Numerical Simulation for Droplet Combustion Using Lagrangian Hydrodynamics

A predictive model of spray combustion must incorporate models for the wide variety of physical environments in a practical combustor. In regions where droplets are closely spaced, combustion resembles a diffusion flame; where they are well separated, an envelope or wake flame results. The relative velocity field between the fuel droplets and oxidizer in influences boundary layer development about the droplet, recirculating flow patterns, and droplet shape and stability. A model must encompass these interacting temporal and spatial effects as well as complicated combustor boundaries. The objective of the current work is to develop the triangular gridding method for describing the individual and collective properties of vaporizing and burning fuel droplets

Numerical simulations of fuel droplet flows using a Lagrangian triangular mesh

The incompressible, Lagrangian, triangular grid code, SPLISH, was converted for the study of flows in and around fuel droplets. This involved developing, testing and incorporating algorithms for surface tension and viscosity. The major features of the Lagrangian method and the algorithms are described. Benchmarks of the algorithms are given. Several calculations are presented for kerosene droplets in air. Finally, extensions which make the code compressible and three dimensional are discussed

Auroral Plasma Lines: A First Comparison of Theory and Experiment

In this preliminary report on low-energy (0.3 to 3 eV) secondary electrons in the auroral E layer (90 to 150 km), we compare intensities of plasma lines observed with the Chatanika radar to theoretical predictions obtained from a detailed numerical model. The model calculations are initiated with a flux of energetic auroral primary electrons which enter the atmosphere and lose energy to electrons, ions, and neutrals through a combination of elastic and inelastic collisions. This flux is chosen in order that the total calculated ionization rate matches one that is deduced from the radar measurements. From these same calculations the steady state secondary electron flux is deduced as a function of altitude, energy, and pitch angle. This flux is used to calculate plasma line intensities which are then compared with observed intensities. Initial comparisons suggest that the plasma line theory, when applied to low altitudes, must include the effect of electron-neutral collisions. When this is done, the good agreement obtained between theory and experiment indicates the promise of this approach for the study of low-energy auroral electrons

Jet-Induced Explosions of Core Collapse Supernovae

We numerically studied the explosion of a supernova caused by supersonic jets present in its center. The jets are assumed to be generated by a magneto-rotational mechanism when a stellar core collapses into a neutron star. We simulated the process of the jet propagation through the star, jet breakthrough, and the ejection of the supernova envelope by the lateral shocks generated during jet propagation. The end result of the interaction is a highly nonspherical supernova explosion with two high-velocity jets of material moving in polar directions, and a slower moving, oblate, highly distorted ejecta containing most of the supernova material. The jet-induced explosion is entirely due to the action of the jets on the surrounding star and does not depend on neutrino transport or re-acceleration of a stalled shock. The jet mechanism can explain the observed high polarization of Type Ib,c and Type II supernovae, pulsar kicks, very high velocity material observed in supernova remnants, indications that radioactive material was carried to the hydrogen-rich layers in SN1987A, and some others observations that are very difficult or impossible to explain by the neutrino energy deposition mechanism. The breakout of the jet from a compact, hydrogen- deficient core may account for the gamma-ray bursts and radio outburst associated with SN1998bw/GRB980425.Comment: 14 pages, LaTeX, aaspp4.sty, epsf.sty, submitted to ApJ Let

Thermonuclear Supernovae: Simulations of the Deflagration Stage and Their Implications

Large-scale three-dimensional numerical simulations of the deflagration stage of a thermonuclear supernova explosion show the formation and evolution of a highly convoluted turbulent flame in a gravitational field of an expanding carbon-oxygen white dwarf. The flame dynamics is dominated by the gravity-induced Rayleigh-Taylor instability that controls the burning rate. The thermonuclear deflagration releases enough energy to produce a healthy explosion. The turbulent flame, however, leaves large amounts of unburnt and partially burnt material near the star center, whereas observations imply these materials only in outer layers. This disagreement could be resolved if the deflagration triggers a detonation.Comment: 17 pages, 5 figures. To appear in Science, January 200

Baryon number segregation at the end of the cosmological quark-hadron transition

One of the most interesting questions regarding a possible first order cosmological quark--hadron phase transition concerns the final fate of the baryon number contained within the disconnected quark regions at the end of the transition. We here present a detailed investigation of the hydrodynamical evolution of an evaporating quark drop, using a multi-component fluid description to follow the mechanisms of baryon number segregation. With this approach, we are able to take account of the simultaneous effects of baryon number flux suppression at the phase interface, entropy extraction by means of particles having long mean-free-paths, and baryon number diffusion. A range of computations has been performed to investigate the permitted parameter-space and this has shown that significant baryon number concentrations, perhaps even up to densities above that of nuclear matter, represent an inevitable outcome within this scenario.Comment: 33 pages, Latex file, 6 postscript figures included in the text (psfig.tex). To appear in Phys. Rev. D1

Spontaneous Transition of Turbulent Flames to Detonations in Unconfined Media

Deflagration-to-detonation transition (DDT) can occur in environments ranging from experimental and industrial systems to astrophysical thermonuclear (type Ia) supernovae explosions. Substantial progress has been made in explaining the nature of DDT in confined systems with walls, internal obstacles, or pre-existing shocks. It remains unclear, however, whether DDT can occur in unconfined media. Here we use direct numerical simulations (DNS) to show that for high enough turbulent intensities unconfined, subsonic, premixed, turbulent flames are inherently unstable to DDT. The associated mechanism, based on the nonsteady evolution of flames faster than the Chapman-Jouguet deflagrations, is qualitatively different from the traditionally suggested spontaneous reaction wave model, and thus does not require the formation of distributed flames. Critical turbulent flame speeds, predicted by this mechanism for the onset of DDT, are in agreement with DNS results.Comment: 4 pages, 3 figures; accepted to Physical Review Letter

Predicting decoherence in discrete models

The general aim of this paper is to supply a method to decide whether a discrete system decoheres or not, and under what conditions decoherence occurs, with no need of appealing to computer simulations to obtain the time evolution of the reduced state. In particular, a lemma is presented as the core of the method.Comment: 8 pages, 2 figure