Numerical investigations of nonideal explosions

This study focusses on nonideal free-field blast waves arising from the finite rate of energy addition in a distributed nonspherical source region. The behavior of the blast wave is determined in a compressible medium surrounding a centrally ignited flammable mixture during and after the propagation of a heat addition wave which models the detonative or deflagrative combustion process. Numerical integration of the nonsteady, two-dimensional Euler equations is performed by incorporating a heat addition “two-gamma” working fluid model to represent the flame or detonation wave. This study also investigated the behavior of a blast wave generated by the burst of a high-pressure ellipsoid. The effect of the heat addition waves on the near and far field blast waves was studied by determining the relevant blast parameters, peak overpressure, and positive phase impulse, as well as the detailed evolution of the gas dynamic variables with time and distance. The blast parameters which correspond to low velocity flames compare favorably with those obtained using linear acoustic monopole theory. The results indicate that pressure relief associated with the presence of a compressible inert medium surrounding the combustion products after the flame propagates to the interface of nonspherical clouds sevrely restricts the intensity of the explosion and thereby reduces the damages that can be produced by the blast wave.     

Astrophysical explosions: from solar flares to cosmic gamma-ray bursts

Astrophysical explosions result from the release of magnetic, gravitational or thermonuclear energy on dynamical time scales, typically the sound-crossing time for the system. These explosions include solar and stellar flares, eruptive phenomena in accretion discs, thermonuclear combustion on the surfaces of white dwarfs and neutron stars, violent magnetic reconnection in neutron stars, thermonuclear and gravitational collapse supernovae and cosmic gamma-ray bursts, each representing a different type and amount of energy release. This paper summarizes the properties of these explosions and describes new research on thermonuclear explosions and explosions in extended circumstellar media. Parallels are drawn between studies of terrestrial and astrophysical explosions, especially the physics of the transition from deflagration-to-detonation.     

Surface Coulomb explosions: The influence of initial charge distributions

Using molecular dynamics simulations, we have studied the Coulomb explosion processes on silicon surfaces. Three different initial shapes of ion distributions are used to model the possible localized charge distributions generated in highly charged ion (HCI)-surface impact. The three distinct distributions include a hemispherical, a flat disk, and a long and thin cylindrical geometrical. At t = 0, 100 singly-charged Si ions are embedded in the Si(111) surfaces. In about 100 fs, the strong repulsive electrostatic forces in these three systems cause Coulomb explosions and thus create three different shapes of craters on the surfaces. All simulations are carried to 1.6 ps, at which point the size and shape of the craters are nearly stabilized. The detailed analysis of the ejected ions, atoms, and the substrates reveals the dynamical consequences of the different initial conditions. For these 100 ions, the differences in the total number of the ejected particles, ranging from 245 to 317 particles, appear to be determined by the initial shape of the ionized region and not by the initial repulsive energy restored in the charged region. Contrary to intuition, a long and thin cylindrical distribution is the most efficient pattern for ejecting particles. The underlying mechanism is that ions with this initial configuration transfer more energy to the surrounding atoms. In all three cases, the number of ejected neutral particles are much greater then the number of ions (6–10 times as many atoms as ions). Among the ejected particles, a small percent of particles are found to return the surface at a later time. The angular distribution of ejected particles are also analyzed. While the differences in the distributions of polar angle of the Si atoms of the three configurations is small, the differences in distributions of the ions portray a strong shape dependence in the polar angle.