Parsons Neal, Levin Deborah A, van Duin Adri C T, Zhu Tong
Department of Aerospace Engineering, The Pennsylvania State University, 233 Hammond Building, University Park, Pennsylvania 16802, USA.
Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, 136 Research East, University Park, Pennsylvania 16802, USA.
J Chem Phys. 2014 Dec 21;141(23):234307. doi: 10.1063/1.4903782.
The Direct Simulation Monte Carlo (DSMC) method typically used for simulating hypersonic Earth re-entry flows requires accurate total collision cross sections and reaction probabilities. However, total cross sections are often determined from extrapolations of relatively low-temperature viscosity data, so their reliability is unknown for the high temperatures observed in hypersonic flows. Existing DSMC reaction models accurately reproduce experimental equilibrium reaction rates, but the applicability of these rates to the strong thermal nonequilibrium observed in hypersonic shocks is unknown. For hypersonic flows, these modeling issues are particularly relevant for nitrogen, the dominant species of air. To rectify this deficiency, the Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT) method is used to accurately compute collision and reaction cross sections for the N2(Σg+1)-N2(Σg+1) collision pair for conditions expected in hypersonic shocks using a new potential energy surface developed using a ReaxFF fit to recent advanced ab initio calculations. The MD/QCT-computed reaction probabilities were found to exhibit better physical behavior and predict less dissociation than the baseline total collision energy reaction model for strong nonequilibrium conditions expected in a shock. The MD/QCT reaction model compared well with computed equilibrium reaction rates and shock-tube data. In addition, the MD/QCT-computed total cross sections were found to agree well with established variable hard sphere total cross sections.
直接模拟蒙特卡洛（DSMC）方法通常用于模拟高超音速地球再入流，它需要精确的总碰撞截面和反应概率。然而，总截面往往是根据相对低温的粘度数据外推确定的，因此对于高超音速流中观测到的高温，其可靠性未知。现有的DSMC反应模型能够准确再现实验平衡反应速率，但这些速率在高超音速激波中观察到的强热非平衡情况下的适用性尚不清楚。对于高超音速流，这些建模问题对于空气中的主要成分氮气尤为重要。为了纠正这一缺陷，采用分子动力学/准经典轨迹（MD/QCT）方法，利用基于ReaxFF拟合最近先进的从头算计算开发的新势能面，精确计算高超音速激波预期条件下N2(Σg+1)-N2(Σg+1)碰撞对的碰撞和反应截面。结果发现，对于激波中预期的强非平衡条件，MD/QCT计算的反应概率表现出更好的物理行为，并且预测的离解比基线总碰撞能量反应模型少。MD/QCT反应模型与计算得到的平衡反应速率和激波管数据相比表现良好。此外，还发现MD/QCT计算的总截面与既定的可变硬球总截面吻合良好。
