具有转动与振动非平衡的双原子气体自适应统一气体动理学格式
Adaptive unified gas-kinetic scheme for diatomic gases with rotational and vibrational nonequilibrium
AI总结：
本文提出具有转动与振动非平衡的自适应统一气体动理学格式（AUGKS），通过离散与连续分布函数动态耦合及非结构化速度空间，在保持多尺度模拟精度的同时提升计算效率并降低内存需求。
摘要：
在航空航天工程和微机电系统的应用中，例如高速飞行器和低气压封装，会遇到局部克努森数大幅变化下的多尺度非平衡物理。要在单一计算中准确描述所有流动区域的流动物理，需要一种真正的多尺度方法。自适应统一气体动理学格式（AUGKS）正是为此类多尺度流动模拟而发展的。AUGKS 采用离散速度空间来准确捕捉多尺度 UGKS 中的非平衡物理，并自适应地采用遵循 Chapman-Enskog 展开的连续分布函数，以高效恢复 GKS 中的近平衡流动区域。UGKS 和 GKS 通过来自离散和连续气体分布函数的通量在单元界面处动态连接，从而避免了它们之间的任何缓冲区。本研究发展了具有转动和振动非平衡的 AUGKS 方法，捕捉了不同流动区域中的真实气体效应。为准确捕捉气动加热，还引入了热流修正。采用非结构化离散粒子速度空间以进一步提高计算性能。数值测试包括 Sod 管、正激波结构、二维圆柱和三维球体及空间飞行器周围的高速流动，以及从连续流到背景真空的非定常喷管羽流，验证了当前格式。与原始 UGKS 相比，当前格式加快了计算速度，降低了内存需求，并在多尺度流动模拟中保持了同等精度，为非平衡流动模拟提供了有效工具，尤其适用于中低速流动。
英文摘要：
Multiscale non-equilibrium physics at large variations of local Knudsen number are encountered in applications of aerospace engineering and micro-electro-mechanical systems, such as high-speed flying vehicles and low pressure of the encapsulation. An accurate description of flow physics in all flow regimes within a single computation requires a genuinely multiscale method. The adaptive unified gas-kinetic scheme (AUGKS) is developed for such multiscale flow simulation. The AUGKS applies discretized velocity space to accurately capture the non-equilibrium physics in the multiscale UGKS, and adaptively employs continuous distribution functions following Chapman-Enskog expansion to efficiently recover near-equilibrium flow region in GKS. The UGKS and GKS are dynamically connected at the cell interface through the fluxes from the discretized and continuous gas distribution functions, which avoids any buffer zone between them. In this study, the AUGKS method with rotation and vibration non-equilibrium is developed. The real gas effect in different flow regimes has been captured. To capture aerodynamic heating accurately, the heat flux modifications are also included. Unstructured discrete particle velocity space is adopted to further improve the computational performance. Numerical tests, including Sod tube, normal shock structure, high-speed flow around the two-dimensional cylinder and three-dimensional sphere and space vehicles, and an unsteady nozzle plume flow from the continuum flow to the background vacuum, have been conducted to validate the current scheme. In comparison with the original UGKS, the current scheme speeds up the computation, reduces the memory requirement, and maintains the equivalent accuracy for multiscale flow simulation, which provides an effective tool for non-equilibrium flow simulations, especially for the flows at low and medium speed.
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