【作者】 吴晓妍；

【导师】 秦丰华；

【作者基本信息】 中国科学技术大学 ， 流体力学， 2021， 硕士

【摘要】 气固相互作用是许多物理问题的基础,描述气体分子反射速度分布规律的散射核是其核心。经典散射核模型存在自由参数,且不考虑气固温度差异的影响。随着微尺度流动、稀薄环境下高超声速流动等研究的深入,对气固相互作用模型提出了更高的要求。本文从分子尺度直接模拟气固相互作用过程,进而重建散射核模型,考察气固温度差异的影响,拓展气固相互作用模型的适用范围,并将其应用于高速稀薄气体流动模拟。首先,采用GPU双层并行的分子动力学(MD)方法,直接模拟了不同温度的气体分子与固定温度壁面的相互作用过程,获得气体分子反射速度与入射状态之间的关系。进而应用机器学习方法,以Cercignani-Lampis-Lord(CLL)模型为基础,从直接模拟数据中重建出更精确的、考虑气固温度差的修正散射核(MCLL)模型,并分析了模型参数、气体分子散射特性随气固温度比的变化规律。结果表明,在所讨论温度范围内,与MD模拟结果相比较,修正散射核模型能准确重现反射速度与同方向入射速度的概率密度分布;气固温度差异对散射核的影响体现在特征温度调整为气体温度、调节系数随入射速度及气固温度比变化等两个方面。随着气固温度比变大,气体分子入射速度对散射特征的影响增强,当温度比超过2.5时切向反射速度分布不再受温度影响,而法向分布仍持续缓慢变化。其次,采用直接模拟Monte Carlo(DSMC)方法计算了高速稀薄气体对斜直壁面的流动,分析了不同散射核模型的影响。应用标准CLL模型、考虑真实空气高温非平衡效应的计算结果表明,调节系数的具体取值基本上不改变流动结构,但激波角随着切向动量调节系数变大而略增大,壁面滑移速度降低,壁面附近振动温度显著升高。进而,考察了 Maxwell模型、标准CLL模型、MCLL模型存在斜直壁面对单原子气体高速流动的影响,其中标准CLL模型的调节系数由MD数据学习确定。不同模型计算得到的流动结果差异仍然主要存在于壁面附近,与MCLL模型结果相比,Maxwell模型结果随调节系数在较大范围变化,调节系数越小,差异越明显;CLL模型在压力分布、滑移速度、壁面热通量等方面基本一致,但壁面附近温度明显偏低。这表明考虑局部气固温度差效应的MCLL模型能更准确的描述高超声速流动中壁面效应。更多还原

【Abstract】 The gas-solid interaction is the basis of many physical problems,and the scattering kernel that describes the distribution of the reflection velocity of gas molecules is important.The classical scattering kernel models have free parameters without considering the influence of the gas-solid temperature difference.With the deepening of the research on micro-scale flow and hypersonic flow in the rarefied environment,higher requirements are put forward for the gas-solid interaction model.This paper directly simulates the gas-solid interaction process from the molecular scale,and then reconstructs the scattering kernel to investigate the influence of the gas-solid temperature difference,expands the scope of the application of the gas-solid interaction model,and applies it to the simulation of high-speed rare gas flow.Firstly,using the GPU dual-layer parallel molecular dynamics(MD)method,the scattering of gas with different temperatures on solid surface with specified temperature is directly simulated and the relationship between the reflection velocity of the gas molecules and the incident state is obtained.Furthermore,based on the Cercignani-Lampis-Lord(CLL)model,a more accurate modified scattering kernel with the effect of the gas-solid temperature difference is reconstructed from the MD simulation data using the machine learning method.The dependency of parameters in scattering kernel on the gas-solid temperature ratio and the scattering characteristics of gas molecules are analyzed.The results show that within the temperature range discussed,compared with the MD simulation results.The characteristic temperature in the modified kernel is the gas temperature,instead of the wall temperature which is used in traditional kernels.And the adjustment coefficients depend on both the incident velocity and gas-solid temperature ratio.The larger the gas-solid temperature ratio,the stronger the influence of the molecular incidence velocity on the scattering characteristic.When the temperature ratio is larger than 2.5,the probability density of reflection velocity in tangential direction almost keeps constant but that in normal direction still changes slowly with the temperature.Secondly,the hypersonic rarefied gas flow around the oblique split is calculated by the direct simulation Monte Carlo(DSMC)method,and the effects of different scattering kernels are analyzed.The calculation results of the standard CLL model considering the high temperature non-equilibrium effect of real air show that the value of the adjustment coefficients basically don’t change the flow structure,but the shock angle increases slightly with the increase of the tangential momentum adjustment coefficient,the wall slip velocity decreases,and the vibration temperature near the wall increases significantly.Furthermore,the effects of Maxwell model,standard CLL model and the Modified CLL(MCLL)model on the hypersonic flow of monoatomic gas around oblique wedge are investigated,in which the adjustment coefficients of the standard CLL model are determined by MD data.The differences of the flow result calculated by different models still exist mainly near the wall.Compared with the results of the MCLL model,the results of Maxwell model vary in a larger range with the adjustment coefficient,and the smaller the adjustment coefficient is,the more obvious the difference is;the CLL model is basically consistent in pressure distribution,slip velocity and wall heat flux,but the temperature near the wall is obviously lower.This shows that the MCLL model considering the local gas-solid temperature difference can more accurately describe the wall effect in hypersonic flow.更多还原