激波串是实现超声速内流减速增压的一种流动结构，是一系列激波/边界层干扰耦合作用的复杂现象，存在于超燃冲压发动机隔离段、超声速扩压器等部件内。激波串结构形态关于流道中心线不对称是激波串最典型的特征之一，激波串在管道内运动过程还存在自激振荡、位置突跳、迟滞等行为，激波串非对称现象与运动特性耦合会对发动机的后续燃烧特性造成严重影响，因此激波串结构非对称特性与运动特性是影响超燃冲压发动机性能的重要基础问题。

本文利用直连式试验平台和自由射流式试验平台以及数值仿真方法，对等直矩形隔离段内斜激波串非对称性及激波串运动过程中的迟滞特性进行了研究。利用简化建模、激波熵产生分析以及量级分析的方法，揭示了斜激波串非对称机制。结合高速纹影摄像和动态压力分析，获得了运动激波串与隔离段内斜楔发生干扰的详细过程，并深入分析了典型流态转变过程中的迟滞现象和规律。

首先，利用实验和数值仿真研究了均匀来流条件下的斜激波串形态特征。结果表明，马赫数3.0工况斜激波串在自激振荡状态下始终处于非对称形态，振荡状态下激波串头激波（最上游两道激波）起始点位置和激波角小幅波动变化，而激波串的分离区振荡与下游前传的强压力扰动耦合作用会引发主分离区位置切换。进一步对斜激波串主分离区特性的研究表明，激波串非对称形态主要由头激波的非对称性决定。

进一步对头激波波系建立了简化的准二维模型，应用最小熵产生原理分析了准定常条件下的简化头激波，结果表明非对称头激波比对称形态的头激波熵产更小、更利于激波串的稳定，斜激波串结构因此变得不对称。斜激波串非对称程度除了受最小熵产生原理决定外，还受到激波/边界层干扰的分离区最小尺度限制，两者共同决定了非对称度系数。结合对边界层方程的量级分析，通过数据拟合发现斜激波串非对称度系数与激波串上游马赫数平方与壁面摩擦系数乘积成正比，马赫数越高非对称性越强。试验及数值结果证实了模型分析的合理性。

针对下游反压变化条件的激波串运动特性，本文系统试验研究了激波串受到上游斜楔流动干扰时的流动特性。结果表明，斜楔对隔离段上游流动的干扰会诱发反压上升及下降过程中的激波串位置迟滞，证实了前期数值研究有关激波串位置迟滞的发现。在激波串后退过程中，受背景激波和下游反压的共同作用，斜楔后缘大尺度回流区没有消退，受此影响而发生激波串位置迟滞。同时伴随着出现流场震荡特性的路径依赖性，激波串后退过程中激波串振荡减弱。本文进一步试验考察了迟滞效应受斜楔高度和宽度的影响，发现迟滞效应随斜楔高度降低而减弱，两侧或中间开口斜楔在一定宽度范围内的迟滞效应比与全宽斜楔稍强，但当斜楔宽度小于0.3倍隔离段宽度后迟滞会突然消失。

着眼于反压升高及降低过程中激波串迟滞所表现的大尺度分离区的路径依赖性问题，进一步解耦背景激波和下游反压与斜楔后台阶回流区流动的相互作用，分别对入射激波/后台阶流动干扰、变几何凹腔流动两种基本干扰流场进行了实验研究。结果表明，两种干扰流场形态均发生了同类型的迟滞，下游干扰以逆压梯度的方式干扰后台阶回流区剪切层的再附过程，导致回流区内部压力升高、维持了大尺度特性，并由此引发了分离区汇合与分离的迟滞，斜楔干扰诱发的激波串位置迟滞机理与此相同。

本文对矩形隔离段内的斜激波串非对称特性以及运动过程中的迟滞特性开展了比较系统的研究，包括主分离区切换特性、准定常非对称的斜激波串热力学行为、斜楔干扰运动激波串的迟滞特性，这些研究结果为进一步揭示激波串流动机理、为发动机控制设计与优化提供有益参考。

A shock train is a complex system of shock waves and compression waves that decelerates a supersonic flow in a duct. It is essentially a system of coupled shock wave /boundary-layer interactions. It exists in the isolator of a scramjet engine, supersonic diffuser and so on. The asymmetry is a typical feature of the shock train pattern. When the asymmetry occurs, the core flow is inclined to one side and a dominant separation zone(DS) will appears on one other side. When the back pressure changes, the shock train will move in the duct. The jumps and hysteresis in the position of the shock train will occur during the moving process, which may couple with the asymmetry and self-excited oscillations of shock train. These may change the performance of the combustor. It can be seen that the asymmetry and motion properties of the shock train are basic issues affecting the performance of the scramjet engine.

In this paper, the oblique shock train asymmetry and the hysteresis of the shock train during the motion of the shock train are studied by using the direct-connect facility, the hypersonic propulsion test facility, and numerical simulation. Using a simplified model of head shocks, thermodynamic analysis and order analysis, the features of asymmetric shock train patterns are discussed. Combining high-speed schlieren imaging and dynamic pressure measurement, the detailed process of interactions between moving shock train and a ramp is exhibited, and the hysteresis phenomenon during the transition of different flow regimes are analyzed in detail.

Firstly, the morphological characteristics of asymmetric oblique shock train and switching characteristics of the DS were studied by using a direct-connect facility with inflow Mach number of 3.0. The results show that the oblique shock train is always in an asymmetrical shape even with the self-excited oscillation at inflow Mach number 3.0 conditions. Due to the influence of self-excitation, the differences of starting position and shock angles of two head shocks change slightly. The coupling effect of self-oscillation of the shock train and the downstream strong pressure disturbance will cause the dominant separation zone to switch. Further research on the characteristics of the main separation zone of the oblique shock train shows that the asymmetry of the shock train is mainly determined by the asymmetry of the head shocks.

Further, a simplified quasi-two-dimensional model is established to describe the head shocks of oblique shock train. An analysis of entropy production is performed for the simplified model under quasi-steady conditions. The results show that asymmetrical head shocks have a smaller entropy production than symmetric head shocks and are more conducive to the stability of the shock train. Therefore, the oblique shock train becomes asymmetric. The degree of asymmetry of the oblique shock train is determined not only by the principle of minimum entropy production but also by the minimum size limitation of the separation bubble associated with shock-wave/boundary-layer interactions. By performing order analysis of the boundary layer equation, it is found that the asymmetry intensity of the oblique shock train is proportional to the product of the square of the Mach number and the wall friction coefficient. It is consistent with the numerical results.

Secondly, the effects of a ramp on the motion characteristics of the shock train are experimentally studied. The results show that the full ramp can induce flow hysteresis in the position of shock train during the moving process, which confirms our previous numerical findings on the hysteresis in the shock train position. When the shock train moves downstream, the large-scale recirculation zone downstream the ramp did not disappear due to the combined effects of the background shock and the back pressure. As a result, the hysteresis in shock train position occurs. At the same time, oscillation characteristics of the shock train show the path dependence. Further studies show that the hysteresis range is significantly affected by the height and width of the ramp. Decreases the ramp height the hysteresis effects are decreased. Decreasing the width of the ramp from both sides or the middle opening will increase the flow hysteresis range. Nevertheless, the hysteresis will disappear suddenly after the width of the ramp is less than 0.3 times the width of the isolator.

Focusing on the path dependence of the large-scale separation zone exhibited in the ramp induced hysteresis in shock train position, the effects of the incident shock and back pressure on the hysteresis is further decoupled. Two baseline experiments, incident shock/backward-facing step flow interactions and variable geometry cavity flow, are conducted to study the mechanism of shock train hysteresis. The results show that the same kind of hysteresis occurs in large-scale separation zone patterns of the two baseline experiments. It is found that the incident shock and back pressure act as reverse pressure gradient to exert influence on the reattachment of the backward-facing step flow. This results in a high pressure value in the recirculation zone and maintain a large-scale recirculation zone, and causes the hysteresis phenomenon. This mechanism is the same as the hysteresis in shock train position induced by ramp.

In this paper, the asymmetry characteristics of oblique shock train and the hysteresis phenomenon during the moving process of the shock train are systematically studied. The switching characteristics of the dominant separation zone in the unsteady state and the thermodynamic behavior of asymmetric oblique shock train in quasi-steady state are exhibited. The interactions between the shock train and a ramp are investigated in detail. The results will provide a useful reference for understanding the shock train flow and will be beneficial for the design and optimization of the scramjet engine.