Robert T. Fisher

Three-Dimensional Simulations of the Deflagration Phase of the Gravitationally Confined Detonation Model of Type Ia Supernovae

We report the results of a series of three-dimensional (3D) simulations of the deflagration phase of the gravitationally confined detonation mechanism for Type Ia supernovae. In this mechanism, ignition occurs at one or several off-center points, resulting in a burning bubble of hot ash that rises rapidly, breaks through the surface of the star, and collides at a point opposite the breakout on the stellar surface. We find that detonation conditions are robustly reached in our 3D simulations for a range of initial conditions and resolutions. Detonation conditions are achieved as the result of an inwardly directed jet that is produced by the compression of unburnt surface material when the surface flow collides with itself. A high-velocity outwardly directed jet is also produced. The initial conditions explored in this paper lead to conditions at detonation that can be expected to produce large amounts of 56Ni and small amounts of intermediate-mass elements. These particular simulations are therefore relevant only to high-luminosity Type Ia supernovae. Recent observations of Type Ia supernovae imply a compositional structure that is qualitatively consistent with that expected from these simulations.

On the Hydrodynamic Interaction of Shock Waves with Interstellar Clouds. II. The Effect of Smooth Cloud Boundaries on Cloud Destruction and Cloud Turbulence

The effect of smooth cloud boundaries on the interaction of steady planar shock waves with interstellar clouds is studied using a local adaptive mesh refinement technique with an axisymmetric Godunov hydrodynamic scheme. A three-dimensional calculation is also done to confirm the two-dimensional results. We find that smooth cloud boundaries significantly affect cloud morphology and retard cloud destruction. After shock passage, a sharp density jump formsdue to velocity gradientsgeneratedinthe smooth cloud boundary.Werefer to this density jump as a‘‘slip surface’’because the velocity isshearedparalleltoits surface. Theformation of aslip surface leads tocompletecloud destruction because of the Kelvin-Helmholtz and Rayleigh-Taylor instabilities. We construct analytic models of cloud drag and vorticity generation that compare well with the numerical results. Small shreds formed by the instabilities have significant velocity dispersions of 10%‐20% of the ambient shock velocity. They could be related to the small cold H i clouds recently observed by Stanimirovic´ & Heiles. The dependence of the velocity dispersion on region size, the so-called line width‐size relation, is found to be time-dependent. In the early stages, the line width‐size relation is more or less flat because of the significant small-scale fluctuations generated by the KelvinHelmholtz instability. In the later stages, the small-scale fluctuations tend to damp, leading to a line width that increases with size. The possibility of gravitational instability triggered by shock compression is discussed. We show thatgravitationalcollapsecanbeinducedinaninitiallyuniformcloudbyaradiativeshock( �< 4/3)onlyif itisnot too strong and nonthermal motions are weak. Subject headingg hydrodynamics — ISM: clouds — shock waves — supernova remnants — turbulence

Universal intermittent properties of particle trajectories in highly turbulent flows

We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers in the range R{lambda}in[120:740]. Lagrangian structure functions from all data sets are found to collapse onto each other on a wide range of time lags, pointing towards the existence of a universal behavior, within present statistical convergence, and calling for a unified theoretical description. Parisi-Frisch multifractal theory, suitably extended to the dissipative scales and to the Lagrangian domain, is found to capture the intermittency of velocity statistics over the whole three decades of temporal scales investigated here.

Introduction to FLASH 3.0, with application to supersonic turbulence

FLASH is a flexible, modular and parallel application code capable of simulating the compressible, reactive flows found in many astrophysical environments. It is a collection of inter-operable modules which can be combined to generate different applications. FLASH is gaining increasing recognition as a community code with a fairly wide external user base. Unlike other component-based codes that have historically met with varying degrees of success. FLASH started out as a more traditional scientific code and evolved into a modular one as insights were gained into manageability, extensibility and efficiency. As a result, the development of the code has been, and continues to be, driven by the dual goals of application requirements and modularity. In this tutorial paper, we give an overview of the FLASH code architecture and capabilities. We also include an example of a customized application adapted from the sample applications provided with the code distribution.

Terascale turbulence computation using the FLASH3 application framework on the IBM Blue Gene/L system

Understanding the nature of turbulent flows remains one of the outstanding questions in classical physics. Significant progress has been recently made using computer simulation as an aid to our understanding of the rich physics of turbulence. Here, we present both the computer science and the scientific features of a unique terascale simulation of a weakly compressible turbulent flow that includes tracer particles. (Terascale refers to performance and dataset storage use in excess of a teraflop and terabyte, respectively.) The simulation was performed on the Lawrence Livermore National Laboratory IBM Blue Gene/L™ system, using version 3 of the FLASH application framework. FLASH3 is a modular, publicly available code designed primarily for astrophysical simulations, which scales well to massively parallel environments. We discuss issues related to the analysis and visualization of such a massive simulation and present initial scientific results. We, also discuss challenges related to making the database available for public release. We suggest that widespread adoption ofan open dataset model of high-performance computing is likely to result in significant advantages for the scientific computing community, in much the same way that the widespread adoption of open-source software has produced similar gains over the last 10 years.

Large-scale simulations of buoyancy-driven turbulent nuclear burning

An critical uncertainty in modeling thermonuclear supernovae is the degree of enhancement of the burning rate by turbulence during the subsonic burning (deflagration) phase. As turbulent combustion in the laboratory is still an active area of research, this remains a challenging problem. A unique feature of turbulent combustion in supernovae is that the driving of the turbulence arises from the strong buoyancy of the burned material. We discuss the large-scale fully three dimensional studies under way. These studies have the goals of characterizing the essential length scales of flame surface structure and thereby developing specific requirements that models of small-scale structure must meet. We discuss some preliminary results of our study concerning the scale-dependence of flame surface structure.

Fragmentation and Star Formation in Turbulent Cores

We examine the conditions under which binary and multiple stars may form out of turbulent molecular cloud cores using high resolution 3-D, adaptive mesh refinement (AMR) hydrodynamics (Truelove et al., 1997, 1998; Klein, 1999). We argue that previous conclusions on the conditions for cloud fragmentation have limited applicability, since they did not allow for the nonlinear density and velocity perturbations that are ubiquitous in molecular cloud cores. Over the past year, we have begun to simulate the evolution of marginally stable, turbulent cores. These models have radii, masses, density contrasts, turbulent linewidths, and projected velocity gradients consistent with observations of low-mass molecular cloud cores. Our models are evolved in time under self-gravitational hydrodynamics with AMR using a barotropic equation of state that models the transition from an isothermal to an adiabatic equation of state. We examine several properties of the resulting protostellar fragments and discuss the qualitative nature of the fragmentation process in realistic cloud cores: the transition from single to binary and multiple stars; the formation of misaligned binary systems; and the role played by filament formation in the formation of stars.