Optimum performances of longitudinal convective fins with symmetrical and asymmetrical profiles

In the present work, the heat transfer performance of optimized dissipators with longitudinal fins of asymmetrical cross section is investigated and compared with that of optimized dissipators with symmetrical fins. In particular, the problem of optimizing the shape and the spacing of the fins of a thermal dissipator cooled by a fluid in laminar flow is studied by assigning two different polynomial lateral profiles to the fins. A finite element model is proposed to determine velocity and temperature distributions and is employed in a genetic algorithm to find the dissipator geometries which make the heat transfer coefficient as high as possible under different conditions. Some examples of optimized geometries are finally shown and discussed.     

一类非线性系统载荷识别的组合迭代法

针对一类非线性系统提出了一种新的载荷识别方法,组合迭代法.该方法通过有限元方法和主动控制方法组合迭代来实现一类非线性系统的载荷识别.首先将非线性系统的有限元模型模态缩减成简化模型,由简化模型组成主动控制的被控对象;然后在选定的控制律下,设计控制调节器,使该系统监测点的响应功率谱密度达到预定谱,从而得到系统激励,即被识别的载荷;最后由非线性有限元响应验证载荷的合理性.对圆锥壳-包带组合系统载荷识别的数值研究表明了组合迭代法的有效性.该方法为导弹、宇宙飞船、航天飞机、火箭等航天航空结构振动试验的载荷识别提供指导作用,将促进航天航空事业的发展.     

考虑空气耦合的薄膜结构动力分析

假定空气是可压缩、无粘性和有势的,推导了薄膜结构的非线性刚度矩阵和空气与薄膜结构耦合作用的气动力表达式。根据薄膜结构的受力特点,建立了薄膜结构与空气耦合系统的运动方程。采用有限元方法对无限域的空气进行简化,建立了空气与薄膜结构的耦合有限元模型,模拟了空气对薄膜结构的附加质量和声致阻尼影响。考虑薄膜结构的几何非线性,对结构在空气中的自由振动和受迫简谐振动进行了系统分析,同时探讨了流体环境对薄膜结构动力响应的影响因素。     

Static Finite Element Validation of a Flexible Micro Air Vehicle

The flexible-wing approach has proven to be a successful method for designing micro air vehicles. The wing’s passive deformation under wind loads can allow for gust rejection, delayed stall, or improved longitudinal stability. As such, an accurate structural model of the flexible wing can provide greater understanding of the aforementioned phenomena. This paper seeks to formulate a static finite element wing model, with a particular emphasis on accuracy. The wing is broken into three different types of elements: beams, plates, and membranes. Individual element types are characterized and validated by constructing simple structures from the appropriate material, and then comparing experimental and numerical deformation fields. Experimental results are found through a visual image correlation system. The elements are then combined to form the complete wing model, which is also validated through experiments. The resulting finite element model is found to be very accurate, able to predict the complicated structural response of a composite wing. Due to observations made during standard wind tunnel testing, the structural response of a typical membrane MAV wing in steady level pre-stall flight is thought to be quasi-static. As such, the finite element model formulated in this work will be indispensable towards future numerical static aeroelastic optimization research efforts aimed at improving the efficiency, agility, and sensitivity of practical micro air vehicles.     

Analysis of turbulent heat transfer from sinusoidal profile finned dissipators

 In the present work the effectiveness of thermal dissipators cooled by a fluid in turbulent flow is studied by varying the fins spacing and shape. The profile of the fin is described by a sinusoidal function depending on two parameters that define its amplitude and number of oscillations. The velocity distribution in the fluid, and the temperature distribution in the dissipator and in the fluid are determined with the help of a finite element method. The model allows to study the variations of the heat transfer coefficient due to the fluid dynamic conditions imposed by the fin profile. Lastly, a combination of geometrical parameters is proposed, which yields the best dissipator performance under particular conditions. Received on 29 June 2000     

基于能量辨识的非线性静电-结构系统的自由度缩减建模方法

给出了一种基于系统能量函数辨识的静电致动微薄板系统自由度缩减建模方法.从Von Karman应变-位移关系式出发,推导出以广义模态坐标为变量的系统动能、应变能以及电容函数的函数表达式.为了将应变能以及电容函数写成广义模态坐标的多变量多项式形式,利用一系列经静态非线性结构有限元计算的结果,拟合得到多变量多项式的未知系数.由Lagrangian方程获得原系统的自由度缩减模型.利用该模型对器件的静/动态特性进行仿真,其计算费用很低.与有限元结果比较,验证了建模方法的正确性.     

Optimization of heat transfer through finned dissipators cooled by laminar flow

In the present work, the problem of optimizing the shape and the spacing of the fins of a thermal dissipator cooled by a fluid in laminar flow is studied. For a particular finned conduit, the velocity and temperature distributions on the transversal section are determined with the help of a finite element model and a global heat transfer coefficient is calculated. A polynomial lateral profile is proposed for the fins and the geometry is optimized in order to make the heat transfer coefficient as high as possible with the smallest dimensions or the lowest hydraulic resistance to the flow. The optimum fin profile and spacing, obtained by means of a genetic algorithm, are finally shown for different situations. Increases of 45% are obtained in the heat transfer coefficient referring to the maximum values which can be obtained with rectangular fin profiles.     

Modes computation and analysis for long multi-span bundle conductors of power transmission lines with galloping control devices

A new type of element which is suitable for solving the modes of the galloping long multi-span bundle conductor structures is presented. The element is composed of all sub-conductor segments between two spacers. Based on the linearized governing differential equations of the conductors, the mass matrix and stiffness matrix of the element in consideration of the constrained relations imposed on the conductors by spacers are derived. The dynamic characteristics of the galloping control devices can be directly added to the element. The modes for an actual power line structure are computed by using the element formula and FEM procedures, where seven cases of different galloping control device allocations are considered. Compared with the measured data, the method is shown to be reliable and effective. Analysis and discussions of the computational results are given. Some hints that are helpful to further investigation of galloping are also obtained.     

THE EFFECT OF FLOW PULSATIONS ON CORIOLIS MASS FLOW METERS

It has been reported that the accuracy of Coriolis mass flow meters can be adversely affected by the presence of pulsations (at particular frequencies) in the flow. A full analysis of the transient performance of a commercial Coriolis meter is only possible using finite element techniques. However, this is a transient, nonlinear problem in which the space and time variables are not (strictly) separable and the finite element techniques for tackling such problems make it desirable to have an analytical solution for a simplified meter, against which the finite element solution can be compared. This paper reports such a solution. The solution will also provide guidance for experiments. Existing analytical solutions for the performance of Coriolis meters in steady flow (a complex eigenvalue problem) are not easily extended to the transient flow case. The paper thus begins with the presentation of an alternative solution for steady flow through a simple, straight tube, Coriolis meter and it is notable that this solution gives a simple analytical expression for the experimentally observed small change in the resonant frequency of the meter, with flow rate, as well as an analytical expression for the meter sensitivity. The analysis is extended to the transient case, using classical, forced vibration, modal decomposition techniques. The solution shows that, unlike the steady flow case where the detector signals contain components at the drive frequency and the second mode frequency (Coriolis frequency), for pulsatile flow the detector signals will in general contain components involving at least four frequencies. It is demonstrated that the meter error depends on the algorithm used to estimate the phase difference from the detector signals. The particular flow pulsation frequencies which could possibly lead to large meter errors are identified.     

TEC结构的三维非线性瞬态温度场分析

热电制冷器(TEC)以其体积小、作用速度快及无噪音等机械制冷无法替代的优点在航空航天和电子工业等领域得到了越来越广泛的应用。本文根据TEC的导热特点,推导了TEC结构稳态温度场的解析解,建立了其瞬态非线性温度场分析的微分方程。利用伽辽金法导出TEC结构热分析的有限元方程,对非线性热分析的有限元方程进行了求解,得到了TEC的稳态温度场和瞬态响应温度场。算例结果表明,本文提出的TEC结构热分析有限元模型具有较高的精度,能够有效地分析TEC的非线性瞬态温度场。     

SUPG finite element method based on penalty function for lid-driven cavity flow up to $$Re = 27500$$

A streamline upwind/Petrov–Galerkin(SUPG)finite element method based on a penalty function is proposed for steady incompressible Navier–Stokes equations.The SUPG stabilization technique is employed for the formulation of momentum equations. Using the penalty function method, the continuity equation is simplified and the pressure of the momentum equations is eliminated. The lid-driven cavity flow problem is solved using the present model. It is shown that steady flow simulations are computable up to Re = 27500, and the present results agree well with previous solutions. Tabulated results for the properties of the primary vortex are also provided for benchmarking purposes.     

基于hp自适应伪谱法的组合动力可重复使用运载器轨迹优化

针对传统化学火箭难以重复使用、发射成本高的问题,提出了一种水平起飞/降落的新一代可重复使用运载器飞行方案,并对其动力模式设计和上升段轨迹优化方法等关键技术进行深入研究。设计了一种涡轮\冲压发动机结合火箭\冲压发动机的组合动力模式,建立了发动机推力与高度、马赫数等变量间的耦合模型,根据动力形式将上升段轨迹分为两段并采用全局搜索法确定动力切换的最佳时机。根据分段结果,分别以燃料最省和终端速度最大为指标,利用hp自适应伪谱法对两段进行轨迹优化设计。该算法基于双层策略求解最优控制问题,兼备伪谱法和有限元法的优点,与打靶法、伪谱法和间接法相比,初值更易选取,收敛速度更快。     

简化Navier—Stokes方程的IFT理论的各向异性张力有限元法

本文首先讨论简化Navier-Stokes IFT方程组的有限元离散方式,然后对其广义解进行分析,并从而利用与之相匹配的各向异性张力单元对流函数—涡量方程进行计算。通过平板层流和台阶绕流两个算例的分析,证明这种与IFT理论相匹配的有限单元算法是成功的。     

The finite dynamic annular element for the vibration analysis of variable thickness discs

According to the gyro-periodicity of dynamic displacements, the two-dimensional problems of circular plates with variable thickness are simplified into one-dimensional ones in this paper. Taking the expanded form of frequency power series of the dynamic displacement functions as the dynamic shape functions of the finite annular element, the mass and stiffness matrices as well as their one-order revised matrices are given succinctly. The dynamic method is used to analyse the vibration characteristics of a bladed disc assembly and is compared with conventional finite element method and experiment, and is proved to be superior to other numerical methods.     

Simplified prediction of creep buckling of cylinders under external pressure. Part 1: finite element validation

In this paper, a simplified method is proposed for the prediction of creep buckling. This simplified approach relies upon a model which yields an analytical evaluation of creep buckling times for cylinders under external pressure. This model is fully developed herein, and a ‘closed-form’ solution is given for the evaluation of the critical creep collapse time. The collapse mechanism is assumed to be due to the formation of a plastic hinge which induces an unstable post-buckling of the ring. The analytical ‘closed-form’ creep collapse time is then compared to finite element buckling predictions using the quasi-axisymmetric COMU shell element in the INCA code of the CASTEM system. The model is then applied to four different cylinders under external pressure and compared to finite element predictions; the cylinders' radius-to-thickness ratio varies between 50 and 550. It is shown that the proposed model performs well for this type of prediction: in all cases, the times to failure predicted by the model are lower than the finite element predictions. These predictions prove to be rather conservative for thicker cylinders. It is shown that creep buckling is a very dangerous failure mode. If the shape of the structure is observed as a function of time, nothing seems to happen during a very long ‘incubation’ period; when the initial imperfection reaches some critical value, buckling then suddenly occurs. This phenomenon is shown by the two methods of evaluation presented herein.     

Dynamic modeling and control of a rotating piezo-bonded beam-mass system

A variational finite element method is used to derive the dynamic equations for a high speed rotating beam-mass system with embedded piezoelectric materials. Two piezoelectric layers are attached on the beam surfaces. The layer on the top surface is used for sensing and monitoring. The layer on the bottom surface is used for feedback control. The objective is to develop a simple finite element based dynamic model which can be used for structural characterization, monitoring and control. The structural dynamics introduced by the gyroscopic effect is included in the model. The nodal displacements of the structure are monitored by the output voltage level of the sensing layer. The voltage induced by the sensing layer is negatively fed back to the control layer for the adjustment of nodal displacements. Several simulation examples are presented to validate the effectiveness of the proposed finite element based control model.     

Finite element computations of two-dimensional arterial flow in the presence of a transverse magnetic field

A finite element solution of the Navier-Stokes equations for steady flow under the magnetic effect through a double-branched two-dimensional section of a three-dimensional model of the canine aorta is discussed. The numerical scheme involves transforming the physical co-ordinates to a curvilinear boundary-fitted co-ordinate system. The shear stress at the wall is calculated for a Reynolds number of 1000 with the branch-to-main aortic flow rate ratio as a parameter. The results are compared with earlier works involving experimental data and found to be in reasonable qualitative agreement. The steady flow, shear stress and branch flow under the effect of a magnetic field have been discussed in detail.     

A numerical method for subcritical and supercritical open channel flow calculation

A marching finite volume method is presented for the calculation of two-dimensional, subcritical and supercritical, steady open channel flow including the usually neglected terms of slope and bottom friction. The channel flow will be assumed to be homogeneous, incompressible, two-dimensional and viscous with wind and Coriolis forces neglected. A hydrostatic pressure distribution is assumed throughout the flow field. The numerical technique used is a combination of the finite element and finite difference methods. A transformation is introduced through which quadrilaterals in the physical domain are mapped into squares in the computational domain. The governing system of PDEs is thus transformed into an equivalent system applied over a square grid network. Comparisons with other numerical solutions as well as with measurements for various open channel configurations show that the proposed approach is a comparatively accurate, reliable and fast technique.     

安装蜂窝板动力学特性分析及主动控制试验研究

通过对蜂窝芯的等效化处理,建立了ANSYS的壳单元模型,并作有限元模态分析,然后与其试验模态分析结果比较,有限元分析结果和试验结果基本一致。通过ANSYS的PSD随机功率谱分析和模态分析所得振型确定了贴片位置,对蜂窝板进行了压电智能结构振动主动控制试验研究,得到了较好的振动抑制效果。分析结果为仪器安装蜂窝板的设计和实现智能结构控制提供了重要参考依据。     

Frictional Hertzian contact problems under cyclic loading using static reduction

In this study we investigate an axisymmetric Hertzian contact problem of a rigid sphere pressing into an elastic half-space under cyclic loading. A numerical solution is sought to obtain a steady state, which demands a large amount of computer memory and computing speed. To achieve a tractable problem, the current numerical model uses a “static reduction” technique, which employs only a contact stiffness matrix rather than the entire stiffness of the problem and is more accurate than the approach used by most finite element codes. Investigation of the tendency of contact behavior in the transient and steady states confirms that a steady state exists, showing converged energy dissipation. The dependence of dissipation on load amplitude shows a power law of load amplitude less than 3, which may explain some deviations in the experimental findings.