659 research outputs found
Astrophysical fluid simulations of thermally ideal gases with non-constant adiabatic index: numerical implementation
An Equation of State (\textit{EoS}) closes the set of fluid equations.
Although an ideal EoS with a constant \textit{adiabatic index} is the
preferred choice due to its simplistic implementation, many astrophysical fluid
simulations may benefit from a more sophisticated treatment that can account
for diverse chemical processes. Here, we first review the basic thermodynamic
principles of a gas mixture in terms of its thermal and caloric EoS by
including effects like ionization, dissociation as well as temperature
dependent degrees of freedom such as molecular vibrations and rotations. The
formulation is revisited in the context of plasmas that are either in
equilibrium conditions (local thermodynamic- or collisional excitation-
equilibria) or described by non-equilibrium chemistry coupled to optically thin
radiative cooling. We then present a numerical implementation of thermally
ideal gases obeying a more general caloric EoS with non-constant adiabatic
index in Godunov-type numerical schemes.We discuss the necessary modifications
to the Riemann solver and to the conversion between total energy and pressure
(or vice-versa) routinely invoked in Godunov-type schemes. We then present two
different approaches for computing the EoS.The first one employs root-finder
methods and it is best suited for EoS in analytical form. The second one leans
on lookup table and interpolation and results in a more computationally
efficient approach although care must be taken to ensure thermodynamic
consistency. A number of selected benchmarks demonstrate that the employment of
a non-ideal EoS can lead to important differences in the solution when the
temperature range is K where dissociation and ionization occur. The
implementation of selected EoS introduces additional computational costs
although using lookup table methods can significantly reduce the overhead by a
factor .Comment: 17 pages, 10 figures, Accepted for publication in A&
The Effect of Expansion on Mass Entrainment and Stability of Super-Alfv\'enic Jets
We extend investigations of mass entrainment by jets, which previously have
focused on cylindrical supermagnetosonic jets and expanding trans-Alfv\'enic
jets, to a set of expanding supermagnetosonic jets. We precess these jets at
the origin to excite the helical mode of the Kelvin-Helmholtz (or KH)
instability, in order to compare the results with predictions from linear
stability analysis. We analyze this simulation set for the spatial development
of magnetized mass, which we interpret as jet plus entrained, initially
unmagnetized external mass. As with the previous simulation sets, we find that
the growth of magnetized mass is associated with the growth of the KH
instability through linear, nonlinear, and saturated stages and with the
expansion of magnetized material in simulated observations of the jet. From
comparison of measured wavelengths and wave speeds with the predictions from
linear stability analysis, we see evidence that the KH instability is the
primary cause for mass entrainment in these simulations, and that the expansion
reduces the rate of mass entrainment. This reduced rate can be observed as a
somewhat greater distance between the two transition points separating the
three stages of expansion.Comment: 18 pages, 6 figures, AASTeX, to appear in Nov 1 issue of ApJ (vol
543), postscript versions of Figures 3 and 5 are available at
http://crux.astr.ua.edu/~rosen/supcon/rh.htm
Fedosov supermanifolds: II. Normal coordinates
The study of recently introduced Fedosov supermanifolds is continued. Using
normal coordinates, properties of even and odd symplectic supermanifolds
endowed with a symmetric connection respecting given sympletic structure are
studied.Comment: 12 pages, Late
Modèles linéaires stochastiques théoriques pour la réponse des petits bassins
En rendant aléatoires les intrants du modèle déterministe en cascade de réservoirs linéaires de Nash-Dooge, on obtient des modèles linéaires stochastiques adaptés aux petits bassins, qui peuvent être formulés comme des systèmes dynamiques stochastiques linéaires simples représentés par des équations différentielles stochastiques (EDS). Les processus du système, la précipitation et les pertes dues à l'évapotranspiration (cette dernière étant considérée comme un intrant négatif), sont respectivement modélisés par un processus composé de Poisson et par un bruit blanc gaussien à moyenne nulle superposé à une moyenne déterministe. Pour la réponse superficielle et la réponse souterraine, on propose des modèles stochastiques en cascades de Nash-Dooge à n réservoirs linéaires égaux et à deux réservoirs en parallèle. Des travaux récents sur la genèse des débits ont conduit à mettre au point un modèle dynamique grossier, plus plausible conceptuellement, formé de régimes à réponse rapide et à réponse lente parallèles. Ce modèle est élaboré en attribuant au réservoir lent toutes les pertes d'évapotranspiration, les fluctuations de celle-ci étant modélisées par un bruit gaussien coloré à moyenne nulle et en rationalisant un modèle d'infiltration linéarisé fonction d'un écoulement à régime lent précédant une précipitation. En fait, cette contribution vise à donner une portée plus générale à la théorie déterministe de Nash-Dooge basée sur l'hydrogramme unitaire, afin de l'étendre à une théorie linéaire stochastique de réponse d'un bassin.By randomizing the inputs to the deterministic Nash-Dooge linear reservoir cascade, linear stochastic conceptual response models suitable for small catchments are formulated as simple linear stochastic dynamical systems within the formalism of stochastic differential equations (SDE’s). The system driving processes, rainfall and evapotranspiration losses, the latter regarded as a negative input, are modeled respectively as a compound Poisson process and a mean zero white Gaussian noise superposed on a deterministic mean. Elementary stochasticized Nash-Dooge cascades of n equal linear reservoirs and two reservoirs in parallel are given as potential models of surface and subsurface response. On consideration of recent discoveries concerning streamflow generation, a more conceptually plausible coarse-grained dynamical model of parallel quick and slow response regimes is developed by confining all evapotranspiration losses to the slow reservoir, modeling evapotranspiration fluctuations as mean zero colored Gaussian noise and rationalizing a linearized infiltration model dependent on slow regime outflow just prior to an event. In essence, the effort is directed towards generalizing the deterministic Nash-Dooge theory of the unit hydrograph to a linear stochastic theory of catchment response
B-Meson Distribution Amplitudes of Geometric Twist vs. Dynamical Twist
Two- and three-particle distribution amplitudes of heavy pseudoscalar mesons
of well-defined geometric twist are introduced. They are obtained from
appropriately parametrized vacuum-to-meson matrix elements by applying those
twist projectors which determine the enclosed light-cone operators of definite
geometric twist and, in addition, observing the heavy quark constraint.
Comparing these distribution amplitudes with the conventional ones of dynamical
twist we derive relations between them, partially being of Wandzura-Wilczek
type; also sum rules of Burkhardt-Cottingham type are derived.The derivation is
performed for the (double) Mellin moments and then re-summed to the non-local
distribution amplitudes. Furthermore, a parametrization of vacuum-to-meson
matrix elements for non-local operators off the light-cone in terms of
distribution amplitudes accompanying independent kinematical structures is
derived.Comment: 18 pages, Latex 2e, no figure
A Particle Module for the PLUTO Code: I - an implementation of the MHD-PIC equations
We describe an implementation of a particle physics module available for the
PLUTO code, appropriate for the dynamical evolution of a plasma consisting of a
thermal fluid and a non-thermal component represented by relativistic charged
particles, or cosmic rays (CR). While the fluid is approached using standard
numerical schemes for magnetohydrodynamics, CR particles are treated
kinetically using conventional Particle-In-Cell (PIC) techniques.
The module can be used to describe either test particles motion in the fluid
electromagnetic field or to solve the fully coupled MHD-PIC system of equations
with particle backreaction on the fluid as originally introduced by
\cite{Bai_etal.2015}. Particle backreaction on the fluid is included in the
form of momentum-energy feedback and by introducing the CR-induced Hall term in
Ohm's law. The hybrid MHD-PIC module can be employed to study CR kinetic
effects on scales larger than the (ion) skin depth provided the Larmor gyration
scale is properly resolved. When applicable, this formulation avoids to resolve
microscopic scales offering a substantial computational saving with respect to
PIC simulations.
We present a fully-conservative formulation which is second-order accurate in
time and space and extends to either Runge-Kutta (RK) or
corner-transport-upwind (CTU) time-stepping schemes (for the fluid) while a
standard Boris integrator is employed for the particles. For highly-energetic
relativistic CRs and in order to overcome the time step restriction a novel
sub-cycling strategy that retains second-order accuracy in time is presented.
Numerical benchmarks and applications including Bell instability, diffusive
shock acceleration and test particle acceleration in reconnecting layers are
discussed.Comment: 27 pages, 16 figures. Accepted for publication in ApJ Supplement
serie
Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds
Two three-dimensional magnetohydrodynamical simulations of strongly
magnetized conical jets, one with a poloidal and one with a helical magnetic
field, have been performed. In the poloidal simulation a significant sheath
(wind) of magnetized moving material developed and partially stabilized the jet
to helical twisting. The fundamental pinch mode was not similarly affected and
emission knots developed in the poloidal simulation. Thus, astrophysical jets
surrounded by outflowing winds could develop knotty structures along a straight
jet triggered by pinching. Where helical twisting dominated the dynamics,
magnetic field orientation along the line-of-sight could be organized by the
toroidal flow field accompanying helical twisting. On astrophysical jets such
structure could lead to a reversal of the direction of Faraday rotation in
adjacent zones along a jet. Theoretical analysis showed that the different
dynamical behavior of the two simulations could be entirely understood as a
result of dependence on the velocity shear between jet and wind which must
exceed a surface Alfven speed before the jet becomes unstable to helical and
higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September
Cosmological Implications of the Tetron Model of Elementary Particles
Based on a possible solution to the tetron spin problem, a modification of
the standard Big Bang scenario is suggested, where the advent of a spacetime
manifold is connected to the appearance of tetronic bound states. The metric
tensor is constructed from tetron constituents and the reason for cosmic
inflation is elucidated. Furthermore, there are natural dark matter candidates
in the tetron model. The ratio of ordinary to dark matter in the universe is
calculated to be 1:5.Comment: 23 page
The KINDRA project – towards Open Science in Hydrogeology for higher impact
Groundwater knowledge and research in the European Union is often scattered and non-standardised. Therefore, KINDRA is conducting an EU-wide assessment of existing groundwater-related practical and scientific knowledge based on a new Hydrogeological Research Classification System (HRC-SYS). The classification is supported by a web service, the European Inventory of Groundwater Research (EIGR), which acts not only as a knowledge repository but also as a tool to help identify relevant research topics, existing research trends and critical research challenges. These results will be useful for
producing synergies, implementing policies and optimising water management in Europe. This article presents the work of the project during the first two years in relation to a common classification system and an activity for data collection and training delivered by the EFG’s National Associations in 20 European countries
Charged particles in random magnetic fields and the critical behavior in the fractional quantum Hall effect
As a model for the transitions between plateaus in the fractional Quantum
Hall effect we study the critical behavior of non-interacting charged particles
in a static random magnetic field with finite mean value. We argue that this
model belongs to the same universality class as the integer Quantum Hall
effect. The universality is proved for the limiting cases of the lowest Landau
level, and slowly fluctuating magnetic fields in arbitrary Landau levels. The
conjecture that the universality holds in general is based on the study of the
statistical properties of the corresponding random matrix model.Comment: 11 pages, Revtex 3.0, no figures, to appear in PR
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