In this paper, the mode transition process of an over-under exhaust system for a turbine based combined cycle (TBCC) is investigated experimentally. The method of characteristics (MOC) and truncation method are employed to design the single expansion ramp nozzle (SERN) at the cruise condition, and the final configuration of the exhaust system is determined by resolving the turbine nozzle performance. The crank-link mechanisms are applied to convert the rotation modes of the splitter and cowl to the linear motion modes in the experimental model. The flowfield structure and pressure distribution along the symmetric plane are measured in the experiment. Furthermore, the computational fluid dynamics (CFD) approach is also adopted to simulate the flowfield of the exhaust system for comparison. The results denote that although the flowfield is greatly complicated as the interaction of the exhaust system plume with the external freestream, the internal flowfield of the exhaust system is independent of the external flow. The flowfield of the entire exhaust system is occupied by the turbine nozzle flow at the initial condition, while it is dominated by the ramjet nozzle flow at the end of the mode transition. At the afterburner and throttle stages of the turbine, the shear layer deflects toward the turbine nozzle flowfield, and the influence of the ramjet nozzle jet on the turbine nozzle flowfield is increased. The internal flow of the turbine nozzle is not influenced by the ramjet nozzle plume at the afterburner stage, while it is greatly affected by the ramjet nozzle plume at the throttle stage. Instead, the shear layer deflects toward the ramjet nozzle flowfield at the transitional stage, and the effect of the turbine nozzle flow on the ramjet nozzle flowfield is increased. Furthermore, the calculated flowfield structure and pressure distributions agree well with the experimental results, and the numerical method can be capable of capturing the flowfield feature of the exhaust system.

在本文中，对基于涡轮的联合循环（TBCC）的上下排气系统的模式转换过程进行了实验研究。采用特性方法（MOC）和截断方法来设计巡航状态下的单个膨胀坡道喷嘴（SERN），并通过解决涡轮喷嘴的性能来确定排气系统的最终配置。应用曲柄连杆机构将分离器和整流罩的旋转模式转换为实验模型中的线性运动模式。实验中测量了沿对称平面的流场结构和压力分布。此外，还采用计算流体动力学（CFD）方法来模拟排气系统的流场以进行比较。结果表明，尽管流场非常复杂，因为排气系统羽流与外部自由流之间的相互作用，但排气系统的内部流场与外部流无关。整个排气系统的流场在初始状态下由涡轮喷嘴流占据，而在模式转换结束时由冲压喷嘴流占主导。在涡轮的加力燃烧器和节气门阶段，剪切层向涡轮喷嘴流场偏转，并且冲压喷嘴喷嘴射流对涡轮喷嘴流场的影响增加。涡轮喷嘴的内部流量在加力阶段不受冲压喷嘴喷嘴羽流的影响，而在节流阶段受冲压喷嘴喷嘴羽流的影响很大。代替，剪切层在过渡阶段向冲压喷嘴流场偏转，并且涡轮喷嘴流对冲压喷嘴流场的影响增加。此外，计算得到的流场结构和压力分布与实验结果吻合得很好，数值方法能够捕获排气系统的流场特征。