Some numerical studies of exotic shock wave behavior

For shock waves propagating in materials with nonideal equations of state, a variety of nonstandard phenomena can occur. Here, we present numerical studies of two such exotic shock effects: (i) "anomalous" behavior, in the terminology of Zel’dovich and Raizer; and (ii) a search for "acoustic emission instabilities." The motivation is in part the possibility of such phenomena in the implosion of inertial confinement fusion (ICF) pellet materials, whose equations of state are currently far from well known. In shock wave theory, anomalous materials are those whose isentropes have regions of negative curvature (in the plane of pressure versus specific volume) through which the shock adiabatic passes. The existence of such regions is significant because they can interfere with the steepening of compressive pulses into shocks, lead to the formation of rarefactive shock waves, and even cause shocks to "split." A van der Waals fluid with a large heat capacity is one example of a material possessing such anomalous properties. Acoustic emission instability—the second exotic shock mechanism considered—may occur when the slope of the shock adiabatic lies below a critical value. In this phenomenon, perturbations of a two-dimensional planar shock front can render it unstable, and lead to the downstream emission of acoustic waves. In addition to the van der Waals fluid, an equilibrium dissociation model for strong shocks in diatomic hydrogen is shown to fulfill the theoretical criteria for this instability, but its numerical verification has been hard to achieve, suggesting that further study is needed. Both classes of phenomena may be expected to play a role in ICF compression scenarios