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Investigations of the Wave Structures of New Concept RAM Propulsion Systems and Their MHD Control

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Tutor: ChenZhiHua
School: Nanjing University of Technology and Engineering
Course: Mechanics
Keywords: Scramjet,Ram accelerator,Magnetohydrodynamics,Lorentz force,Flowcontrol,Shock wa
CLC: O381
Type: PhD thesis
Year:  2013
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For the new types of ram propulsion systems, such as scramjet, oblique detonation wave engine and ram accelerator etc., their flow fields are very complex and have high transient features, the investigations of such fields are still inadequate to satisfy the practical requirement, a series of problems need to be solved, such as to expand their operation ranges to a wide range of Mach numbers and the infulences of high intensity shock waves influences etc. On the other hand, the strong shock waves or detonation waves of such propulsion systems can lead to a significant ionization of the air flow, which allows us to use the MHD (Magnetohydrodyamics) for the control and optimization of the flowfield. In this dissertation, based on the Euler equations with the chemical and MHD source terms, with the employ of the hybrid Roe/HLL scheme, Adaptive Mesh Refinement (AMR) Cartesian grid system and Immersed Boundary Method (IBM) to investigate the wave and flow structures of the new types of ram propulsion systems and their MHD control.Firstly, through the simulation of the detonation initiation accelerated by the collision of the diffraction shock waves, the numerical methods were verified, and our results revealed the mechanism that the wake shock waves diffract behind the square obstacles and collide to accelerate the induction of the detonation waves.The process of the MHD control of the double wedged inlet flowfield of a typical scramjet was also simulated. The numerical results show that, with the application of Lorentz force at the forebody of the first ramp, the lcoations of two oblique shock wave fronts at the inlet can be controlled, and the directions of the Lorentz force have significant effect on the MHD control process, moreover, the two oblique shock waves at off-design conditions can be restored to the desired location with proper magnitude and direction of Lorentz force, but the flowfield structures are slightly different from the desired case.The MHD control of the oblique detonation wave (ODW) induced by both single and double wedges has been investigated, the numerical results showed that MHD control can also return the location of a stable ODW front to its designed location. However, with the MHD control located on the surface of the first ramp, the second shock wave that generated at the corner of the double wedge could not be controlled. When the angle of the second ramp is large enough, the stable ODW becomes unstable and it cannot be manipulated with the MHD control. It is hard for the MHD control returning the unstable ODW front, but the front turns to be stable with proper MHD control. With the high order WENO scheme, we simulated the non-reactive flow field of a typical ram accelerator. The wake flow, including the shock wave structures, have been shown clearly, which agree well with corresponding experimental results. It is validated that WENO scheme is suitable for capturing the hypersonic flowfield and shock waves inside the ram accelerator. The generation and development of shock waves and relative parameters around the projectile have been presented, which can be used as a guide and reference for the cold launch experiments of ram accelerator.For the thermally choked ram accelerators, the influences of the gas reaction rate, the velocity and the projectile shapes on the states and performances of ram accelerator have been investigated numerically. It has been shown that the thermally choked mode occurs only when the variation of the reaction rate and velocity within certain limit, and a proper projectile shape can extend this limit. Furthermore, in a thermally choked mode, the flame front can be stable just behind the shoulder of a boat-shaped projectile, which generates the largest stable thrust. While the thrust generated with the flame stable at the projectile base vibrates due to the vortex shedding.Lastly, we investigated the flow field of the superdetonative mode of the ram accelerators, we found that with the increase of the velocity, there would be ODWs appeared at the shoulder and the head of the projectile, respectively, which can accelerate the projectile. For a typical superdetonative mode, the higher the reaction rate is, the larger its thrust becomes. The Lorentz force can extend the velocity area of the typical superdetonative mode from the cases that the velocity is too low or too high. As the velocity is too low, the Lorents force accelerates the flow of the projectile head, which makes the ODW staleb at the rear shoulder, however, for a certain incoming velocity, without the action of Lorentz force, the drag will be generated due to the appearance of vortexes, however, with the use of Lorentz force the stable ODW can be induced at the front wedge of projectile, and results to the generation of both thrust and stable flow.
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