Particle jetting has been widely observed in volcanic eruptions, supernovae, explosion of landmines, thermobaric explosion, high-speed intruder striking granular media, and particle jets impacting targets. Generally, a typical configuration of this phenomenon involves particle rings or shells dispersed by the explosion of central charges and the resultant expanding cloud of disseminated materials comprises of large particle agglomerates which rapidly protrude to finger- or spike-like particle jets. The formation of explosively driven particle jetting is a subject of fundamental and applied research interests to better understand shock interaction with particles and collective response of particles in extreme conditions as well as engineering applications.
To date, published literature seems to agree on some fundamental features of the particle jetting phenomenon. Conversely, various shortfalls have been experienced in previously demonstrated approaches; for instance, the particle jetting becomes recognizable during the shock interaction timescale and occurs in the absence of inner and outer casings. Besides, the most determinant factors driving the jetting are still far from clear.
A thorough review of existing literature shows that the relationship between the shock propagation and contact-fabric in granular materials has not been fully proposed, let along the influence of fabric on the jetting structure. Therefore, it would be crucial to reveal the underlying mechanism of particle jetting. To this end, a group of researchers from the State Key Laboratory of Explosive Science and Technology, Beijing Institute of Technology, China: Dr. Kun Xue, Dr. Jiaqi Liu and Dr. Chunhua Bai, in collaboration with Dr. Chun Feng and Dr. Yixiang Gan at the Chinese Academy of Sciences, Beijing, China and the School of Civil Engineering, The University of Sydney, Australia; respectively looked carefully at the formation of the hierarchical particle jetting pattern numerically by discrete element method (DEM) coupled with finite element method (FEM). Their goal was to uncover the underling mechanism of particle jetting which ought to be based on the thorough understanding of the interactions between multiple waves and particles as indicated by many experimental observations. Their work is published in the research journal, Chemical Engineering Science.
The researchers adopted a numerical framework (CDEM®) which incorporated both finite element model (FEM) and discrete element model (DEM) where they modeled the detonation of central explosive and the expansion of detonation gases by FEM, and particles by DEM. The detonation pressures were then calculated by the FEM solver exerted on the particles in contact with the explosive domains through contacts at the interfaces/boundaries.
The research team observed that the external jetting arose from the spallation of an outer layer pulled away by inward propagating rarefaction waves. Meanwhile an inner compact band re-compressed by a secondary shock remained densely packed while expanding outward. The authors also noted that the fragmentation of the inner compact particle band, preceding the internal particle jetting, was caused by the profuse spiral shear failures expanding from the inner radius to the outer radius. The resultant jetting structure was seen to depend on the shear-band spacing and the grouping of the clockwise and counterclockwise shear bands as well.
In summary, the study by Kun Xue and colleagues presented a coupled FEM-DEM numerical framework for the direct simulation of the formation of jetting structure driven by the explosive dispersal. Remarkably, the framework was capable of resolving the detonation of explosive, the interaction between the detonation gases and particles, as well as the interaction between particles using realistic multibody collision model. Overall, the propagation of multiple shock waves and rarefaction wave in particles was accurately captured, which is of essence to the evolution of particle shells/rings under explosive loadings.
Kun Xue, Jiaqi Liu, Chun Feng, Yixiang Gan, Chunhua Bai. Explosively driven hierarchical particle jetting. Chemical Engineering Science, volume 202 (2019) page 250–269.Go To Chemical Engineering Science