SCI Publications

A classical potential function for simulations of poly(vinylidene fluoride) (PVDF) based upon quantum chemistry calculations on PVDF oligomers has been developed. Quantum chemistry analysis of the geometries and conformational energies of 1,1,1,3,3-pentafluorobutane (PFB), 1,1,1,3,3,5,5,5-octofluoropentane (OFP), 2,2,4,4-tetrafluoropentane (TFP), and 2,2,4,4,6,6-hexafluoroheptane (HFH) was undertaken. In addition, an ab initio investigation of the energies of CF_4-CF₄ and CH_4-CF₄ dimers was performed. The classical potential function accurately reproduces the molecular geometries and conformational energies of the PVDF oligomers as well as intermolecular interactions between CH₄ and CF₄. To validate the force field, molecular dynamics simulations of a PVDF melts have been carried out at several temperatures. Simulation results are in good agreement with extant experimental data for PVT properties for amorphous PVDF as well as for PVDF chain dimensions.

Multigrid methods have proved robust and highly desirable in terms of the iteration speed in solving elastohydrodynamic lubrication (EHL) problems. Lubrecht, Venner and Ehret, amongst others, have shown that multigrid can be successfully used to obtain converged solutions for steady problems. steady problems.

A detailed study reinforces these results but also shows, in some cases, that while multigrid techniques give initial rapid convergence, the residuals - having dropped to a low level - may reach a stalling point, mainly due to the cavitation region. The study will explain this behaviour in terms of the iterative scheme and show how, if this happens, the errors in the fine grid solution can be reduced further. Example results of both steady and transient EHL problems (including a thermal viscoelastic case) are shown with further developments into adaptive meshes considered.

Multigrid methods have proved robust and highly desirable in terms of the iteration speed in solving elastohydrodynamic lubrication (EHL) problems. Lubrecht, Venner and Ehret, amongst others, have shown that multigrid can be successfully used to obtain converged solutions for steady problems, steady problems. A detailed study reinforces these results but also shows, in some cases, that while multigrid techniques give initial rapid convergence, the residuals - having dropped to a low level - may reach a stalling point, mainly due to the cavitation region. The study will explain this behaviour in terms of the iterative scheme and show how, if this happens, the errors in the fine grid solution can be reduced further. Example results of both steady and transient EHL problems (including a thermal viscoelastic case) are shown with further developments into adaptive meshes considered. © 2000 Elsevier Science B.V. All rights reserved.

The Center for the Simulation of Accidental Fires and Explosions (C‐SAFE) at the University of Utah is focused on providing state‐of‐the‐art, science‐based tools for the numerical simulation of accidental fires and explosions, especially within the context of handling and storage of highly flammable materials. The objective of the C‐SAFE effort is to provide a scalable, high‐performance system composed of a problem‐solving environment in which fundamental chemistry and engineering physics are fully coupled with non‐linear solvers, optimization, computational steering, visualization and experimental data verification. The availability of simulations using this system will help to better evaluate the risks and safety issues associated with fires and explosions. Our five‐year product, termed Uintah 5.0, will be validated and documented for practical application to accidents involving both hydrocarbon and energetic materials.

The molecular structures and energetic stabilities of the three pure polymorphic forms of crystalline HMX were calculated using a first-principles electronic-structure method. The computations were performed using the local density approximation in conjunction with localized “fireball” orbitals and a minimal basis set. Optimized cell parameters and molecular geometries were obtained, subject only to preservation of the experimental lattice angles and relative lattice lengths. The latter constraint was removed in some calculations for β-HMX. Within these constraints, the comparison between theory and experiment is found to be good. The structures, relative energies of the polymorphs, and bulk moduli are in general agreement with the available experimental data.

Using the BLYP and B3LYP level of density functional theory, four possible decomposition reaction pathways of HMX in the gas phase were investigated: N-NO₂ bond dissociation, HONO elimination, C-N bond scission of the ring, and the concerted ring fission. The energetics of each of these four mechanisms are reported. Dissociation of the N-NO₂ bond is putatively the initial mechanism of nitramine decomposition in the gas phase. Our results find the dissociation energy of this mechanism to be 41.8 kcal/mol at the BLYP level and 40.5 kcal/mol at the B3LYP level, which is comparable to experimental results. Three other mechanisms are calculated and found at the BLYP level to be energetically competitive to the nitrogennitrogen bond dissociation; however, at the B3LYP level these three other mechanisms are energetically less favorable. It is proposed that the HONO elimination and C-N bond scission reaction of the ring would be favorable in the condensed phase.