弹性特征对跨声速抖振边界的影响研究

跨声速抖振直接影响飞机结构强度和疲劳寿命,易引发飞行事故。对于刚性静止翼型,其跨声速抖振特性仅与来流状态参数（马赫数、迎角和雷诺数）有关。该文通过释放翼型的俯仰自由度,采用非定常雷诺平均N-S方程建立了流场和结构的耦合求解方法,研究翼型结构弹性特征对跨声速抖振始发边界的影响。研究发现,结构固有频率对弹性支撑翼型的抖振边界影响较大,系统会在更小的来流迎角下失稳;而质量比对抖振边界影响较小。弹性支撑翼型的抖振始发迎角较刚性静止翼型的抖振起始迎角降低了近1°,因此翼型的弹性特征是跨声速抖振研究的重要因素。     

Microtab对翼型跨音速抖振始发特性的影响

研究了microtab对翼型跨音速抖振始发特性的影响。采用SST湍流模型对RANS方程进行封闭,以NACA0012翼型为对象,用CFD方法计算了在其后缘附近安装microtab后,翼型的跨音速流场。对比分析了在翼型后缘附近的上表面、下表面单独安装和上下表面对称安装microtab后,对翼型跨音速抖振始发特性的影响,并研究了microtab的高度和弦向安装位置对跨音速抖振边界的控制作用。结果表明：在翼型后缘附近的上表面加装microtab能够提高抖振始发攻角,并且随着microtab高度的增加,抖振始发攻角有增大的趋势。在翼型后缘附近的下表面加装microtab能够提高抖振始发时的升力系数,且抖振始发时的升力系数随着microtab高度的增加而增大。microtab的弦向安装位置的变化对于抖振始发攻角影响较小,但是对抖振始发时的升力系数有明显的影响。         

一种基于充气气囊的垂尾抖振抑制新方法研究

通过数值模拟探索了一种运用充气气囊抑制双垂尾抖振的新方法。该文方法利用充气气囊可迅速充气变形的特点,在三角翼上翼面靠近顶点沿涡核的位置设置气囊。在小迎角下气囊不凸起,从而保证机翼前缘涡的强度以产生非线性涡升力;当大迎角抖振现象较严重时,迅速对气囊充气形成凸起,该凸起通过对前缘分离涡的强度和涡空间位置的影响,减弱涡破裂对双垂尾的非定常气动载荷激励,达到抑制抖振的目的。对某三角翼双垂尾布局模型的计算结果表明：气囊可以使前缘涡的涡核弯曲、扭转,减弱了前缘涡的强度,使前缘涡破裂点位置提前,在大迎角范围可将垂尾绕翼根的弯矩值显著减小,并且减小了垂尾表面压力脉动的幅度和对应的功率谱密度的峰值。因此,该文所探索的利用充气气囊抑制抖振的方法是一种简单可靠,并且值得进一步研究的技术途径。     

NACA0012翼型抖振现象实验研究

通过在风洞侧壁安装动态压力传感器测量侧壁脉动压力的方法针对NACA0012翼型开展了零攻角状态下跨声速抖振特性的实验研究,并测量了翼型沿弦向与展向的压力分布。从时域和频域两个方面分析了脉动压力结果,得到了NACA0012翼型的跨声速抖振起始马赫数和抖振频率等抖振特性。研究结果表明：风洞二维性较好,侧壁区域的压力情况能够反映核心流压力变化情况;抖振发生在实验马赫数Ma_exp = 0.88~0.89范围内;随着马赫数的增大,抖振频率有所增加。     

基于节段模型测振试验的大跨度桥梁抖振响应预测

传统大跨度桥梁抖振响应计算是基于颤抖振理论,并借助节段模型测压或测力试验进行,目前鲜有通过节段模型测振试验实现桥梁抖振响应预测的报道。基于抖振分析理论推导了考虑三维效应的两波数抖振力,并根据综合传递函数的概念提出了基于节段模型测振试验的大跨度桥梁抖振响应预测方法。以某流线型箱梁悬索桥为例,通过节段模型及全桥气弹模型试验研究了该预测方法的精度及可行性。结果表明:结构展宽比对综合传递函数识别精度存在影响,展宽比越大,识别精度越高。即使在湍流积分尺度并未远大于结构宽度的前提下,增大模型展宽比可有效减弱三维效应的影响。基于节段模型测振试验识别综合传递函数的方法可用于大跨度桥梁抖振响应的预测,该方法预测结果偏于保守。     

考虑桥塔风场效应的斜拉桥抖振时域分析

与以往的抖振时域分析中仅模拟主梁风场不同,本文则同时模拟了主梁风场和桥塔风场,探讨了桥塔风场效应对主梁及桥塔抖振响应的影响,计算中采用了风洞中实测的紊流风速谱。空间相关性是影响桥梁抖振响应的一个重要因素,在杭州湾跨海大桥风洞试验中,对风洞空间相关性进行了测量研究,并在风场模拟时将衰减因子取为实测值。数值计算结果与风洞试验结果进行了对比,吻合较好,分析比较还表明:考虑桥塔抖振效应会显著增大桥塔的横桥向抖振响应,而对桥塔的顺桥向振动以及主梁振动影响不大。     

前缘激波脱体的大迎角翼面颤振工程计算方法

本文利用二阶线化理论对当地流活塞理论作了修正 ,利用激波脱体的临界折角确定翼型上声速线位置 ,并用插值法估计脱体激波到声速线之间局部亚音速区的当地气流参数 ,声速线后的当地气流参数仍由膨胀波公式计算 ,从而形成了适合于激波脱体流态的小展弦比大迎角翼面颤振工程估算的一整套方法。如考虑翼前缘圆角的存在 ,全部Mn >1的小展弦翼面都可用本文方法统一进行颤振分析。经Ma =1.1～ 2 .5的带迎角舵模型风洞颤振试验验证 ,表明本文方法具有一定的精度 ,能满足工程设计要求。     

基于主动格栅脉动流场的抖振力测试与相关性研究

为了探究钝体断面在紊流作用下抖振力的相关性机理,以矩形桥梁断面为例,在主动格栅产生的脉动流场中,通过测压试验并结合不同相关性模型拟合方法,研究了在不同脉动频率下抖振力的空间分布特性。对比探究矩形桥梁断面在静止状态以及运动状态抖振力空间相关性的变化,并利用matlab进一步分解自激力与抖振力,以此来分析自激力对抖振力展向相关性的影响;同时,通过天平测力试验(同步测压)对比了两种不同测力方法的试验结果。试验结果表明抖振阻力跨向相关性伴随着频率和跨向间距的增大而减小,在气动自激力的影响下,抖振力的跨向相关性能得到一定程度的提高,并高于来流脉动风速的相关性。     

斜风作用下大跨度斜拉桥双悬臂状态抖振性能

通过气弹模型风洞试验对斜风作用下湛江海湾大桥施工阶段最长双悬臂状态的抖振性能进行了研究,结果显示:对于大跨度斜拉桥梁抖振响应法向风不一定是最不利的情况;抖振响应随风偏角呈非单调变化,最大值可能在0°~15°风偏角范围内发生;抖振响应随风速的增加近似地按两次曲线增加,并且基本固有模态对抖振响应的贡献占到了主要成份;最长双悬臂状态塔顶纵向抖振响应显著地大于裸塔塔顶纵向抖振响应,而两者随风偏角的变化形态也互不相同.试验和分析结果的比较表明,只要解决好诸如气动导纳等气动参数的取值问题,可以对大跨度桥梁抖振响应作出合理的理论预测.     

考虑桥塔风效应的多塔斜拉桥抖振响应分析

该文以6塔斜拉桥——嘉绍大桥为研究对象,基于ANSYS瞬态动力学分析功能进行了嘉绍大桥风致抖振响应的非线性时域分析,研究了多塔斜拉桥主梁和桥塔在强风作用下的抖振响应特性,并详细考察了自激力和桥塔风效应对主梁和桥塔抖振响应的影响。分析结果表明：1) 主梁与桥塔的风致抖振响应与结构的振动特性联系紧密,其抖振响应由于主梁与桥塔的动力耦合作用呈现出一定的独特性;2) 考虑自激力后主梁竖向抖振响应明显减小,而对主梁横桥向和扭转抖振响应影响相对较小。同时,自激力对桥塔的横桥向抖振响应基本没有影响,但对桥塔的顺桥向抖振响应起到了明显的抑制作用;3) 桥塔风效应对主梁的竖向和扭转抖振响应以及桥塔的顺桥向抖振响应基本没有影响,但会对主梁和桥塔的横桥向抖振响应产生较大影响。     

飞机垂尾抖振极限载荷预测方法

为满足先进战斗机高速、大机动飞行性能的要求,在飞机设计的强度估算与校核中必须考虑抖振载荷的影响,以保证飞机结构在全包线飞行状态中具有足够的强度。按照飞机飞行状态进行抖振飞行子状态划分建立起抖振载荷数据库的数据存储关系,提出根据有限测点处测试的抖振响应重构飞机垂尾结构抖振载荷的方法,并基于所选抖振飞行子状态载荷概率分布模型预测出抖振极限载荷。最后以某飞机垂尾上某节点的抖振极限载荷预测为例,演示用本文方法预测飞机垂尾抖振极限载荷流程。     

Prediction of airblast loads on structures behind a protective barrier

Experimental results and numerical simulations have demonstrated that a protective barrier can effectively reduce blast load and, therefore, protect structures from an external explosion. However, there are no formulae in the open literature that can be used to estimate the blast loads on a structure behind a barrier. In this paper, pseudo-analytical formulae based on numerical results are derived to estimate the reflected pressure-time history on a rigid wall behind a protective barrier. Numerical simulations of blast wave propagation are carried out to estimate the peak reflected pressure and the impulse on a rigid wall behind a blast barrier. The shock wave front arrival time and positive phase duration are extracted from the numerical results. Pseudo-analytical formulae, which are derived from the best-fitted curves of the numerical results, are suggested. These formulae can be used with those given in TM5-1300 or other methods for blast pressure estimation in the no-barrier case, to estimate pressure-time histories at various building locations behind a protective barrier.     

Hybrid S2/Carbon Epoxy Composite Armours Under Blast Loads

Civil and military structures, such as helicopters, aircrafts, naval ships, tanks or buildings are susceptible to blast loads as terroristic attacks increases, therefore there is the need to design blast resistant structures. During an explosion the peak pressure produced by shock wave is much greater than the static collapse pressure. Metallic structures usually undergo large plastic deformations absorbing blast energy before reaching equilibrium. Due to their high specific properties, fibre-reinforced polymers are being considered for energy absorption applications in blast resistant armours. A deep insight into the relationship between explosion loads, composite architecture and deformation/fracture behaviour will offer the possibility to design structures with significantly enhanced energy absorption and blast resistance performance. This study presents the results of a numerical investigation aimed at understanding the performance of a hybrid composite (glass/carbon fibre) plate subjected to blast loads using commercial LS-DYNA software. In particular, the paper deals with numerical 3D simulations of damages caused by air blast waves generated by C4 charges on two fully clamped rectangular plates made of steel and hybrid (S2/Carbon) composite, respectively. A Multi Materials Arbitrary Lagrangian Eulerian (MMALE) formulation was used to simulate the shock phenomenon. For the steel plates, the Johnson-Cook material model was employed. For the composite plates both in-plane and out-of-plane failure criteria were employed. In particular, a contact tiebreak formulation with a mixed mode failure criteria was employed to simulate delamination failure. As for the steel plates the results showed that excellent correlation with the experimental data for the two blast load conditions in terms of dynamic and residual deflection for two different C4 charges. For the composite plates the numerical results showed that, as expected, a wider delamination damage was observed for the higher blast loads case. Widespread tensile matrix damage was experienced for both blast load cases, while only for 875?g blast load fiber failure damage was observed. This agrees well with the experimental data showing that the composite panel was not able to resist to the 875?g blast load.     

结合面切向接触刚度的概率统计模型

该文考虑结合面上微凸体接触为椭圆形接触, 运用弹性力学空间半无限体受载荷变形理论, 并根据椭圆形接触面上压力分布的Hertz理论, 推导出椭圆形接触面的长、短半轴计算式和单个微凸体接触时的切向接触刚度表达式. 建立了结合面上微凸体椭圆形接触面的长、短半轴呈二维正态分布、高度呈正态分布情况下的宏观切向接触刚度模型, 得到了相应的宏观切向接触刚度表达式. 通过数值计算给出了切向接触刚度随结合面的法向载荷、切向载荷、椭圆微凸体的离心率、微凸体分布的相关系数、微凸体测量高度和微凸体分布的标准方差等各影响因素的变化情况, 增大法向载荷可以提高结合面的切向接触刚度, 而切向载荷的增大会导致切向接触刚度的 减小.     

Transient wave propagation and early short time transient responses of laminated composite cylindrical shells

The generalized ray method (GRM) and the method of reverberation ray matrix (MRRM) have been successfully used to study the transient elastic wave transmission in the beams, planar trusses, space frames and infinite layered media. In this paper, the GRM and MRRM are extended to investigate the early short time transient responses of laminated composite cylindrical shells with finite size under impact load. Using the Laplace transformation, the ray groups transmitting in the laminated cylindrical shells under the shock load are yielded by means of the boundary conditions. The reverberation-ray matrix representing the multi-reflected and scattered waves in the laminated cylindrical shells with finite structural size is deduced. Using FFT algorithm, the transient response corresponding to each ray group can be derived. Through the numerical simulations, it is seen that the early short time transient accelerations of the laminated composite cylindrical shells under impact loads are very large, but the early short time transient shear strain and displacement are very small. Furthermore, the effects of the stacking sequence, thickness of the composite cylindrical shells and different shock loads on the early short time transient responses of the laminated composite cylindrical shells with finite size are also analyzed.     

An inverse approach for pressure load identification

An inverse approach for the identification of pressure loading on a structure has been proposed and developed. In this approach, surface measurements of structural response (e.g. strain, displacement and velocity field measurements, such as can be measured with 3D digital image correlation) are utilized as input data and are combined with numerical simulations to identify the pressure load on a structure. The inverse approach has been verified by numerical benchmarks involving pressure identification under quasi-static as well as dynamic impulse loading conditions, and also been validated by an experiment involving a quasi-static pressure load. The results indicate that the proposed inverse method can identify not only the magnitude of the quasi-static pressure but also the impulsive pressure loading history. The developed inverse approach offers an opportunity to apply inverse analysis techniques to identify interactive pressure loads (such as those resulting from a blast wave) on structures in explosive events.     

On boundary-layer transition in transonic flow

Boundary-layer transition in transonic external flow is addressed theoretically. The transonic area is rich in different flow structures, and transition paths, and the work has wide potential application in transonic aerodynamics, including special reference to the example of flow transition over an engine nacelle. The investigation is intended partly to aid, compare with, and detect any limitations of, a quasi-parallel empirical methodology for design use in the area, especially with respect to the transonic range, and partly to develop an understanding and possible control of the nonlinear natural or by-pass properties of the compressible transition present. The mechanisms behind three major factors, (a) substantial external-flow deceleration, (b) rapid boundary-layer thickening, (c) three-dimensional nonlinear interactions, are identified; these three are involved in the specific application above and in more general configurations, depending on the disturbance background present. It is found also that some similarities exist with the phenomenon of buffeting on transonic airfoils, and the relevant physics and governing equations throughout are identified. Sensitive nonlinear effects are important in all the factors (a)-(c), especially a resonance linkage between shock buffeting and boundary-layer thickening, and nonlinearly enhanced three-dimensional growth triggered by slight three-dimensional warping for instance, peculiar to the transonic range. The latter enhanced growth is perhaps the most significant finding. The implications, in the general setting as well as for the nacelle-flow context in particular, are also presented.