A new methodology is proposed to couple Molecular Dynamics (MD) and Direct Simulation Monte Carlo (DSMC) methods to simulate high Knudsen number (Kn) flows. For this purpose a two-dimensional hybrid ...MD-DSMC code is developed. In this method gas–surface interactions are modeled using MD, and gas–gas interactions are modeled using DSMC method. Two-way coupling between MD and DSMC is implemented by employing buffer zones for both MD and DSMC regions. Bootstrap sampling and energy minimization algorithms are employed for dynamic coupling of these two methods since MD utilizes real number of molecules during simulation whereas DSMC utilizes a lesser number of simulated molecules. The hybrid methodology combines the advantages of both methods; it has the capability of modeling the gas–surface interaction accurately considering the effect of the presence of neighboring real number of gas molecules, while in the bulk it utilizes DSMC with only the simulated number of molecules thus increasing the computational efficiency significantly compared to pure MD codes. As a result comparatively large domain sizes can be simulated with realistic behavior at the walls. The utility of the hybrid method is demonstrated by simulating high Kn flows through a micro-channel, micro-nozzle and micro-scale shock tube. The effect of partial accommodation of gas molecules with the wall is seen to be captured dynamically with this approach.
The first measurements of multiple, high-pressure shock waves in cryogenic deuterium-tritium (DT) ice layered capsule implosions on the National Ignition Facility have been performed. The strength ...and relative timing of these shocks must be adjusted to very high precision in order to keep the DT fuel entropy low and compressibility high. All previous measurements of shock timing in inertial confinement fusion implosions T. R. Boehly et al., Phys. Rev. Lett. 106, 195005 (2011), H. F. Robey et al., Phys. Rev. Lett. 108, 215004 (2012) have been performed in surrogate targets, where the solid DT ice shell and central DT gas regions were replaced with a continuous liquid deuterium (D2) fill. This report presents the first experimental validation of the assumptions underlying this surrogate technique.
Wall effects in a micro-scale shock tube are investigated using the Direct Simulation Monte Carlo method as well as a hybrid Molecular Dynamics–Direct Simulation Monte Carlo algorithm. In the Direct ...Simulation Monte Carlo simulations, the Cercignani–Lampis–Lord model of gas–surface interactions is employed to incorporate the wall effects, and it is shown that the shock attenuation is significantly affected by the choice of the values of tangential momentum accommodation coefficient. A loosely coupled Molecular Dynamics–Direct Simulation Monte Carlo approach is then employed to demonstrate incomplete accommodation in micro-scale shock tube flows. This approach uses fixed values of the accommodation coefficients in the gas–surface interaction model, with their values determined from a separate dynamically similar Molecular Dynamics simulation. Finally, a completely coupled Molecular Dynamics–Direct Simulation Monte Carlo algorithm is used, wherein the bulk of the flow is modeled using Direct Simulation Monte Carlo, while the interaction of gas molecules with the shock tube walls is modeled using Molecular Dynamics. The two regions are separate and coupled both ways using buffer zones and a bootstrap coupling algorithm that accounts for the mismatch of the number of molecules in both regions. It is shown that the hybrid method captures the effect of local properties that cannot be captured using a single value of accommodation coefficient for the entire domain.
This paper presents an exploration of potential mitigation methods for the gas fuel fill tube in Inertial Confinement Fusion (ICF) implosions at the National Ignition Facility (NIF), and the impact ...of hydrodynamic growth seeded from other target imperfections using a specialized low convergence implosion experiment. Enhanced x-ray self- emission of this experiment design allows the impact of hydrodynamic growth through the deceleration phase of the implosion to be examined.
Experiments are presented comparing the perturbation visible during the implosion deceleration that are seeded by the fill tube, through varying the initial geometry in otherwise similar implosions. We further extend the experiment to explore the impact of isolated high atomic number 'dots' of 5 and 20 µm diameter. These isolated dots are compared in two different ‘High Density Carbon’ ablator designs in a gold hohlraum. The experiment series finds a correlation to number of high frequency self-emission features observed in deceleration and degradation in total Deuterium-Deuterium neutron yield.
Two-dimensional direct simulation Monte Carlo (DSMC) was used to investigate the effect of wall accommodation on different modes of operation of a nozzle under high Knudsen number conditions. ...Substantial performance enhancement was seen at higher values of converging-diverging angles, because a reduction in the surface area-to-volume ratio decreased the wall effects. Though the normal energy accommodation had a negligible influence on the nozzle performance, it was seen to be a critical factor for accurate prediction of gas temperature along the wall. At higher Knudsen numbers, the flow tended to become one-dimensional even for the case of complete accommodation.
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