Particle jet formation during explosive dispersal of solid particles

When a layer of solid particles is explosively dispersed, a multiphase instability often occurs which leads to the formation of coherent clusters of particles or jet-like particle structures, which are aerodynamically stable. Figure 1 (cf. video 1) shows an example of the formation of such particle jets for the case of explosive dispersal of a packed bed of sand particles contained within a glass sphere. The particles are accelerated radially by the detonation of a ball of plastic explosive placed at the center of the spherical bed. The general sequence of events that occurs during explosive dispersal of dry particles has been described by Milne et al.¹ After detonation of the explosive charge, a spherical shock wave travels outwards radially, compacting the particle bed. When the wave reaches the surface of the particle bed, a blast wave is transmitted outwards and an expansion wave reflects back into the bed. At this point, the tension induced at the particle interface causes a thin spall layer of particles to be torn from the surface of the particle bed. Around the time that the expansion wave reaches the inner surface of the particle bed, perturbations to the particle density have been observed with flash radiography.¹ The compacted particle bed is observed to break up into fragments that have a scale which is comparable to the thickness of the compacted layer. These fragments then move on ballistic trajectories, shedding particles and forming the narrow jets observed in Fig. 1.

Several different mechanisms likely contribute to the jet formation, depending on the material properties of the particles and the state of the particle bed after shock compaction. For example, Milne et al.² have suggested that the fracturing of the compacted particle bed may involve a dynamic balance of surface energy and local kinetic energy, as proposed earlier by Grady² for the fragmentation of condensed matter.

Explosive dispersal of a liquid such as water generates jets of liquid droplets. Dispersal of the same volume of liquid generates a larger number of jets, in comparison with solid particles, although the droplets fragment and the jets dissipate sooner. Dispersal of a solid particle bed saturated with liquid generates more jets than either material alone, which is clearly evident in dispersal experiments using the cylindrical geometry shown in Fig. 2. Figure 3 shows that the dispersal of a wet particle bed results in almost an order of magnitude more jets in this cylindrical geometry. These jet formation effects have yet to be predicted by the current generation of multiphase models.

Support for this work was provided by Defense Research and Development Canada - Suffield, and the Defense Threat Reduction Agency under grant HDTRA1-11-1-0014 (program manager Suhithi Peiris). One of the authors (D.F.) would like to thank Alec Milne and Aaron Longbottom for useful discussions.