nccrFOAM suite: Nonlinear coupled constitutive relation solver in the OpenFOAM framework for rarefied and microscale gas flows with vibrational non-equilibrium

The nccrFOAM suite is a collection of nonlinear coupled constitutive relation (NCCR) solvers for rarefied and microscale gas flows with vibrational non-equilibrium, in conjunction with the conservation laws implemented in the foam-extend framework which is an open-source solver with a General Public License (GPL 3). nccrFOAM solvers are developed as an extension to the dbnsTurbFoam solver by implementing additional algebraic constitutive relations for the non-conserved quantities of the stress tensor and heat flux vector. In contrast to Navier-Stokes-Fourier (NSF) solvers that employ first-order constitutive relations to calculate non-conserved quantities and consequently suffer from obvious shortcomings when simulating gas flows in high non-equilibrium, the second-order NCCR framework presents a novel alternative to simulate rarefied and microscale gas flows in a better and a more intuitive manner. In addition to the solver for monoatomic gases, the solvers are for the first time implemented for diatomic and polyatomic gases with translational-rotational and vibrational degrees of freedom based on second-order constitutive models in a three-dimensional framework. Towards this, a new foam-extend library has been written based on the two-temperature formulation to handle the translational-rotational and vibrational modes. The new foam-extend solver was validated for several representative problems, and an exhaustive list of tutorials is documented. The solver will certainly benefit the rarefied and microscale gas dynamics and hypersonics communities at large interested in flows involving high degrees of rarefaction, speed, and temperature variations.nccrFOAM套件是一套面向振动非平衡态稀薄与微尺度气体流动的非线性耦合本构关系（nonlinear coupled constitutive relation, NCCR）求解器集合，依托采用通用公共许可证（General Public License, GPL 3）的开源框架foam-extend实现守恒律求解。nccrFOAM求解器以dbnsTurbFoam求解器为基础进行扩展开发，通过为应力张量与热流矢量的非守恒量新增代数本构关系得以实现。与采用一阶本构关系计算非守恒量、在模拟高非平衡态气体流动时存在显著缺陷的Navier-Stokes-Fourier（NSF）求解器不同，二阶NCCR框架提供了一种更优质、更直观的全新方案，可用于稀薄与微尺度气体流动的仿真。除针对单原子气体的求解器外，本套件首次在三维框架下基于二阶本构模型，实现了具备平动-转动与振动自由度的双原子及多原子气体求解器。为此，研究团队基于双温度公式开发了全新的foam-extend库，用于处理平动-转动与振动模态。这款全新的foam-extend求解器已通过多个典型问题的验证，并配套提供了详尽的教程文档。该求解器将为关注高稀薄度、高速与强温度变化流动的稀薄与微尺度气体动力学及高超声速领域的广大科研人员提供有力支持。