Previous experimental studies have shown that when a layer of solid particles is explosively dispersed, the particles often develop a non-uniform spatial distribution. The instabilities within the particle bed and at the particle layer interface likely form on the timescale of the shock propagation through the particles. The mesoscale perturbations are manifested at later times in experiments by the formation of coherent clusters of particles or jet-like particle structures, which are aerodynamically stable. A number of different mechanisms likely contribute to the jet formation including shock fracturing of the particle bed and particle-particle interactions in the early stages of the dense gas-particle flow. Aerodynamic wake effects at later times contribute to maintaining the stability of the jets. The experiments shown in this fluid dynamics video were carried out in either spherical or cylindrical geometry and illustrate the formation of particle jets during the explosive dispersal process. The number of jet-like structures that are generated during the dispersal of a dry powder bed is compared with the number formed during the dispersal of the same volume of water. The liquid dispersal generates a larger number of jets, but they fragment and dissipate sooner. When the particle bed is saturated with 1 arXiv:1110.3090v2 [physics.flu-dyn]

The purpose of this study was to compare the distribution of inert part~cles on polyvinylchloride (PVC) panels with earlier results from field studies. The panels were prepared in order to test heterogeneity (crevices similar in shape, but differing in size) and complexity (combination of crevices of different sizes) effects. The experiments were carried out using PVC particles and preserved Placopecten magellanjcus larvae in a flume under controlled flow conditions (mean velocities of 3, 5 and 10 cm S-') and using silicon-coated panels. The heterogeneous panels had different comblnations of flat surfaces and 1, 10 and 100 mm crevices. The density of particles per unit surface area adhering to the panels was higher at 3 and 5 cm S-' than at 10 cm S -' for all experiments. The density of adhering particles decreased with increasing panel complexity. Within panels with only 1 mm crevices, the density of particles was significantly higher inside the crevices than on smooth surfaces. However the density of particles was significantly lower in 10 and 100 mm crevices than on adjacent flat surfaces within panels with only 10 and l00 m m crevices, respectively. Furthermore, the 1 and 10 mm crevices nested inside 100 mm crevices collected fewer particles than those outside 100 mm crevices within panels with many scales of crevices. The patterns of distribution for dead larvae on heterogeneous panels in the flume corresponded to those of the inert PVC particles. However, the distribution patterns of inert particles in the flume did not correspond to those of living bivalve spat observed in an earlier fleld study. This suggests that larval behaviour contributed to the selection of settlement location for these bivalve larvae, though flume hydrodynamical conditions may differ from those in the field The results also indicate that the hydrodynamic processes affecting settlement on heterogeneous substrates are scale-dependent and that processes occurring at scales of 10 and 100 mm influence processes occurring at smaller, 1 mm, scales.

Abstract. Models for explosives containing metal particles must consider the complex gas-particle flow that is generated following detonation of the explosive. Previous experimental studies have shown that when a layer of solid particles is explosively dispersed, the particles often develop a non-uniform spatial distribution. The instabilities within the particles and at the particle layer interface likely form on the timescale of the detonation propagation through the particles. The mesoscale perturbations are manifested at later times in experiments by the formation of clusters of particles or coherent jet-like particle structures which persist for some distance during the dispersal process. These particle jets influence the particle-gas mixing and hence particle reaction rates at the macro scale. The particle instabilities that occur in explosively dispersed particles are investigated with a mesh-free computational method (smoothed-particle hydrodynamics). The simulations are compared with experimental results for the dispersal of a cylindrical packed bed of particles surrounding a central explosive charge. Of particular interest is the effect of the particle density and charge/particle mass ratio on the susceptibility of the particles to form jets.