The determination of physical dimensions of a hypersonic three-stage compression system considering panel vibration effects
Introduction
The performance of an air-breathing hypersonic vehicle is closely related with the performance of its intake system [1], [2]. The intake system contains a crucial subsystem, i.e., compression subsystem, which is used to compress the free flow and create appropriate air inlet conditions for the combustor. A compression system, take the multi-stage planar one for instance, is formed by several compression ramps, which commonly adopt thin-panel structures in consideration of the low-weight requirement. However, the thin-panel structure is liable to be excitated by various vibration sources such as hypersonic flow (fluid-structural interaction), engine (thrust variation) and noise, and the induced panel vibration has significant effect on the performance of air intake system. Thus, for the design of a compression system, it is essential to obtain vibration characteristics of compression ramps and clarify their effects on the intake performance.
According to the geometric configurations, the intake system can be classified into axisymmetric, two-dimensional planar and three-dimensional ones [3]. The axisymmetric one adopts axisymmetric compression surfaces and is well adaptive to axisymmetric scramjets and vehicles, like HRE engine of NASA [4], GTX engine of NASA [5], SABRE engine of Reaction Engines Ltd. [6], and so on. The two-dimensional one always adopts planar compression surfaces consist of several ramps and can be found in various well-known lifting body vehicles like X-43A [7] and X-51A [8], [9] of NASA. One of the advantages of two-dimensional intake system is its well adaptability to the integration design of intake and fuselage. The three-dimensional compression intake system is the one with additional lateral compression surfaces like the Strutjet engine of GenCorp Aerojet [10], and it can also be found as inward turning type like Rectangular-to-Elliptical-Shape-Transition intake of NASA [11]. The additional compression and turning processes in such three-dimensional intake system contribute to the improvement of compression efficiency.
In this work, an air-breathing hypersonic vehicle with non-axisymmetric lifting body configuration is involved, and a rocket based combined cycle with three-stage planar compression intake system is adopted. The performance of intake system is always evaluated by the total pressure recovery and mass flow coefficient at the outlet. Such type of compression system contains external and internal compression sections, and the external one is always composed by several ramps and has more configurational parameters to be determined. Configurational parameters like the shock angles, compression angles and the length projections of compression ramps are always designed based on the so-called “shock-on-lip” [12], [13] and Oswatitsch [14] criterions. The “shock-on-lip” criterion means all the shock waves are converged to the cowl lip with no spillage and indicates the maximum mass flow coefficient (should be 100%), while the Oswatitsch criterion indicates the maximum total pressure recovery [15]. However, in real conditions, the viscous effect will lead to the reduction of intake performance, thus viscosity corrections by corresponding CFD simulations are essential subsequent steps. A great number of researches about complex fluid-structural phenomena, e.g., turbulent boundary layers [16], [17], aeroelastic effects [18], [19], dynamic aeroelastic effects [20], [21], aerothermoelastic effects [22], [23], shock-wave-boundary-layer interactions [24], [25], [26], [27] and involved thermochemical [28] and high temperature non-equilibrium effects [29], [30] also contribute to the cognition of viscous effects and the effective design of intake system.
For the intake system of air-breathing hypersonic vehicles, the widely adopted thin panel structure is liable to be excitated by various vibration sources such as hypersonic flow, engine and noise. Take the hypersonic flow for instance, the structural vibration that induced by hypersonic flows is always considered as the fluid-structural or aeroelastic interaction between hypersonic flow and intake panels. As discussed above, the static aeroelastic [18], [19], dynamic aeroelastic [20], [21] and aerothermoelastic [22], [23] phenomena of hypersonic intake system are widely studied by the time-consuming and sometimes unaffordable CFD analysis. The shape deformation and vibration of intake panels under such extreme conditions has high uncertainty and non-negligible influence on the engine performance, and the unpredicted deformation might cause serious consequences like the unstart of intake, chocking of combustor or failure of structures [18], [31], [32]. Thus, the structural design of intake system should consider objectives and constraints in two perspectives, i.e., the structural and intake performance. For the structural performance, the static and vibration mechanic analyses under certain hypersonic conditions need to be carried out. For the intake performance, the flow fields and the intake performance parameters, i.e., the total pressure recovery and mass flow coefficients, with panel deformations and vibrations should be simulated and obtained. A clear structural design path of external compression and intake system considering the various vibration sources, high uncertainty and complex design objectives and constraints is essential while still absent at present.
The ideal panels should have enough stiffness to reduce the effects of vibrations and the stiffness reinforcing design can be realized by increasing panel thickness, strengthening ribs and so on. The structural design path, take thickness for instance, should have three branch paths, i.e., avoiding too high stress of structure, averting resonance of structure and maintaining intake performance. As discussed above, there are several vibration sources like the fluid-structural interaction, engine thrust variation and so on for a hypersonic vehicle, from the structural point of view the available thickness of compression ramp panels should be determined by considerations of all the vibration sources, which is actually difficult to realize. Thus, in this work, the mode frequency under real constraint conditions and deformations under aerodynamic force of panels with different thicknesses are obtained first by mode and static mechanic analysis, respectively; then, the effects of forced panel vibrations, i.e., vibration frequency and amplitude on the intake performance are clarified; finally, the available panel thickness is determined by fluid-structural analysis from the structural and flow point of views and a design path of compression system is developed by considering both the structural and intake performance.
Section snippets
The hypersonic vehicle and multi-stage compression system
Fig. 1 shows the hypersonic vehicle and its intake system. Fig. 1(a) shows the configuration and the trajectory of the vehicle. The vehicle has length of about 30 m, width of about 5 m, and wingspan of about 15 m. The flight altitude is between 0 to 60 km and the Mach number is between 0 to 8. In this work, the air-breathing combined engine is adopted and the engine performance is closely dependent on the air intake system. The intake system is composed of compression section and isolator
Numerical models
In this work, three types of numerical simulations should be conducted: the mode analysis of compression ramps, the flow field analysis with vibrational structural boundaries, and the fluid-structural interaction analysis. The numerical models for each simulation are introduced below.
Mode analysis
As discussed above, the mode of compression ramp panel is analyzed under two types of constraint conditions, CoA and CoB. Fig. 7, Fig. 8 show the mode shape (normalized displacement pattern) of the six lowest order mode under CoA and CoB for CI with 5 mm panel thickness, respectively. The values in Fig. 7, Fig. 8 are the normalized displacements and no real physical meaning is involved. From the figures one can find that CoA and CoB have very similar mode shape, the vibration area for CoA is
Conclusions
For an air-breathing hypersonic vehicle, the thin-panel structure is always adopted by the compression ramp of air intake system, the panel vibration that excitated by hypersonic flow, engine and noise has decisive effect on its performance. In this paper, a three-stage compression intake system is designed under Mach number of 6 and flight altitude of 24 km. The mode vibration characteristics of compression ramps' panels are obtained and the influence of panel vibration frequency and amplitude
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study is supported by the National Natural Science Foundation of China (51806175).
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