32,110 research outputs found
Drived diffusion of vector fields
A model for the diffusion of vector fields driven by external forces is
proposed. Using the renormalization group and the -expansion, the
dynamical critical properties of the model with gaussian noise for dimensions
below the critical dimension are investigated and new transport universality
classes are obtained.Comment: 11 pages, title changed, anisotropic diffusion further discussed and
emphasize
Constraint preserving boundary conditions for the Z4c formulation of general relativity
We discuss high order absorbing constraint preserving boundary conditions for
the Z4c formulation of general relativity coupled to the moving puncture family
of gauges. We are primarily concerned with the constraint preservation and
absorption properties of these conditions. In the frozen coefficient
approximation, with an appropriate first order pseudo-differential reduction,
we show that the constraint subsystem is boundary stable on a four dimensional
compact manifold. We analyze the remainder of the initial boundary value
problem for a spherical reduction of the Z4c formulation with a particular
choice of the puncture gauge. Numerical evidence for the efficacy of the
conditions is presented in spherical symmetry.Comment: 18 pages, 8 figure
Coagulation reaction in low dimensions: Revisiting subdiffusive A+A reactions in one dimension
We present a theory for the coagulation reaction A+A -> A for particles
moving subdiffusively in one dimension. Our theory is tested against numerical
simulations of the concentration of particles as a function of time
(``anomalous kinetics'') and of the interparticle distribution function as a
function of interparticle distance and time. We find that the theory captures
the correct behavior asymptotically and also at early times, and that it does
so whether the particles are nearly diffusive or very subdiffusive. We find
that, as in the normal diffusion problem, an interparticle gap responsible for
the anomalous kinetics develops and grows with time. This corrects an earlier
claim to the contrary on our part.Comment: The previous version was corrupted - some figures misplaced, some
strange words that did not belong. Otherwise identica
Accretion modes in collapsars - prospects for GRB production
We explore low angular momentum accretion flows onto black holes formed after
the collapse of massive stellar cores. In particular, we consider the state of
the gas falling quasi-spherically onto stellar-mass black holes in the
hypercritical regime, where the accretion rates are in the range 0.001 - 0.5
solar masses per second and neutrinos dominate the cooling. Previous studies
have assumed that in order to have a black hole switch to a luminous state, the
condition l >> r_g c, where l is the specific orbital angular momentum of the
infalling gas and r_g is the Schwarszchild radius, needs to be fulfilled. We
argue that flows in hyperaccreting, stellar mass disks around black holes are
likely to transition to a highly radiative state when their angular momentum is
just above the threshold for disk formation, l ~ 2 r_g c. In a range where l
lies between r_g c and 2 r_g c, a dwarf disk forms in which gas spirals rapidly
into the black hole due to general relativistic effects, without any help from
horizontal viscous stresses. For high rotation rates with l greater than 2 r_g
c, the luminosity is supplied by large, hot equatorial bubbles around the black
hole. The highest neutrino luminosities are obtained for l ~ 2 r_g c, and this
value of angular momentum also produces the most energetic neutrinos, and thus
also the highest energy deposition rates. Given the range of l explored in this
work, we argue that, as long as l is greater than 2 r_g c, low angular momentum
cores may in fact be better suited for producing neutrino--driven explosions
following core collapse in supernovae and gamma ray bursts.Comment: Revised version following referee's comments. References added.
Accepted for publication in Ap
Numerical framework for transcritical real-fluid reacting flow simulations using the flamelet progress variable approach
An extension to the classical FPV model is developed for transcritical
real-fluid combustion simulations in the context of finite volume, fully
compressible, explicit solvers. A double-flux model is developed for
transcritical flows to eliminate the spurious pressure oscillations. A hybrid
scheme with entropy-stable flux correction is formulated to robustly represent
large density ratios. The thermodynamics for ideal-gas values is modeled by a
linearized specific heat ratio model. Parameters needed for the cubic EoS are
pre-tabulated for the evaluation of departure functions and a quadratic
expression is used to recover the attraction parameter. The novelty of the
proposed approach lies in the ability to account for pressure and temperature
variations from the baseline table. Cryogenic LOX/GH2 mixing and reacting cases
are performed to demonstrate the capability of the proposed approach in
multidimensional simulations. The proposed combustion model and numerical
schemes are directly applicable for LES simulations of real applications under
transcritical conditions.Comment: 55th AIAA Aerospace Sciences Meeting, Dallas, T
Reconciling threshold and subthreshold expansions for pion-nucleon scattering
Heavy-baryon chiral perturbation theory (ChPT) at one loop fails in relating
the pion-nucleon amplitude in the physical region and for subthreshold
kinematics due to loop effects enhanced by large low-energy constants. Studying
the chiral convergence of threshold and subthreshold parameters up to fourth
order in the small-scale expansion, we address the question to what extent this
tension can be mitigated by including the as an explicit degree
of freedom and/or using a covariant formulation of baryon ChPT. We find that
the inclusion of the indeed reduces the low-energy constants to more
natural values and thereby improves consistency between threshold and
subthreshold kinematics. In addition, even in the -less theory the
resummation of corrections in the covariant scheme improves the results
markedly over the heavy-baryon formulation, in line with previous observations
in the single-baryon sector of ChPT that so far have evaded a profound
theoretical explanation.Comment: 10 pages, 4 tables, Mathematica notebook with the analytic
expressions for threshold and subthreshold parameters included as
supplementary material; journal versio
Attosecond probing of instantaneous AC Stark shifts in helium atoms
Based on numerical solutions of the time-dependent Schr\"odinger equation for
either one or two active electrons, we propose a method for observing
instantaneous level shifts in an oscillating strong infrared (IR) field in
time, using a single tunable attosecond pulse to probe excited states of the
perturbed atom. The ionization probability in the combined fields depends on
both, the frequency of the attosecond pulse and the time delay between both
pulses, since the IR field shifts excited energy levels into and out of
resonance with the attosecond probe pulse. We show that this method (i) allows
the detection of instantaneous atomic energy gaps with sub-laser-cycle time
resolution and (ii) can be applied as an ultrafast gate for more complex
processes such as non-sequential double-ionization
-Pb deep inelastic scattering
Nuclear-medium effects in the weak structure functions and
in the charged current neutrino and antineutrino induced deep
inelastic reactions in Pb have been studied. The calculations have been
performed in a theoretical model using relativistic nuclear spectral functions
which incorporate Fermi motion, binding and nucleon correlations.
We also consider the pion and rho meson cloud contributions calculated from a
microscopic model for meson-nucleus self-energies. Using these structure
functions, the results for the differential cross section have been obtained
and compared with the CERN Hybrid Oscillation Research apparatUS (CHORUS) data.
The results for the ratios ,
, ,
, and (i=2,3)
have also been obtained and a few have been compared with some of the
phenomenological fits.Comment: 19Pages, 12 Fig
Ising exponents in the two-dimensional site-diluted Ising model
We study the site-diluted Ising model in two dimensions with Monte Carlo
simulations. Using finite-size scaling techniques we compute the critical
exponents observing deviations from the pure Ising ones. The differences can be
explained as the effects of logarithmic corrections, without requiring to
change the Universality Class.Comment: 7 pages, 2 postscript figures. Reference correcte
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