Study On Virbartion Characteristic Of Panel Structures Of Ramjet Inlet Affected By High Speed Fluid Flows

Geometry and structure of ramjet engine are simple, formed mainly with panel structures. Exceeded vibration arises during ramjet engine operation, which affects the safety of ramjet operation significantly. To realize the root cause of vibration, the problem is studied from two aspects, one is fluid-structure interaction of panel structures, and another is forced vibration by fluid exciting forces, with nonlinear dynamics, numerical simulation and experiment analysis.First, nonlinear panel flutter equations is deduce for Kelvin viscoelastic panel structure and simplified aerodynamic theory for supersonic fluid flows, considering panel structure is mainly form of ramjet engine. Stability, flutter and hysteretic characteristics are numerically studied. The results imply the complex dynamic characteristic of panel flutter system, presenting buckling, limit cycle oscillation, quasi-periodic and chaos type characteristic. When viscoelastic damping ratio increases, the system stability region shrinks and chaos is suppressed. The frequency jumping phenomena is suppressed too.Then, from the fluid-structure fully coupling point of view, the dynamic mesh method and coupling method related to numerical simulation of fluid-structure interaction problem are studied. The dynamic mesh method based on spring simulation is enhanced with scale parameter to adapt for large deformation of structures. And, the coupling scheme based on linear or spline interpolation prediction-correction method and subcycling method is proposed to eliminate the lag error which results in divergence of weak coupling scheme. The scheme is able to stabilize numerical computation and keep low level of computing costs.In the following, the scheme is used in researches on panel flutter problem where supersonic fluid flows through passage formed with two panels, considering inner fluid flows and elastic panel structures of ramjet engine. And, height of passage and dynamic pressure coefficient are set as parameters. The results demonstrate difference between panel flutter characteristic with inner flow and outer flow. Stability boundary is smaller than that of outer flow case. With decrease of passage height, the interaction between panel vibration and supersonic fluid flows is strengthened, while flutter frequency and amplitude increase, and higher harmonic frequencies arise. The panel presents unsymmetric limit cycle oscillation and quasi-periodic vibration. With increase of dynamic pressure coefficient, the panel lost stability and flutter amplitude and frequency increase, which possess similar phenomena with outer flow case.By analyzing the propagation and reflection of shock waves and expansion waves in supersonic fluid passage, the cause of strengthened interaction is confirmed as secondary affection of disturbance arising from panel vibration. With decrease of passage height, shock waves, compression waves and expansion waves are reflected between panels, which strengthen the fluid loading on elastic panel, thus the interaction. If the height decrease lower enough, characteristic of panel flutter becomes complex because of disturbance between those waves.In addition, damping effects on the passage panel flutter are studied too. With Rayleigh damping on panel structures, flutter amplitude and frequency decrease rapidly. With increase of damping, higher harmonic frequencies vanish, and limit cycle oscillations become symmetric again. These imply the effectiveness of damping on suppressing panel flutter.Besides coupling vibration, the fluid excitation in ramjet engine is essentially complex, forced vibration and implied resonance induced by which may result in catastrophic damage of engine. Dynamic characteristic of and fluid flows in a ramjet engine inlet are studied which indicate the dense distribution of modal frequencies and closing relation between fluid flows and exiting pressure (pressure in combustor). With increase of exiting pressure, shockwaves in inlet moved upstream until being pushed out of throat under certain pressure conditions which result in unstart of inlet.Self-excited oscillation, downstream disturbance induced oscillation and coupling between them are also studied. It demonstrates the nonlinearity of oscillation, and relation between self-excited oscillation and acoustic modes. Complex nonlinear properties arise for the interaction of different kinds of oscillation mechanism. Superposition of oscillation frequencies and structural mode frequencies imply resonance of structures.Also, practical ramjet engine experiment data from cold flows and combustion condition are analyzed. The vibration on combustion is larger than that on cold flows. In cold flows experiments, ramjet engine is excited by mainly turbulent fluctuation pressures. In combustion conditions, the oil pressure fluctuation induces combustion oscillation, thus fluid flows in engine and vibration of structures. Vibration is closely related to fluid flows. Because of nonlinearity of fluid flows, vibrating frequencies of structures include the base exciting frequencies, harmonic frequencies and composition frequencies, which is similar with results of numerical simulation above.