A study of numerical methods for hyperbolic conservation laws with stiff source terms

The proper modeling of nonequilibrium gas dynamics is required in certain regimes of hypersonic flow. For inviscid flow this gives a system of conservation laws coupled with source terms representing the chemistry. Often a wide range of time scales is present in the problem, leading to numerical difficulties as in stiff systems of ordinary differential equations. Stability can be achieved by using implicit methods, but other numerical difficulties are observed. The behavior of typical numerical methods on a simple advection equation with a parameter-dependent source term was studied. Two approaches to incorporate the source term were utilized: MacCormack type predictor-corrector methods with flux limiters, and splitting methods in which the fluid dynamics and chemistry are handled in separate steps. Various comparisons over a wide range of parameter values were made. In the stiff case where the solution contains discontinuities, incorrect numerical propagation speeds are observed with all of the methods considered. This phenomenon is studied and explained

Information Theoretic Operating Regimes of Large Wireless Networks

In analyzing the point-to-point wireless channel, insights about two qualitatively different operating regimes--bandwidth- and power-limited--have proven indispensable in the design of good communication schemes. In this paper, we propose a new scaling law formulation for wireless networks that allows us to develop a theory that is analogous to the point-to-point case. We identify fundamental operating regimes of wireless networks and derive architectural guidelines for the design of optimal schemes. Our analysis shows that in a given wireless network with arbitrary size, area, power, bandwidth, etc., there are three parameters of importance: the short-distance SNR, the long-distance SNR, and the power path loss exponent of the environment. Depending on these parameters we identify four qualitatively different regimes. One of these regimes is especially interesting since it is fundamentally a consequence of the heterogeneous nature of links in a network and does not occur in the point-to-point case; the network capacity is {\em both} power and bandwidth limited. This regime has thus far remained hidden due to the limitations of the existing formulation. Existing schemes, either multihop transmission or hierarchical cooperation, fail to achieve capacity in this regime; we propose a new hybrid scheme that achieves capacity.Comment: 12 pages, 5 figures, to appear in IEEE Transactions on Information Theor

Critical Collapse of an Ultrarelativistic Fluid in the Γ→1\Gamma\to 1 Limit

In this paper we investigate the critical collapse of an ultrarelativistic perfect fluid with the equation of state $P=(\Gamma-1)\rho$ in the limit of $\Gamma\to 1$. We calculate the limiting continuously self similar (CSS) solution and the limiting scaling exponent by exploiting self-similarity of the solution. We also solve the complete set of equations governing the gravitational collapse numerically for $(\Gamma-1) = 10^{-2},...,10^{-6}$ and compare them with the CSS solutions. We also investigate the supercritical regime and discuss the hypothesis of naked singularity formation in a generic gravitational collapse. The numerical calculations make use of advanced methods such as high resolution shock capturing evolution scheme for the matter evolution, adaptive mesh refinement, and quadruple precision arithmetic. The treatment of vacuum is also non standard. We were able to tune the critical parameter up to 30 significant digits and to calculate the scaling exponents accurately. The numerical results agree very well with those calculated using the CSS ansatz. The analysis of the collapse in the supercritical regime supports the hypothesis of the existence of naked singularities formed during a generic gravitational collapse.Comment: 23 pages, 16 figures, revised version, added new results of investigation of a supercritical collapse and the existence of naked singularities in generic gravitational collaps

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.

The Athena Astrophysical MHD Code in Cylindrical Geometry

A method for implementing cylindrical coordinates in the Athena magnetohydrodynamics (MHD) code is described. The extension follows the approach of Athena's original developers and has been designed to alter the existing Cartesian-coordinates code as minimally and transparently as possible. The numerical equations in cylindrical coordinates are formulated to maintain consistency with constrained transport, a central feature of the Athena algorithm, while making use of previously implemented code modules such as the Riemann solvers. Angular-momentum transport, which is critical in astrophysical disk systems dominated by rotation, is treated carefully. We describe modifications for cylindrical coordinates of the higher-order spatial reconstruction and characteristic evolution steps as well as the finite-volume and constrained transport updates. Finally, we present a test suite of standard and novel problems in one-, two-, and three-dimensions designed to validate our algorithms and implementation and to be of use to other code developers. The code is suitable for use in a wide variety of astrophysical applications and is freely available for download on the web

A Study of Numerical Methods for Hyperbolic Conservation Laws with Stiff Source Terms

The proper modeling of nonequilibrium gas dynamics is required in certain regimes of hypersonic flow. For inviscid flow this gives a system of conservation laws coupled with source terms representing the chemistry. Often a wide range of time scales is present in the problem, leading to numerical difficulties as in stiff systems of ordinary differential equations. Stability can be achieved by using implicit methods, but other numerical difficulties are observed. The behavior of typical numerical methods on a model advection equation with a parameter-dependent source term is studied. Two approaches to incorporate the source terms are utilized: MacCormack type predictor-corrector methods with flux limiters and splitting methods in which the fluid dynamics and chemistry are handled in separate steps. Comparisons over a wide range of parameter values are made. On the whole, the splitting methods perform somewhat better. In the stiff case, a numerical phenomenon of incorrect propagation speeds of discontinuities is observed and explained. Similar behavior was reported by Colella, Majda, and Roytburd (SIAM J. Sci. Stat. Comput. 7, 1059 (1986)) on a model combustion problem. Using the model scalar equation, we show that this is due to the introduction of nonequilibrium values through numerical dissipation in the advection step

Consistent thermodynamic derivative estimates for tabular equations of state

Numerical simulations of compressible fluid flows require an equation of state (EOS) to relate the thermodynamic variables of density, internal energy, temperature, and pressure. A valid EOS must satisfy the thermodynamic conditions of consistency (derivation from a free energy) and stability (positive sound speed squared). When phase transitions are significant, the EOS is complicated and can only be specified in a table. For tabular EOS's such as SESAME from Los Alamos National Laboratory, the consistency and stability conditions take the form of a differential equation relating the derivatives of pressure and energy as functions of temperature and density, along with positivity constraints. Typical software interfaces to such tables based on polynomial or rational interpolants compute derivatives of pressure and energy and may enforce the stability conditions, but do not enforce the consistency condition and its derivatives. We describe a new type of table interface based on a constrained local least squares regression technique. It is applied to several SESAME EOS's showing how the consistency condition can be satisfied to round-off while computing first and second derivatives with demonstrated second-order convergence. An improvement of 14 orders of magnitude over conventional derivatives is demonstrated, although the new method is apparently two orders of magnitude slower, due to the fact that every evaluation requires solving an 11-dimensional nonlinear system.Comment: 29 pages, 9 figures, 16 references, submitted to Phys Rev

Are gauge shocks really shocks?

The existence of gauge pathologies associated with the Bona-Masso family of generalized harmonic slicing conditions is proven for the case of simple 1+1 relativity. It is shown that these gauge pathologies are true shocks in the sense that the characteristic lines associated with the propagation of the gauge cross, which implies that the name ``gauge shock'' usually given to such pathologies is indeed correct. These gauge shocks are associated with places where the spatial hypersurfaces that determine the foliation of spacetime become non-smooth.Comment: 7 pages, 5 figures, REVTEX 4. Revised version, including corrections suggested by referee

L'habitat : être au bon endroit au bon moment ?

La variabilité spatiale et temporelle est une des caractéristiques des milieux lotiques en raison notamment des fluctuations du débit et du niveau de l'eau. Dans cet environnement, le poisson recherche en permanence un compromis entre la variabilité du milieu et l'accomplissement de ses besoins vitaux comme la nécessité de se reproduire, de se protéger des prédateurs, et de s'alimenter à différents stades de son développement. Son habitat (par définition, le milieu géographique propre à la vie d'une espèce animale ou végétale) résulte de cette recherche de compromis. L'héritage phylogénique fait peser sur l'espèce un certain nombre de contraintes abiotiques, biologiques et comportementales qui fixent le cadre de ses besoins en terme d'habitat. Mais les individus développent également des tactiques qui sont des réponses adaptatives aux modifications du milieu, et qui peuvent se traduire par des comportements alternatifs. Certains résultats font également penser qu'il y a possibilité d'apprentissage et de choix. L'habitat est donc une notion essentiellement dynamique, une référence spatiale et temporelle : la position qu'occupe à un instant donné un individu parvenu à un certain stade de développement, cherchant à optimiser le nécessaire compromis entre différentes contraintes biologiques et écologiques, dans un milieu lui-même très variable. Dans ce contexte, l'habitat ne peut être défini seulement par les caractéristiques physiques du milieu, comme ce fut souvent le cas jusqu'ici. Il faut rechercher une typologie qui tienne compte des comportements biologiques et quatre grands types d'organisation spatio-temporelle sont proposés : — la zone de stabulation ou zone de repos pour l'individu qui cherche avant tout un abri temporaire vis-à-vis du milieu ou de prédateurs, — la zone d'activité qui est l'échelle de référence pour les cycles nycthéméraux et lunaires correspondant notamment à la recherche de nourriture, — la niche ontogénique qui correspond à l'ensemble des milieux dont une espèce a besoin pour accomplir son cycle biologique. Cette échelle qui inclut les déplacements nécessaires à la reproduction et au développement, est d'étendue variable selon que les poissons auront un comportement de type sédentaire ou nomade, — l'échelle de la métapopulation qui correspond aux différents bassins hydrographiques dans lesquels l'espèce est présente