HYPER SONIC VEHICLES

A numerical study has been carried out to investigate the full flow path and aerodynamic characteristics of a hypersonic vehicle at a 7.0 free stream Mach number. Results indicate that the inlet started and unstated operations have remarkable effects on the flow pattern of the full flow path. When the inlet operates in a started mode, the transverse pressure gradient generated by the fore body alters the air captured characteristics and the entering flow quality of the inlet. Furthermore, the expansion process of the nozzle jet flow is obviously affected by the external flow field around the after body with the cross section shape transiting from a near rectangle at the exit of the nozzle to a near triangle at the tail of the vehicle. 

When the inlet operates in an unstated mode, the aerodynamic instability can be observed in the full flow path of the vehicle. Due to the oscillation of the external compressed shock wave and nozzle jet flow, the aerodynamic characteristics of the vehicle vary periodically with the lift-drag ratio changing from 0.25 to 2.09. Finally, by comparing to the experimental data, the reliability of the CFD is verified. The insufficient net thrust obtained by hyper sonic vehicles poses a formidable challenge to the design of various aerodynamic components in the propulsion system and the air frame. One promising technology that is capable of meeting the challenge is the air frame/propulsion integration. Since the 1960s, a lot of research studies have been dedicated to the integration of the propulsion system with the hypersonic airframe, including the integration of the inlet with the fore body, the integration of the nozzle with the afterbody, the optimization of the full flow path, the effects of the scram jet operating states on the aerodynamics of the airframe and so on. 

Unfortunately, there are few papers that focus on the effects of the inlet started/unstarted condition on the flow pattern of the full flow path and the aerodynamic characteristics of the vehicle. It is well known that hyper sonic inlets should always operate in a started mode in the whole flight envelope. However, because of the immaturity of the current design methods and the uncertainties of the engine thrust regulations as well as the complex flight conditions, the unstated condition of the inlet occurs inevitably during the development of hyper sonic air-breathing propulsion system. 

According to Refs.[9-12], a prominent feature of unstarted hyper sonic inlets is of an unsteady flow which can threaten the stable operation of the propulsion system and even lead to unsteady aerodynamic force and moment on the vehicle. Moreover, the unsteady aerodynamic characteristics will bring troubles to the flight control and the structure safety of the vehicle. Therefore, from both the academic and the practical point of view, it arises an urgent need for thorough investigation on the aerodynamic characteristics while the inlet is either in started or in unstated mode. Recently, the influences of the inlet started/unstated operations on the vehicle aerodynamic characteristics were experimentally studied by Ref. Some useful conclusions were drawn, but the complex phenomena still remain unclear because the pressure measurement adopted was of steady type and the FPS (frames per second) of the CCD camera was insufficient. In order to gain a deep insight into the complex phenomena, the integrated flow field in Ref. has been numerically studied in an unsteady manner in this paper. With the aid of CFD (short for computational fluid dynamics) technology, this paper acquires an intensive knowledge of the aerodynamic characteristics and the flow patterns of the full flow path while the inlet operates either in a started or in an unstarted state. Besides, the reliability of the CFD method is also verified by the experimental data in Ref.