Numerical Analysis of Shock Compression of Heterogeneous Medium Including Air Bubble

Local hot spots in heterogeneous medium can be generated by shock reflection, diffraction, micro-jetting, shear bands, viscous flow effects, etc. in the shock wave propagation. Local hot spot gives several effects to materials. In the process of shock compression, the void in condensed matter makes materials highly reactive. Explosives and energetic materials can be sensitized by local hot spots where pressure and temperature are high enough to accelerate the reaction even under low shock pressure. Numerical analysis for the shock compression of heterogeneous medium has been performed by two- dimensional Eulerian hydrodynamic code where finite difference scheme was applied with artificial viscosity called Donor Acceptor method where no heat conduction and diffusion were assumed. First, shock wave propagation in water with a spherical air bubble was computed. Measured Hugoniot data of water is used. Air was assumed to be ideal gas. When the incident shock pressure is 0.1^-GPa, micro jetting was observed. The velocity of jet is slower than the shock velocity in water. The pressure in the water at impact point by water jet becomes several Giga Pascals behind the shock front in water. However, temperature increase in water around bubble was small on impact of water jet because water is precompressed. When the incident shock pressure is 1^-GPa or higher, water jet is appeared as shown in Fig. 1, and the velocity of water jet was much higher than the shock propagation velocity in water. Then the water jet impacts before the arrival of shock front in water at axis. Both pressure and temperature increase in water by the impact of water jet were quite high. The cases of bubble collapse in water show the importance of difference between shock propagation velocity and impact velocity of jet. When the jet velocity is slower than the shock propagation velocity in water, locally high pressure zone is appeared but no high temperature zone is observed. Shock propagation mechanism is quite similar to the optical lens in which shock waves are focused on water by the air lens. Compression of air bubble is isentropic. Finally, size of air bubble is negligibly small and disappeared.