Hydrodynamic simulation and thermodynamic characterization of functionally graded material induced isentropic compression: Towards optimum density profile

Abstract

Hydrodynamic simulations of dynamic compression experiments reveal that heating as well as entropy production in the target are much lower along quasi-isentropes, generated using impactors employing functionally graded material (FGM), than in shock compression. The possibility of achieving quasi-isentropic compression using FGM, in both gas gun and explosive driven systems, was explored in a recent paper. Qualitative analyses of temporal profiles of pressure pulse generated in the target, obtained with various density distributions within FGM impactors, showed that quadratic density variation is most suitable. This paper attempts to re-establish this finding by identifying the signatures of quasi-isentropic compression from basic thermodynamic aspects. It is shown that quadratic density variation is most suitable candidate as it leads to least entropy increase for a specific peak pressure. Further, the optimum density profile, found by genetic algorithm based optimization tool, with density of individual layers as variables, is shown to have good agreement with quadratic density function. Finally, we explore the possibility of using layers of commonly available materials with increasing shock impedances for a generation of isentropic compression. It is shown that ramp pressure wave can be produced by optimizing the layer thicknesses of the materials used.