恒载效应对拱结构自振频率的影响分析

提出用样条有限点法分析拱的自振频率,采用3次B样条函数的线性组合作为位移振型函数,以曲杆变形理论为基础,根据Hamilton原理建立了考虑恒载效应影响时拱的振动频率方程.计算分析了恒载效应对拱结构自振频率的影响.结果表明恒载效应会使拱结构自振频率减小;影响大小与结构自身刚度有关;拱结构截面刚度一定时,拱矢跨比、径厚比越大,恒载效应对自振频率的影响越大.     

偏心井产量变化规律研究

国内外调研发现,虽然边界元法在渗流力学中被广泛应用,但主要用于研究不同类型油藏井底压力曲线特征,在研究不稳定产量方面未见报道.在边界元方法求解不稳定压力动态问题基础上,对边界积分方程作了调整,给出了利用边界元方法进行不稳定产量的计算方法.研究了无因次半径1000的圆形封闭均质油藏一口或多口偏心井产量变化规律,研究发现对于单口偏心井偏心距越大产量越低;对于对称分布二口偏心井开采情形,偏心距为500时产量最高;比较无偏心距单井开采与二口偏心井开采,偏心井总产量早期为单井2倍,但采收率低于无偏心距单井.     

多轴承载下基于Mullins效应的非线性刚度的分析方法

轨道交通所用的橡胶减震元件在多向承载时通常会获得预压缩应力,这会使橡胶减震元件在多向承载过程中表现出Mullins效应.详细研究橡胶材料的Mullins效应原理、超弹本构模型的局限以及Mullins参数对橡胶软化行为的影响特性,为研究和分析带预应力的橡胶减震元件的非线性刚度计算提供一种新的思路和方法.     

一类在轨三维挠性梁模型仿真分析

针对在轨固支-自由三维挠性欧拉-贝努利梁的振动问题.为评估其动力学特性,综合考虑梁的轴力、梁的纵向变形、牵连惯性力、柯氏惯性力,以及梁的纵向变形对另外两个正交方向弯曲变形的影响,采用微元法给出了梁的振动模型.利用模态展开方法求解,采用四阶龙格一库塔法对其进行了数值仿真分析,结果表明三维姿态运动时挠性梁同时表现出软化效应和刚化效应,特别是在大范围运动时刚化效应强于软化效应,并且对于在轨运动的梁,轨道角速度也起着很大的作用,限制了在固定轨道上梁的长度.     

压电式压力传感器热冲击效应的试验研究

论述了瞬变温度对压电式压力传感器具有热冲击作用,并能改变传感器的预紧力、自振频率及灵敏度,形成瞬变温度误差.传感器的特殊膜片结构可以大大减小瞬变温度误差.通过对某2种压电式传感器的模拟热冲击效应的试验研究,阐述了热冲击对传感器的实际影响,并给出了测量数据及分析结果.     

基于无网格法考虑热效应及剪切效应FGM梁的动力学建模与仿真

将无网格点插值法和无网格径向基点插值法用于温度场中旋转柔性功能梯度材料梁的动力学分析.在考虑剪切效应的基础上,在梁本构关系中计及热应变,采用4种离散方法描述梁的变形场,运用第二类Lagrange方程,推导出大范围运动功能梯度材料梁的一次近似刚柔耦合动力学方程,研究不同温度变化下梁的动力学响应.通过动力学仿真得出以下结论：温度场对沿横向对称分布的功能梯度材料梁的横向变形影响较小,对纵向变形的影响较大,且在计算温度荷载作用下的梁末端变形时不应忽略轴向变形影响.     

含支撑阶梯梁自由振动的解析型梁段叠加法

针对多跨阶梯梁尚无简洁解析形式固有频率方程的现状,本文得到含支撑多跨阶梯梁频率方程的解析形式.首先基于Euler-Bernoulli梁理论,获得两端弹性约束单跨梁的频率方程,其表达式由梁坐标的三角函数和双曲三角函数的乘积组成,同时含有两端边界对应的横向刚度、旋转刚度等4个参数.给出至少含一个弹簧刚度约束的6种常见边界条件下梁的频率方程,其形式相对较简洁.然后把阶梯截面多跨梁的自由振动等效为各拆分梁段自由振动叠加的分析模型,结合所提出梁段连接节点处微段满足的动平衡方程,推导出多跨阶梯梁频率方程组的闭合解表达式.对于具有不同边界条件和内部支撑多跨梁的几种情况,算例计算出对应多跨梁的前几阶自振频率和振型图.阶梯形截面多跨梁与等截面多跨梁的频率方程可用统一的形式表示.所得解析结果与已有文献结果比较后发现：所得解析解同有限元结果的相对偏差小于1%,说明本文方法合理有效.阶梯多跨梁的自振频率随支撑刚度值、支撑杆位置和突变截面前后的惯性半径、惯性矩变化而变化.所得解析形式的频率方程在理论上未作近似,因此是精确的,形式上相对简单,具有良好的应用价值,故可用于评价其他数值方法的计算精度.     

索膜结构自振特性的仿真分析

索膜结构属于风敏感结构,其在风荷载作用下的反应,与结构的自振特性密切相关.通过索膜结构的自振特性分析发现,索膜结构的自振频率较低且呈连续密集分布,振型也比较复杂;影响其自振特性的主要结构参数是膜中预应力和结构跨度,随着膜中预应力的增加,自振频率呈较大幅度提高,而随着结构跨度的增大,自振频率却大幅度降低;另外,加劲索的张拉刚度对自振频率特别是第一自振频率影响很小;运用有限元软件ANSYS模拟分析了一个实际算例,验证了以上结论.     

用Timoshenko修正理论研究有梯度界面层双材料梁的振动特性

采用Timoshenko梁修正理论研究了有梯度界面层双材料梁的振动问题,利用静力方程确定了有梯度界面层双材料梁的中性轴位置,在此基础上应用Timoshenko梁修正理论建立了有梯度界面层双材料梁的振动方程,求得其自振频率表达式及其在简谐荷载作用下强迫振动的解析解.讨论分析了梯度界面层高度等因素对有梯度界面层双材料梁的振动影响,并用有限元法验证了Timoshenko梁修正理论.通过实例计算,得到了梯度界面层高度等因素对有梯度界面层双材料梁振动特性有较大影响的结论.     

影响皮托管测风精度的几个因素

影响皮托管测量风速的因素很多,本文主要从气体压缩性、全压孔尺寸、静压孔位置及压力损失等方面对风速测量的影响进行探讨.结果表明,除对风速引入压缩性、压损修正系数外,全压孔内外径之比应选在0.5且内径越小越好,静压孔选在距离全压孔7倍内径位置处,此时测量误差最小.     

Investigations on characteristics of micro/meso scale gas foil journal bearings for 100–200 W class micro power systems using first order slip velocity boundary conditions and the effective viscosity model

In this paper, a modified compressible Reynolds equation for micro/meso scale gas foil journal bearings considering first order slip and effective viscosity under rarefied flow conditions is presented. The influence of rarefaction effect on the load carrying capacity, attitude angle, speed and frequency dependent stiffness and damping coefficients, modal impedance, natural frequencies and unbalance response is studied. From numerical analysis, it has been found that there is significant change in all the static and dynamic characteristics predicted by the no-slip model and model with effective viscosity. There is also a considerable difference between the values predicted by a model with effective viscosity and a model without effective viscosity. For a given eccentricity ratio, the influence of effective viscosity on load carrying capacity and attitude angle is more significant for the typical operating speed range of micro/meso scale gas turbines. The influence of effective viscosity decreases with increase in compliance of bearing structure. Similarly, the influence of effective viscosity on frequency dependent stiffness and damping coefficients increases with excitation frequency ratio. Significant change in natural frequency, modal impedance and unbalance response for model with no slip and slip with effective viscosity is observed. The influence of effective viscosity is found to be significant with increase in Knudsen number.     

Free and forced vibration analysis of viscoelastic damped FG-CNT reinforced micro composite beams

In this paper, free and forced vibration analysis of viscoelastic microcomposite beam reinforced by functionally graded single-walled carbon nanotubes (FG-SWCNTs) is studied using the modified couple stress theory (MCST). The material properties of micro composite beam by generalized rule of mixtures carbon nanotubes are estimated. In addition, these properties are stated as uniform, and functionally graded (FG) distributions in the thickness direction. Energy method and Hamilton’s principle are employed to establish the governing equations of motion for the vibration of viscoelastic damped micro composite beam reinforced by SWCNTs based on the Kelvin–Voigt model. The influences of material length scale parameter, structural damping coefficient and different distributions of SWCNTs on non-dimensional complex natural frequency and amplitude vibration of the viscoelastic micro composite beam are investigated. The results reveal that the lowest vibration amplitude of FG microcomposite beam by the FG-X and the highest occurs by FG-◊. Moreover, in the presence of external periodic load and the absence of structural damping coefficient, the vibration amplitude increases and FG microcomposite beam becomes unstable, even though the amplitude of vibration decreases with increasing structural damping coefficient. It is shown that the natural frequency of SWCNT reinforced composite is more than the frequency of multi-walled carbon nanotubes because SWCNT have more stiffness. In addition, the results illustrate that the experimental data by Lei et al. agree well with those predicted by the MCST in the present work.

     

静电刚度式谐振微加速度计设计

运用梁的横向振动特性分析了梁振动频率与平行板电容形成的静电刚度的关系,并以此设计了静电刚度式谐振微加速度计。在加速度作用下,检测质量产生的惯性力使电容器极板发生位移来改变电容结构的间隙大小,从而使谐振频率发生变化,通过检测频率变化量来测量输入加速度的大小。根据加速度计的工作原理说明检测过程中梁的机械刚度保持不变,只与产生静电刚度的电容间隙变化相关,减小了检测信号对机械误差与残余应力的依赖性。运用加工参数进行理论计算得出加速度计的灵敏度为21．17Hz／gn,在CoventorWare2005中进行仿真表明：加速度计的固有频率为23．94kHz,灵敏度约为20Hz／gn,与理论设计值相近。     

粘弹性夹层圆板自由振动的理论解

研究了粘弹性夹层圆板的自由振动特性.基于经典弹性薄板理论和Kelvin-Voigt粘弹性本构方程,建立了粘弹性夹层圆板振动控制方程.采用分离变量法导出了粘弹性夹层圆板的自然频率及振型解析表达式,计算了固支和简支粘弹性夹层圆板的自然频率,并与有限元计算结果进行比较;讨论了粘弹性夹层圆板的夹心层比率对自然频率及衰减系数的影响.研究表明：（1）随着夹心层厚度的增大,系统频率先增大后减小,高阶时该趋势表现更为明显;（2）随着夹心层厚度的增大,衰减系数一直增大,高阶时该趋势表现更为明显.     

Theory and FEM simulation study of the double-clamped resonant beam with slit-structure

In this paper, we established the vibration model of double-clamped resonant beam with slit structure, and we theoretically analyzed the effect of rectangular slits with round corners on vibration amplitude and natural frequency of the resonant beam. The effect of rectangular slits on detection sensitivity of resonant beam is also analyzed. According to the theoretical analysis, computational studies of slit size and location influence on vibration amplitude, natural frequency and detection sensitivity of the resonant beam were carried out. Meanwhile, stress concentration of proposed slit structure with rounded corners was calculated by finite element method (FEM) and was compared with the stress concentration of slit with right corners. Finally, for resonant beams with slits of different size and location, theoretical calculation results and the FEM simulation results of natural frequency were compared. Theoretical analysis and FEM simulation are in good agreement.     

超声探头外声场在测量管道上的仿真分析

针对超声探头安置于测量管道不同位置时,获取信号不同这一问题,利用有限元软件COMSOL建立了探头外输送管道内的声场分布,研究了在不同声波频率和不同声衰减系数下,管道上的声场分布情况.研究表明:声波频率增大时,指向角减小,管道壁上的主声束范围也随之减小;声衰减系数增大时,指向角不变,但管道壁上主声束范围内的声压值减小.说明接收探头的设计位置应该主要根据声波频率确定.当频率不同时,声压为零的点不同,探头的设计位置应该避开声压为零的点,另外由于主轴处声信号的强度大,为了获得比较好的测量精度,探头位置应该尽量靠近主轴.     

一种基于DCT系数的图像隐藏方法

近年来,随着网络和多媒体技术的快速发展,信息隐藏技术已经成为信息安全领域的一个研究热点。由于JPEG压缩方案中量化是损失信号的主要原因,所以可以在量化后的DCT系数中隐藏信息。文中提出了一种利用对JPEG压缩中的DCT系数进行处理的隐藏图像的方法,将待隐藏图像量化后的DCT系数转化成绝对值小于5的个位数,并将其隐藏在原始图像的量化后的DCT系数中。实验证明该方法能得到不错的隐藏效果,且对原始图像的影响很小。     

分析两端扭转弹簧约束下简支输流管道的动力特性

研究了两端受扭转弹簧约束的简支输流管道的固有频率特性和静态失稳临界流速.根据梁模型横向弯曲振动模态函数,由端部支承和约束边界条件得到了其模态函数的一般表达式.根据动力方程的特征方程,具体分析了约束弹性刚度、流体压强、流速和管截面轴向力等参数对管道固有频率特性和静态失稳临界流速的影响.数值分析表明,约束弹性刚度的增大使管道的固有频率和失稳临界流速明显提高;流体流速、压强和管截面受到的轴向压力的增加使管道的固有频率和失稳临界流速降低.当管道的固有频率和失稳临界流速较低时,可以通过增加端部约束的方法来提高.     

Behaviour of an immersed boundary method in unsteady flows over sharp-edged bodies

The behaviour of the immersed boundary method proposed by Goldstein et al. [Goldstein D, Handler R, Sirovich L. Modelling a no-slip boundary condition with an external force field. J Comput Phys 1993;105:354-66] as a second-order damped control system is investigated. The natural frequency and the damping coefficient are introduced as driving parameters of the method. The comparison between the velocity response at forced points in the startup flow over a square cylinder with the theoretical response of a second-order damped oscillator is performed. The role of each parameter appears clearly. At the beginning of the startup flow, the response time depends directly on the natural frequency, and this parameter determines the level of residual velocities achieved in an unsteady flow. The damping coefficient drives the oscillation of the velocity response at the beginning of the startup flow, but has negligible influence during the establishment and in the unsteady flow. At forced points facing no unsteady perturbation from the flow, the zero-velocity set point is reached asymptotically, as usual in second-order damped-systems. Through the simulation of the flow over a blunt flat plat at Re=1000, it is observed that the initial thickness of the mixing layer due to the separation at the edge may vary during the simulation because the sharpness of the edge increases as the residual velocities decrease. This insight gained on the behaviour of the response allows a time-step optimisation, which, completed with comparisons to reference literature results, confirms the feedback forcing method a competitive tool for accessing near-wall unsteady flow over sharp-edged bodies.