临近空间飞行器在中高层大气高速飞行时，传统气动舵面控制能力因环境大 气趋于稀薄而减弱。工程上，普遍采用反作用力控制系统(RCS)操控这类飞行 器。RCS 通过姿控发动机喷射的喷流与稀薄来流发生相互作用，影响 RCS 的操 控能力。连续喷流与稀薄来流之间的相互作用所形成的跨尺度流场求解是 RCS 面临的重要问题。高超声速条件下的跨尺度流场可以通过耦合基于连续介质流动 假设的计算流体力学(CFD)方法和直接模拟分子运动的直接模拟蒙特卡罗 (DSMC)方法来高效且准确地完成。耦合算法的一个要点是需要使用合理的判 据对流场做区域划分，对满足连续性假设的区域采取求解 Navier-Stokes 方程的 CFD 方法，对不满足连续性假设的区域使用 DSMC 方法计算，在保证计算结果 准确的前提下提高计算效率。因此，对连续喷流和稀薄来流形成的跨尺度流场进 行连续性假设失效研究具有重要意义。

       本文的主要工作内容如下:

       (1)根据连续性假设只在气体分子速度分布函数相对平衡态的 Maxwell 分 布偏差很小的条件下才成立的理论依据，本文从速度分布函数的各阶矩出发，推 导获得了衡量连续性假设失效程度的两个非平衡度参数:基于二阶矩的方差非平 衡度和基于三阶矩的斜偏非平衡度。

       (2)根据方差非平衡度和斜偏非平衡度，深入分析常用连续性假设失效的 宏观判据的适用性。本文揭示 Bird 的 P 参数、Boyd 的当地 Kn 数、Garcia 的 B 参数不能应用于来流与喷流作用流场分析的原因，并结合 Tiwari 的 Φ 参数和 Chen 的无量纲熵产率判据的优点，构造了连续性假设失效的非平衡度判据，其中的非 平衡度截断数值与应用精度的需求关联。

       (3)采用 DSMC 方法，对二维稀薄来流与连续喷流所形成的定常流场进行 数值模拟，根据流动特征对整个流场进行了划分，进一步采用非平衡度判据对流 动的非平衡程度进行了分析讨论，同时比较了非平衡度在几个剖面的定量变化。 在此基础上，比较分析来流飞行高度、来流马赫数对来流与喷流相互作用流场的 影响，包括流场结构的变化和非平衡度的分布。

        (4)进一步计算稀薄来流与双喷流作用的流场。通过改变双喷口间距、双 喷流分子数密度，观察流场的非平衡特征，深入分析双喷流间的相互作用和前后 喷流与来流的相互干扰。

When near space vehicles fly at high speed in the middle and upper atmosphere, the control ability of traditional control surface is weakened due to the thinning of ambient atmosphere. Reaction control system (RCS) is widely used in flight control, but the interaction between the jet produced by the attitude control engine and the rarefied flow affects the control accuracy of RCS. The solution of cross-scale flow field formed by the interaction between continuous jet and rarefied flow is an important issue for RCS. The hypersonic cross-scale flow field can be efficiently and accurately solved by coupling the computational fluid dynamics (CFD) method based on the continuum assumption and the direct simulation Monte Carlo (DSMC) method for direct simulation of molecular motion. One of the key points of coupling algorithm is using a reasonable criterion to divide the flow field into various regions. The CFD method is used to solve the Navier-Stokes equations for the regions that satisfy the continuum assumption, and the DSMC method is used to calculate the regions that do not satisfy continuum assumption, so as to improve the calculation efficiency on the premise of ensuring the accuracy of calculation results. Therefore, it is of great significance to study the failure of continuum assumption in the cross-scale flow field formed by continuous jet and rarefied flow.

The main contents of this paper are as follows:

(1) According to the theoretical basis that the continuum assumption can only be applied if the deviation of the molecular thermal velocity distribution function from Maxwell distribution of the equilibrium state is small. This paper derives two non-equilibrium parameters to measure the failure degree of the continuum assumption: the variance non-equilibrium degree based on the second moment, the skew non-equilibrium degree based on the third moment.

(2) According to variance non-equilibrium degree and skew non-equilibrium degree, the applicability of the macroscopic criteria for continuum breakdown is analyzed. The reason why Bird's P parameter, Boyd's local Knudsen number and Garcia's B parameter cannot be used to analyze the flow field of the interaction between incoming flow and jet is revealed. Based on the advantages of Chen's non-dimensional entropy production rate and Tiwari’s Φ parameter, a criterion measures continuum breakdown is constructed, and its cutoff value is related to the requirement of application accuracy.

(3) The steady flow field formed by two-dimensional rarefied flow and continuous jet flow is simulated by DSMC method. The whole flow field is divided according to the flow characteristics. The non-equilibrium degree of flow is analyzed and discussed based on the non-equilibrium criterion. At the same time, the quantitative changes of the non-equilibrium degree in several sections are compared. On this basis, the influences of the flight height and Mach number on the interaction 

flow field between rarefied flow and continuous jet flow are analyzed, including the change of the flow structure and the distribution of the non-equilibrium degree.

(4) Furthermore, the flow fields formed by rarefied flow and double jets are calculated. The non-equilibrium characteristics of the flow field are observed by changing the distance between two jets and the molecular number density of two jets. The interaction between two jets, the interaction between jets and inflow are analyzed deeply.