【作者】 张云峰；

【导师】 刘占生；

【作者基本信息】 哈尔滨工业大学 ， 动力机械及工程， 2007， 博士

【摘要】 冲压发动机结构简单,主要由壁板结构构成。发动机工作时产生剧烈振动,对发动机的安全运行产生严重影响。本文针对冲压发动机壁板结构振动剧烈的问题,从流体结构耦合振动和流动激励强迫振动的角度,利用非线性动力学、数值模拟以及试验分析的手段进行了研究。首先,针对冲压发动机以壁板为主要结构形式的特点,使用简化的气动力理论,建立了弹性壁板结构在超声速气流作用下的非线性颤振方程,并考虑粘弹材料的减振特性,将Kelvin粘弹阻尼考虑到壁板颤振方程中。通过数值模拟,分析了粘弹结构在超声速气流作用下的稳定性、颤振、滞后等问题。研究表明颤振系统具有复杂的动力学行为,存在屈曲、极限环、概周期以及混沌等失稳运动形式。粘弹阻尼使得系统稳定区域减小,并且可以抑制混沌运动的出现,而阻尼耗散也抑制了颤振频率突跳现象。其次,从结构与流体完全耦合角度出发,研究了流固耦合数值模拟涉及到的动态网格方法和耦合计算方法。对基于弹簧模拟的动网格方法加以改进,通过尺度因子的加入使动网格方法更好地适应结构大变形问题。针对流固耦合数值计算弱耦合方法的稳定性问题,通过基于线性预测、样条插值预测校正方法和子循环方法使流固耦合问题的数值模拟更加稳定,同时将计算耗费保持在较低水平。然后,针对冲压发动机通道内流动以及通道为弹性壁板结构的特点,采用改进的耦合计算方法数值研究了由弹性壁板构成的通道中有超声速气流通过时壁板结构的颤振特性。以通道高度和动压系数为参数的研究结果表明,通道内流动时壁板颤振特性与外流壁板颤振特性有所不同,其稳定边界小于外流壁板颤振稳定边界。随着通道高度的降低,壁板振动与超声速气流之间的耦合加强,颤振频率与颤振幅值均有升高,出现颤振频率的高阶倍频,壁板颤振形式失去对称性,出现了极限环运动、概周期运动等形式。随着动压系数的增大,壁板从稳定向不稳定转变,颤振幅值与颤振频率均升高,其规律与外流壁板颤振特性类似。通过分析激波与膨胀波在超声速流动通道内的传播与反射,表明扰动的二次影响是导致弹性通道壁板颤振区别于外流壁板颤振的原因。随着通道高度的降低,振动的壁板表面产生激波、压缩波与膨胀波,其在通道上下壁板间的反射二次作用于颤振壁板,增强了壁板上的气动载荷,使得耦合增强。当高度降低到一定程度时,由于扰动间相互干扰的增强,导致壁板颤振行为更加复杂。另外,考虑到阻尼减振的特性,文中还对通道壁板在阻尼作用下的颤振问题进行了数值研究。研究发现瑞利阻尼作用下的壁板颤振幅值和频率远小于无阻尼壁板颤振。随着阻尼的增大,高阶倍频响应消失,极限环振动从非对称转变为对称形式。结果表明阻尼是抑制颤振的有效方法。除了可能出现的耦合振动以外,实际发动机中流动激励也非常复杂,其引发的强迫振动以及共振可造成发动机结构破坏。为了解流动激励特性,本文研究了某冲压发动机进气道模型的动力特性及其内部流动状态。结果表明进气道的结构特点使得其模态频率密集,模态振型复杂,发生共振的可能性增大。进气道中的流动与出口压力(燃烧室压力)密切相关,出口压力的增大,可能导致激波被推出喉道,引起进气道不启动。研究了进气道中流动的自激振荡现象、下游压力脉动引起的强迫振荡以及两者间的耦合。结果表明了振荡现象的非线性性质,声模态频率引起的自激振荡。强迫振荡与自激振荡的耦合引起更复杂的非线性振荡现象。而振荡频率范围与结构固有频率范围重合,会引起壁板结构共振。本文还对实际冲压发动机在冷吹与热试车试验时的振动数据和压力数据进行了分析,结果表明热试车时发动机振动水平高于冷吹振动。冷吹时发动机主要受到湍流脉动压力的激振,而热试车时喷油压力脉动引起燃烧脉动,影响发动机中的流动,使得发动机结构开始振动,结构振动与流动状态密切相关。由于流动的非线性性质,结构振动频率中包含各激励频率的基频及其组合频率。其现象与数值模拟结果相似。更多还原

【Abstract】 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.更多还原