Bulletin of the American Physical Society
21st Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 64, Number 8
Sunday–Friday, June 16–21, 2019; Portland, Oregon
Session U2: TMS: Mesoscale Explosive Initiation III |
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Chair: Ryan Wixom, SNL Room: Grand Ballroom II |
Thursday, June 20, 2019 3:15PM - 3:30PM |
U2.00001: Hot spot criticality in shocked octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine over a range of pores sizes and pressures H. Keo Springer, Sorin Bastea, James Gambino Novel continuum hot spot models for explosives now incorporate information on pore size distribution. However, there is a limited understanding of the participation of different sized pores in shock initiation scenarios. The objective of this simulation-based study is to determine the criticality of single hot spots as a function of circular pore diameter and shock pressure. We also study sensitivities to thermal diffusion, the strength model, and the kinetic model. Simulations are performed with the multi-physics hydrocode, ALE3D. A coupled thermochemical code provides the equation-of-states, thermal transport properties, and the chemical kinetics. We also employ a strain, strain-rate, and pressure-dependent strength model. Strain-rate hardening has been shown to enhance shear and temperature localization during pore collapse which can accelerate the time to ignition. Based on our calculations, we develop a simplified ignition criterion and compare to existing hot spot criticality models. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-768077 [Preview Abstract] |
Thursday, June 20, 2019 3:30PM - 3:45PM |
U2.00002: Mesh Dependence of Initiation Threshold in the Presence of Mesoscale Porosity David Hardin, H. Keo Springer, Nirmal Rai, Sushilkumar Koundinyan, G. "Chip" Butler Mesoscale modeling is frequently performed using continuum-level simulation tools to model the behavior of porous energetic materials undergoing dynamic loading from a shock or impact. In this scenario, the pores contained in the computational domain are collapsed due to the mechanical stresses on the material. During this process, a number of different energy localization mechanisms compete to achieve this collapse with the result being a locally heated region of material or ``hot spot''. The shape and temperature distribution of each hot spot is a direct result of the input stress and initial pore size and shape, and the hot spot drives the chemical decomposition of the HE. This work will address the dependence of the calculated initiation threshold of an HE on the underlying computational mesh of the domain. Ideally, this numerical dependence would be minimal relative to the influence of the physical parameters which control energy localization, but this is not always the case. This work will investigate the mesh dependence of initiation threshold predictions across different simulation platforms and discuss the underlying uncertainty that this dependency introduces. [Preview Abstract] |
Thursday, June 20, 2019 3:45PM - 4:15PM |
U2.00003: Addressing the gap between meso(grain) and continuum scales with stochastic burn models and probability density function theory Invited Speaker: David Kittell Within the energetics community, considerable effort is being directed to find a robust scale-bridging link between the unreacted material microstructures and the observed material responses, e.g. detonation and sub- detonative phenomena. One area of active research is mesoscale modeling of explosives initiation (MMEI); here, microstructures are imported directly or as statistical reconstructions into a hydrocode. While MMEI is attractive for simulating the ignition process with ever-increasing model fidelity, a large gap remains between the data being generated at the mesoscale and the calibration of burn model parameters. In this work, we begin to explore and apply scale-bridging techniques found in other fields. This includes particle methods from granular and droplet-laden flows, that use stochastic Langevin-type equations. Further parallels are drawn to turbulent combustion modeling, which leads to preliminary developments using probability density function (pdf) theory by Baer. In order to implement these new scale-bridging concepts, one example of a stochastic burn model is explained in greater detail. Results from the stochastic burn model and numerical method of particle-averaged characteristics (MOPAC) are given to illustrate the approach. Overall, if stochastic continuum-level models are adopted, then the pdf distributions from MMEI could function as the missing scale-bridging link. I.e., the stochastic (random, aleatoric) fluctuations would be sampled from a pdf distribution representative of the thermodynamic states found in an MMEI calculation. Ultimately, the execution of this scale-bridging work will be a community endeavor; to achieve such a capability, research efforts should focus on full-field data mining and pdf evolution in addition to new numerical techniques for hydrocodes. [Preview Abstract] |
Thursday, June 20, 2019 4:15PM - 4:30PM |
U2.00004: Hotspot formation due to crack in HMX crystals Chunyu Li, Alejandro Strachan The shock to detonation transition in composite high energy density materials results from the complex interplay between the propagating wave and microstructural features that lead to the formation of hotspots and the exothermic reactions in these hotspots. We report on large-scale molecular dynamics simulations of the formation of hotspots due to the interaction of a shockwave and internal flaws and pores in crystalline HMX. We characterize the effect of size and orientation of elliptical cracks on the resulting hotspots. Comparing non-equilibrium shock propagation simulations with equilibrium Hugoniotstat simulations for identical shock strength enables us to separate the contribution of different mechanisms to the overall temperature and size of the resulting hotspot. Specifically, we focus on the role of jetting, expansion and recompression, friction and viscous pore collapse. [Preview Abstract] |
Thursday, June 20, 2019 4:30PM - 4:45PM |
U2.00005: Effect of Shape Resolution on the Simulated Energetic Response of Shock Induced Pore Collapse within HMX. Jesus Mares Jr., D Barrett Hardin, G "Chip" Butler, James Vitarelli, Christopher Molek It is widely accepted that the shock initiation of explosive materials is largely dependent upon the localization of energy at material defects commonly referred to as ``hot-spots''. Many studies have investigated the effect of size and distribution of the shock-induced collapse of voids within explosive materials. However, there is limited understanding of the effect of void shape on the collapse process and subsequent energy localization. In this work, we utilize complex Fourier descriptors to characterize a series of 2-dimensional void structures imaged via scanning electron microscopy of pressed HMX material. The shape information of the void structures is then modified by a series of increasing low-pass filters to yield increasingly ``smoothed'' void structures. The shock induced collapse of these modified void structures is then simulated to investigate the effect of the resolution of shape detail on the resulting energy localization of the collapse process. This work intends to evaluate the level of spatial resolution needed to ``adequately'' characterize the void structure of a specified size under various shock loadings. DISTRIBUTION A. Approved for public release: distribution unlimited. (96TW-2019-0047). [Preview Abstract] |
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