大展弦比机翼非线性颤振剪裁设计新方法

针对大展弦比机翼水平弯曲模态参与耦合颤振问题,首先用考虑几何非线性的颤振分析方法研究了某大展弦比机翼的颤振特性,结果表明水平一弯模态参与耦合降低了机翼传统模式的线性颤振速度;然后研究了复合材料的铺层主刚度方向角对机翼非线性振动特性和颤振特性的影响规律,提出了大展弦比机翼非线性颤振剪裁设计的新方法.结果表明主刚度方向角的变化主要引起了水平一弯模态振型的改变,一般表现为主刚度方向角从机翼后梁向后缘偏转,该阶模态的相对扭转振型节线位置向前缘移动;反之,该节线位置后移.进一步非线性颤振分析,发现水平一弯模态振型的变化引起了该阶模态参与耦合颤振速度的明显改变,主要表现为该颤振型的颤振速度随该阶模态的相对扭转振型节线位置前移量的增加而增大.通过两个算例验证了结论的正确性.     

改进的HHT及其在颤振试验数据处理中的应用

颤振试验数据往往具有信噪比低、模态密集及非平稳现象严重等特点,对数据处理精度提出较高的要求.为精确提取颤振模态参数,对颤振边界进行准确预测,提出了一种改进的希尔伯特黄变换算法,算法首先对测试信号进行移频处理,降低模态混叠程度,然后进行EMD分解,并对分解结果进行希尔伯特变换得到相应的瞬时幅值和瞬时相位,最后通过曲线拟合识别颤振模态参数.通过理论分析和数值仿真完成相应的特性研究,并采用实测颤振试验数据进行了分析验证,结果表明,改进的希尔伯特黄变换方法可以较精确地提取颤振模态参数和预测颤振临界速度,从而满足现行颤振试验数据处理的要求.     

一般阻尼动力系统非奇异摄动法

借助矩阵摄动理论,将模态叠加法运用于一般阻尼矩阵的动力学方程求解结构的动响应是一种较为理想的方法.但当系统的外荷载激振频率接近于系统的固有频率时,直接将阻尼矩阵作为摄动矩阵,会使解产生奇异,并导致求解失败或误差过大,这是因为模态坐标下的动力学方程是无阻尼方程.为了解决这一问题,本文考虑在模态坐标的动力学方程中保留一定的阻尼.即将阻尼做分解,代入振动方程,得到不同阶次摄动方程,再将摄动方程变换到模态坐标,即采用非奇异摄动方法.最后通过数值算例,得到一阶、二阶摄动,将其与精确解进行比较.精度明显得到改善,基本趋于精确解.从而验证了本方法的精确性和有效性.     

弹性支撑下风电机组传动系统结构动力分析

考虑水平轴风力发电机组齿轮箱弹性支撑的柔性连接特性,基于集中质量思想和拉格朗日方法,建立风力发电机传动系统多体动力学模型,研究了齿轮箱弹性支撑对传动系统结构动力学特性的影响.利用动力学模型和模态分析方法,得到了由弹性支撑耦合到系统后的模态频率,并获取了在该模态激励下的模态动能分布.采用变参数方法进行传动系统模态对齿轮箱弹性支撑刚度变化的敏感性分析,利用模态叠加法进行齿轮箱体的动响应分析.数值求解结果和分析表明,考虑齿轮箱弹性支撑的传动系统某阶固有频率即为弹性支撑下齿轮箱体振动主模态;弹性支撑线刚度对传动系统低频率固有模态存在一定影响;齿轮箱体振动分析时应考虑1阶和2阶的低频模态较为合理.本研究工作对传动链系统方案可靠性设计和抑制传动链振动的加阻控制提供了一定理论基础.     

机器人轨迹跟踪精度对铣削再生颤振的影响

针对工业机器人低轨迹精度导致的轨迹跟踪误差，分析了切削过程中动态切削厚度和时滞效应对轨迹跟踪误差的响应。考虑轨迹跟踪误差引起的加工底面过程阻尼增加现象，建立包含轨迹误差的动态切削力模型。通过铣削实验验证了该模型，发现了机器人轨迹跟踪误差引发的动态切削厚度变化、时滞效应以及过程阻尼增加等现象能够提高临界轴向切深，扩大铣削加工的稳定域，但会降低表面质量与尺寸精度。本文的理论模型解释了在机器人铣削实验过程中出现的颤振发生又逐渐消失的特殊现象，为提高机器人铣削效率及抑制铣削颤振提供了理论模型。     

弹性支撑梁在移动荷载作用下的响应分析

考虑弹性支撑刚度对弹性支撑梁固有频率及模态的影响,比较前二阶频率受弹性支撑刚度影响的规律及大小.选取二阶模态,利用模态叠加原理,研究弹性支撑梁在移动荷载和移动车辆作用下的动力响应.研究中,一阶模态考虑为简支梁一阶模态与弹性支撑梁的刚体平动相叠加,二阶模态考虑为简支梁二阶模态与弹性支撑梁的刚体转动相叠加.研究结果表明:考...     

声辐射力所致粘弹性组织应变的多物理场 有限元分析研究

目前已经有多种超声弹性成像方法在临床上得到了广泛应用,然而通常情况下,这些方法均是基于组织为纯弹 性的假设,因此忽略了组织粘性对测量结果的影响。本文利用有限元分析的方法,分别用粘弹性力学的理论计算得到软 组织位移形变分布以及声回波分布等。这对于超声粘弹性成像的理论分析具有重要的辅助作用,可以研究模型组织在不 同粘弹性参数和不同频率的声压刺激下对粘弹成像的影响。有限元分析的结果表明,在进行弹性成像研究时不能忽略组 织粘性的影响。     

轴向运动粘弹性夹层梁的横向振动

研究了小扰度下轴向匀速运动粘弹性夹层梁的振动模态和固有频率.基于Kelvin粘弹性本构方程,建立了轴向运动粘弹性夹层梁横向振动控制方程.分别采用Galerkin截断和复模态分析方法,研究两端简支的粘弹性夹层梁的固有频率和模态函数,讨论了轴向运动速度、夹心层与约束层厚度比、初始轴力等参数对夹层梁固有频率、临界速度及稳定性的影响.     

涡激振动行波动力学特征的数值模拟

采用数值模拟分析了水中大长径比结构物涡激振动行波动力学特征,以及水动力学阻尼和模型初始张力对于行波动力学特征的影响.数值模拟模型采用改进的尾流振子模型,该模型考虑了涡激振动横向运动和顺流向运动之间的耦合,考虑了张力沿模型长度的变化和流体的耗散.给出了用于评估行波在整个振动波中所占比例的方法,该方法采用行波椭圆来定量表征行波所占的比例,即行波比.数值模拟结果显示,行波比总体上呈现随流速增加的趋势,但在模态阶数变化的临界流速上突然降低.水动力学阻尼显著影响着行波比,阻尼比越大,行波比越大,行波对于整个振动波的贡献就越大.不同张力情况下,行波比突然下降的临界速度不同.初始张力影响模型的固有频率,影响模态阶数发生转变的流速,从而影响了行波比突然下降的临界流速.     

旋转Timoshenko输流管道的固有频率和稳定性分析

基于Timoshenko梁模型,本文研究了旋转输流管道在自由振动状态下的流固耦合振动特性.考虑流体压力、重力、初始轴应力作用,基于Hamilton原理和欧拉角转换,推导得到了旋转Timoshenko输流管道的偏微分方程.根据Galerkin截断法将运动方程进行离散,通过求解系统的特征方程即可得到输流管一阶复频率的实部和虚部,实部代表固有频率,虚部代表能量变化.在流速较高时,研究发现必须考虑4阶及以上Galerkin截断,才能得到稳定的结果.通过与EulerBernoulli梁模型对比,验证了本文的结果正确性.研究发现针对短粗型管道,Timoshenko梁模型更加精确.此外研究了多种参数对旋转Timoshenko输流管道固有频率和振动稳定性的影响.研究结果表明质量比、流速、剪切系数对Timoshenko输流管道流固耦合振动的稳定性影响显著,而转动惯量、重力、流体压力和初始轴应力在一定程度上也会影响管道振动的频率和稳定性.转速的出现将管道频率分为两个量值,但转速并不影响系统能量变化.     

Organised modes and shock-vortex interaction in unsteady viscous transonic flows around an aerofoil: Part I: Mach number effect

The identification of successive stages in the transition of unsteady viscous transonic flow around an aerofoil is carried out by solving the time-dependent Navier-Stokes equations for a compressible fluid in two-dimensional approach. The numerical simulation is carried out at the Mach number range (0.2-0.98). At a fixed Reynolds number (Re=10,000), it is found that this flow undergoes the following four transition steps: It remains steady up to the Mach number values (0.2-0.35) and afterwards it develops spontaneously, without any imposed artificial perturbation, an inherent unsteadiness corresponding to a near-wake von Kármán instability, in the Mach number range (0.35-0.9). It is found that there exists a critical Mach number between the values (0.90-0.95) for which the flow returns to a steady-state. Furthermore, the flow is found to be governed by two instability processes in the Mach number range (0.75-0.8), where, apart from the von Kármán mode (mode I), a lower frequency mode II appears, due to the formation of weakly supersonic alternating zones in the region upstream of the aerofoil, related to the buffeting phenomenon. A triple role played by the increasing compressibility effects to trigger the instability processes, to maintain and to inhibit them in the transonic flow regime is therefore analysed in detail.     

Influence of an elastic end support on the stability of a nonuniform cantilever subjected to dissipative and nonconservative forces

The influence of an elastic end support on the stability of a damped, linearly tapered cantilever of rectangular cross section subjected to a follower-end-load is studied. The equation of motion is formulated within the Euler-Bernoulli theory for the case of a Kelvin model viscoelastic beam. The effect of external damping is also included in the partial differential equation of motion. The associated adjoint boundary value problem is derived and appropriate adjoint variational principle is introduced. This variational principle is used as the basis for determining approximately the values of the critical load of the system as it depends upon the taper parameters and the stiffness of the elastic end support. It is found that for a given damped, tapered beam the introduction of an elastic support may destabilize the system in certain eases. Furthermore, the critical value of the stiffness of elastic support at which the instability mechanism changes from flutter to divergence or vice versa is decreased as taper increases. For a given support, an increase in taper also decreases the value of the critical load of the system. These reducing effects are more pronounced with increase in thickness taper. The familiar destabilization phenomenon due to small internal damping is observed from the numerical results, but sufficiently large internal damping may have a stabilizing or destabilizing effect depending on the stiffness of the elastic support.     

基于H无穷范数的颤振分析方法

颤振是一种典型的气动弹性动不稳定现象, 求解颤振临界点是气动弹性稳定性分析的重要任务之一.从H_∞控制理论观点出发, 将气动弹性系统视为多输入多输出系统, 并导出其传递函数矩阵. 在颤振临界点附近, 根据系统传递函数矩阵的H_∞–范数趋于无穷大的特点, 发展了相应的颤振临界点搜索方法. 与传统的颤振分析方法相比, 该方法属于完全频域方法, 算法更为简洁, 且具有更高的自动化程度. 数值算例表明, 该方法可以获得正确的颤振临界点.     

Critical and flutter speeds of optical disks

 Critical speeds and aerodynamic flutter instability of various optical disks are experimentally studied in this paper. The two nodal diameter modes of ASMO and CD/DVD disks have the lowest critical speeds at 3800 and 6900 rpm, respectively, where the backward natural frequency vanishes. As the rotational speed increases, aeroelastic disk flutter is observed. Experiments using ASMO disks show that the three nodal diameter mode causes flutter instability at 8750 rpm. At the flutter speed, the vibration amplitude of the flutter mode grows dramatically. The natural frequencies of multiple vibration modes remain almost constant in the post-flutter region, which is called frequency lock-on. CD/DVD disks do not experience flutter up to 14,000 rpm. Received: 5 July 2001/Accepted: 1 November 2001     

风电塔在台风作用下的振动监测与分析

为了得到海上风电塔在“苏迪罗”台风直接作用下的振动响应特性,通过在风电塔上安装低频振动传感器和分量加速度计,对两座5 MW的海上风电塔样机开展振动监测.基于实测数据,对比分析风电塔固有频率、阻尼比等频域特性在台风前后以及台风过程中的变化情况,同时分析了振动幅度、加速度有效值、振动烈度等时域特性随台风的变化情况和台风过程中偏航冲击力的异常情况.监测结果表明:风机塔的振动模态参数对其法兰盘螺栓松动不敏感;法兰盘及其紧固螺栓在台风作用下出现螺栓松动后会表现出明显的局部颤振现象;台风过程中偏航冲击振动更加频繁,而且振动加速度大、持续时间长,对风电塔安全运行威胁较大.研究成果可为风机塔的抗风设计、运营安全监测提供参考.     

Optimal airfoil shapes for viscous transonic flows of Bethe-Zel’dovich-Thompson fluids

High-performance airfoils for transonic viscous flows of dense gases are constructed using an efficient high-order accurate flow solver coupled with a multi-objective genetic algorithm. Dense gases are theoretically characterized by reversed behavior of the speed of sound in isentropic perturbations for a range of temperatures and pressures in the vapor phase. A class of dense gases, namely the so-called Bethe-Zel’dovich-Thompson (BZT) fluids, might exhibit nonclassical gasdynamic behaviors in the transonic and supersonic regimes, such as the disintegration of compression shocks. Utilizing BZT gases as working fluids may result in low drag exerted on airfoils operating at high transonic speeds thanks to an increase in the airfoil critical Mach number. This advantage can be further improved by a proper design of the airfoil shape, also leading to the enlargement of the airfoil operation range within which BZT effects are significant. Such a result is of particular interest in view of the exploitation of BZT fluids for the development of high-efficiency turbomachinery.     

超声速风洞模型插入机构控制系统设计

针对颤振试验模型在超声速风洞启动/关车时会承受较大冲击载荷的问题,采用模型插入机构避免模型承受该载荷。设计了模型插入机构控制系统,介绍了以HNC100作为核心控制器和三级回零保护措施的硬件结构,以及先液压伺服定位再锁紧的控制方法,并给出了试验结果。结果表明,系统运行快速、平稳、可靠。该系统已成功应用于风洞试验,为我国2米量级超声速颤振试验平台的建立奠定了基础。     

On the influence of damping in a problem of dynamic stability optimization

A non-uniform, elastic cantilever, with Kelvin-Voigt internal damping and viscous external damping, subjected to a follower load at its free end, is considered. The mass/stiffness distribution of the column is described by a limited number of orthogonal design functions. For different values of internal and external damping, the mass distribution maximizing the critical load is found. The design improvements are based on analytical sensitivity analysis and the optimal linear combination of the design functions is found by sequential linear programming. Interaction of eigenvalues and the transference of the instability branch during optimization are discussed. Finally, the energy relations for the critical modes are considered.     

Stability of a nonuniform cantilever subjected to dissipative and nonconservative forces

The stability of a tapered cantilever beam subjected to a circulatory force at its free end is investigated. The effects of internal and external damping are included in the partial differential equation of motion. An adjoint variational principle has been used to determine approximately the values of the critical flutter load of the system. Graphs which demonstrate the variation of the critical nutter load with taper, damping and tangency coefficient are presented.     

Active control of supersonic/hypersonic aeroelastic flutter for a two-dimensional airfoil with flap

The flutter, post-flutter and active control of a two-dimensional airfoil with control surface operating in supersonic/hypersonic flight speed regions are investigated in this paper. A three-degree-of-freedom dynamic model is established, in which both the cubic nonlinear structural stiffness and the nonlinear aerodynamic load are accounted for. The third order Piston Theory is employed to derive the aerodynamic loads in the supersonic/hypersonic airflow. Nonlinear flutter happens with a phenomenon of limit...