Simulation shockwave

Holian B L and Lomdahl P S 1998 Plasticity induced by shockwaves in nonequilibrium molecular-dynamics simulations Soienoe 280 2085-8... 

In this section we first present a series of results (Sec. 3.1) which show that molecular dynamics simulations can be used to directly link atomic scale chemistry to the continuum theory of detonations. We then show (Secs. 3.2 and 3.3) that complex initiation behavior can arise even within the simple AB Model I system. Taken together, the results reviewed in this section demonstrate that simulations using REBO potentials provide a powerful probe of the interplay between the continuum properties of shockwaves and the atomic scale chemistry induced in the initiation and propagation of condensed phasn detonations. 

Simulations using Model I were initiated by impacting a flyer plate composed of several layers of a nonenergetic AA molecular solid with the edge of the 2-D semiinfinite energetic AB solid. The latter is initially at rest at near zero temperature and pressure. Figure 4 shows a typical snapshot of a chemically sustained shockwave that can result. A distinct shockfront is visible with reactant molecules at the right and product molecules at the far left. After initiation, the shock front rapidly approaches a constant... 

In these latter studies, strong shockwaves were produced by driving the free edge of the molecular solid with a steadily moving piston as depicted in the lower part of Fig. 3. Two-dimensional simulations were initially carried out to determine the piston driven shock-to-detonation threshold in the perfect crystal. Once this threshold was determined, a crack such as that depicted at the top of Fig. 19 was introduced. Additional simulations were then performed for a series of piston velocities near, but below, the critical piston velocity, Vp, that is necessary to cause detonation in defect-free... 

For strong shockwaves the presence of a single crack such as that depicted in Fig. 19 (top) was found to significantly reduce the shock-to-detonation threshold. This is demonstrated in Fig. 19 (bottom). This figure depicts the shockfront position versus time for a series of simulations... 

Actually, there is not one but two transitions visible in the snapshot of the Model III simulations. This second transition occurs at the interface between the dissociative zone and the rarefaction region as the material transforms from the dissociative phase to A2 and B2 molecular products. It is the second transition that produces the required behavior in the shock Hugoniot for a rarefaction shockwave. This product rarefaction shockfront appears to act as a steadily moving piston producing the observed flat-topped shockwave structure. 

Non-Equilibrium Molecular Dynamics (NEMD) Shockwave Simulations... 

In light of these observations from our MD simulations, we have proposed a simple model [41] describing the role of shockwave interactions with microscopic voids that leads to significant heating, sufficient to thermally initiate chemical reactions in solid explosives, or phase transitions in metals. The key ingredients to this dramatic overshoot in temperature are shown in Fig. 13. The dependencies on both shock strength (piston velocity Up) and onedimensional gap width /, which we observed in atomistic simulations of a two-dimensional unreactive Lennard-Jones solid, for the thermal overshoot AT was well predicted by our straightforward model ... 

We have shown in Section 3.4 that NEMD simulations using ReaxFF allow one to study the initial chemical events induced by shockwaves in RDX. We have also seen that the interaction of voids in the crystals with shocks can lead to large local heating and that this localization of energy can initiate chemical reactions in the AB system. We now turn to analyze the effect of small voids (planar gaps) on the initial chemical events in RDX. 

Equilibrium MD simulations can provide valuable information about the thermal decomposition of energetic materials and can also enable the exploration of phenomena with time-scales much longer than in shockwaves. As an example, we studied the decomposition and subsequent reactions of RDX under various temperatmes (between T = 1200 K and T = 3000 K) and densities (at low density, 0.21 g/cm near normal density, 1.68 g/cm and under compression, 2.11 g/cm ), using MD with RDX interactions given by the reactive potential ReaxFF. 

The main remaining limitation of these kinds of potentials is the lack of terms to represent chemical reactions. Some recent progress along these lines, particularly in the field of energetic materials, has been recently reviewed.[138] The most important developments have been achieved so far using reactive empirical bond order (REBO) potentials introduced by Brenner and coworkers.[139] The REBO potentials have been used mainly for simulations of shockwave propagation in simple diatomic or triatomic molecular crystals however, it is likely that these kinds of potentials will soon be extended to more complex systems. A reactive potential based on somewhat different bond order concepts has been used to calculate the initial shock-wave induced chemical events in RDX.[140] To date, to our knowledge, REBO potentials have not been applied to ionic crystals. 

Many more CTD simulations at the microscale exist and the numerical reproduction of the behavior of pressure-driven flows in microchannels has been obtained often [2, 10]. Some discrepancies have been found between numerical solutions of the Navier-Stokes equations and experimental data obtained in viscosity-dominated shock tube investigations. It was noticed that even using fine grid cells of a few micrometers, the solution of the Navier-Stokes equations still does not match experimental data of shockwave reflection transition over a wedge [2]. It has been pointed out that the solution of shockwave motion at low Reynolds... 

Experiments with a miniature shock tube using low pressures to simulate the effects of small scale have shown qualitative agreement with the proposed model. The effects of scale are even more pronounced than what has been predicted by the model. Experimental and numerical investigations for incident shock Mach number of M = 1.2 have shown significant viscous effects for channel heights below 4 mm even at atmospheric pressure. That shockwaves propagate more slowly at low pressures in a narrow channel has been confirmed, but they may... 

On the other hand, the analysis of experimental shockwave data for water has shown (Ree 1982) that at the limit of high temperatures and pressures intermolecular interactions of water become simpler. In this case, it becomes even possible to use a spherically-symmetric model potential for the calculations of water properties either from computer simulations (Belonoshko and Saxena 1991, 1992) or from thermodynamic perturbation theory in a way similar to simple liquids (Hansen and McDonald 1986). However, such simplifications exclude the possibility of understanding many important and complex phenomena in aqueous fluids on a true molecular level, which is, actually, the strongest advantage and the main objective of molecular computer simulations. 

The Mach number near the downstream position of a micronozzle s throat is lower than that in a conventional nozzle. In the divergent region of the micronozzle, there is a supersonic area instead of the shockwave that usually occurs in conventional-scale nozzles. Results of numerical simulations also show that the position of the... 

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