydrodynamical flow equations in 1D with neutrino emission from weak
equilibrating reactions, and strange quark diffusion across the burning front.
We also include entropy change due to heat released in forming the stable quark
phase. Our numerical results suggest burning front laminar speeds of 0.002-0.04
times the speed of light, much faster than previous estimates derived using
only a reactive-diffusive description. Analytic solutions to hydrodynamical
jump conditions with a temperature dependent equation of state agree very well
with our numerical findings for fluid velocities. The most important effect of
neutrino cooling is that the conversion front stalls at lower density (below
approximately 2 times saturation density). In a 2-dimensional setting, such
rapid speeds and neutrino cooling may allow for a flame wrinkle instability to
develop, possibly leading to detonation.Comment: 5 pages, 3 figures (animations online at
http://www.capca.ucalgary.ca/~bniebergal/webPHP/research.php
Type II critical phenomena of neutron star collapse
We investigate spherically-symmetric, general relativistic systems of
collapsing perfect fluid distributions. We consider neutron star models that
are driven to collapse by the addition of an initially "in-going" velocity
profile to the nominally static star solution. The neutron star models we use
are Tolman-Oppenheimer-Volkoff solutions with an initially isentropic,
gamma-law equation of state. The initial values of 1) the amplitude of the
velocity profile, and 2) the central density of the star, span a parameter
space, and we focus only on that region that gives rise to Type II critical
behavior, wherein black holes of arbitrarily small mass can be formed. In
contrast to previously published work, we find that--for a specific value of
the adiabatic index (Gamma = 2)--the observed Type II critical solution has
approximately the same scaling exponent as that calculated for an
ultrarelativistic fluid of the same index. Further, we find that the critical
solution computed using the ideal-gas equations of state asymptotes to the
ultrarelativistic critical solution.Comment: 24 pages, 22 figures, RevTeX 4, submitted to Phys. Rev.
Raman signatures of classical and quantum phases in coupled dots: A theoretical prediction
We study electron molecules in realistic vertically coupled quantum dots in a
strong magnetic field. Computing the energy spectrum, pair correlation
functions, and dynamical form factor as a function of inter-dot coupling via
diagonalization of the many-body Hamiltonian, we identify structural
transitions between different phases, some of which do not have a classical
counterpart. The calculated Raman cross section shows how such phases can be
experimentally singled out.Comment: 9 pages, 2 postscript figures, 1 colour postscript figure, Latex 2e,
Europhysics Letters style and epsfig macros. Submitted to Europhysics Letter
Quantum turbulence at finite temperature: the two-fluids cascade
To model isotropic homogeneous quantum turbulence in superfluid helium, we
have performed Direct Numerical Simulations (DNS) of two fluids (the normal
fluid and the superfluid) coupled by mutual friction. We have found evidence of
strong locking of superfluid and normal fluid along the turbulent cascade, from
the large scale structures where only one fluid is forced down to the vorticity
structures at small scales. We have determined the residual slip velocity
between the two fluids, and, for each fluid, the relative balance of inertial,
viscous and friction forces along the scales. Our calculations show that the
classical relation between energy injection and dissipation scale is not valid
in quantum turbulence, but we have been able to derive a temperature--dependent
superfluid analogous relation. Finally, we discuss our DNS results in terms of
the current understanding of quantum turbulence, including the value of the
effective kinematic viscosity
Head-on collisions of binary white dwarf--neutron stars: Simulations in full general relativity
We simulate head-on collisions from rest at large separation of binary white
dwarf -- neutron stars (WDNSs) in full general relativity. Our study serves as
a prelude to our analysis of the circular binary WDNS problem. We focus on
compact binaries whose total mass exceeds the maximum mass that a cold
degenerate star can support, and our goal is to determine the fate of such
systems. A fully general relativistic hydrodynamic computation of a realistic
WDNS head-on collision is prohibitive due to the large range of dynamical time
scales and length scales involved. For this reason, we construct an equation of
state (EOS) which captures the main physical features of NSs while, at the same
time, scales down the size of WDs. We call these scaled-down WD models
"pseudo-WDs (pWDs)". Using pWDs, we can study these systems via a sequence of
simulations where the size of the pWD gradually increases toward the realistic
case. We perform two sets of simulations; One set studies the effects of the NS
mass on the final outcome, when the pWD is kept fixed. The other set studies
the effect of the pWD compaction on the final outcome, when the pWD mass and
the NS are kept fixed. All simulations show that 14%-18% of the initial total
rest mass escapes to infinity. All remnant masses still exceed the maximum rest
mass that our cold EOS can support (1.92 solar masses), but no case leads to
prompt collapse to a black hole. This outcome arises because the final
configurations are hot. All cases settle into spherical, quasiequilibrium
configurations consisting of a cold NS core surrounded by a hot mantle,
resembling Thorne-Zytkow objects. Extrapolating our results to realistic WD
compactions, we predict that the likely outcome of a head-on collision of a
realistic, massive WDNS system will be the formation of a quasiequilibrium
Thorne-Zytkow-like object.Comment: 24 pages, 14 figures, matches PRD published version, tests of HRSC
schemes with piecewise polytropes adde
Lattice Boltzmann - Langevin simulations of binary mixtures
We report a hybrid numerical method for the solution of the model H
fluctuating hydrodynamic equations for binary mixtures. The momentum
conservation equations with Landau-Lifshitz stresses are solved using the
fluctuating lattice Boltzmann equation while the order parameter conservation
equation with Langevin fluxes are solved using the stochastic method of lines.
Two methods, based on finite difference and finite volume, are proposed for
spatial discretisation of the order parameter equation. Special care is taken
to ensure that the fluctuation-dissipation theorem is maintained at the lattice
level in both cases. The methods are benchmarked by comparing static and
dynamic correlations and excellent agreement is found between analytical and
numerical results. The Galilean invariance of the model is tested and found to
be satisfactory. Thermally induced capillary fluctuations of the interface are
captured accurately, indicating that the model can be used to study nonlinear
fluctuations
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