Aeronautics and Aerospace Department, von Karman Institute for Fluid Dynamics, Chaussée de Waterloo 72, 1640 Rhode-Saint-Genèse, Belgium.
J Chem Phys. 2018 Nov 7;149(17):174106. doi: 10.1063/1.5030211.
We propose to couple a state-resolved rovibrational coarse-grain model to a stochastic particle method for simulating internal energy excitation and dissociation of a molecular gas. A coarse-grained model for a rovibrational reaction mechanism of an database developed at the NASA Ames Research Center for the N-N system is modified based on variably spaced energy bins. The thermodynamic properties of the modified coarse-grained model allow us to closely match those obtained with the full set of rovibrational levels over a wide temperature range, while using a number of bins significantly smaller than the complete mechanism. The chemical-kinetic behavior of equally and variably spaced bin formulations is compared by simulating internal energy excitation and dissociation of nitrogen in an adiabatic, isochoric reactor. We find that the variably spaced formulation is better suited for reproducing the dynamics of the full database at conditions of interest in the Earth atmospheric entry. Also in this paper, we discuss the details of our particle method implementation for the uniform rovibrational collisional bin model and describe changes to the Direct Simulation Monte Carlo (DSMC) collision algorithm, which become necessary to accommodate our state-resolved reaction mechanism for excitation and dissociation reactions. The DSMC code is then verified against equivalent master equation calculations. In these simulations, state-resolved cross sections are used in analytical form. These cross sections verify micro-reversibility relations for the rovibrational bins and allow for fast execution of the DSMC code. In our verification calculations, we obtain very close agreement for the concentrations profiles of N and N, as well as the translational and rovibrational mode temperatures obtained independently through both methods. In addition to macroscopic moments, we compare discrete internal energy populations predicted at selected time steps via DSMC and the master equations. We observe good agreement between the two sets of results within the limits imposed by statistical scatter, which is inherent to particle-based DSMC solutions. As future work, the rovibrational coarse-grain model coupled to the particle method will allow us to study 3D reentry flow configurations.
我们建议将状态分辨的振转粗粒模型与随机粒子方法耦合，以模拟分子气体的内部能量激发和离解。我们基于变间距的能量-bin 对 NASA Ames 研究中心开发的用于 N-N 体系的振转反应机制的粗粒数据库进行了修改。修改后的粗粒模型的热力学性质允许我们在很宽的温度范围内与完整的振转能级集获得非常接近的结果，同时使用的-bin 数量明显小于完整的机制。通过在绝热等容反应器中模拟氮气的内部能量激发和离解，我们比较了等间距和变间距-bin 配方的化学动力学行为。我们发现，在地球大气进入条件下感兴趣的条件下，变间距配方更适合再现完整数据库的动力学。本文还讨论了我们用于均匀振转碰撞-bin 模型的粒子方法实现的细节，并描述了对直接模拟蒙特卡罗 (DSMC) 碰撞算法的更改，这些更改对于我们用于激发和离解反应的状态分辨反应机制是必要的。然后，DSMC 代码通过与等效主方程计算进行验证。在这些模拟中，状态分辨的截面以解析形式使用。这些截面验证了振转-bin 的微观可逆性关系，并允许快速执行 DSMC 代码。在我们的验证计算中，我们通过两种方法都获得了非常接近的 N 和 N 的浓度分布以及通过独立方法获得的平移和振转模式温度的结果。除了宏观矩之外，我们还比较了通过 DSMC 和主方程在选定时间步预测的离散内部能量群体。我们观察到这两种结果之间的良好一致性，这在基于粒子的 DSMC 解决方案固有的统计分散范围内是不可避免的。作为未来的工作，将振转粗粒模型与粒子方法耦合将使我们能够研究 3D 再入流配置。
