Title: Molecular Dynamics and Monte Carlo Modeling of Hypersonic Gas Phase and Gas-Surface Reactions

Speaker: T. E. Schwartzentruber and P. Norman, University of Minnesota

Date/Time: March 3, 2011, 10:00am       

Location: CSRI Building/Room 90 (Sandia NM)

Brief Abstract: As computer resources continue their rapid growth, advancements in computational chemistry have the potential to accurately predict detailed rate data for use in high-fidelity thermochemical models; data that is difficult to measure experimentally. Efforts to use molecular dynamics (MD) to develop both non-equilibrium dissociation models required in the shock layer, as well as gas-surface interaction models for heterogeneous recombination of dissociated atoms on a heat shield surface, will be summarized.

First an overview of the direct simulation Monte Carlo (DSMC) capability at the University of Minnesota will be given. Second, an accelerated MD algorithm for dilute gases will be presented, called the Event-Driven/Time-Driven (ED/TD) MD method [1]. The method detects and moves molecules directly to their impending collision while still integrating each collision (including multi-body) using conventional Time-Driven (TD) MD with an arbitrary inter-atomic potential. The simulation thus proceeds at time steps approaching the mean-collision-time. Preliminary normal shock wave simulations of monatomic gases (pure Ar and Ar-He mixtures) will be presented in comparison with DSMC predictions and experimental data.

Third, MD simulation techniques to study heterogeneous catalytic recombination employing the ReaxFF inter-atomic potential will be detailed. MD simulations will be presented for molecular beam experiments of O2 impacting Pt(111) [2]. Grand Canonical Monte Carlo (GCMC) simulations employing the ReaxFF potential will be presented for predicting O surface coverage on Pt(111) over a range of temperatures and pressures. Finally, SiO2 surfaces (a main component in thermal protection systems) are equilibrated with a dissociated gas mixture at various temperatures and pressures, establishing surface coverage. Rates of dominant reaction mechanisms, including adsorption, desorption, and E-R/L-H recombination, are then determined by counting individual events and compared with experimental data [3]. These rates are ultimately used in a finite-rate-catalytic wall boundary condition that is implemented in a typical CFD solver.

CSRI POC: Paul Crozier, 505-845-9714