正三角形排列管束结构流弹失稳流体力模型数值研究

研究了流弹失稳流体力模型.选取阻尼机理控制下的流弹稳定性问题为研究对象,在多种入口流速下对正三角形排列管束结构中单管可动情况的流致振动过程进行了模拟.用管子速度和位移的多项式函数作为流体力模型,结合流体力和管子位移数值模拟结果,计算了流体力系数.对不同流速工况下的流体力系数与流速之间的关系进行拟合,将流速的影响引入到流体力模型中.最终得到了与管子速度、位移以及入口流速相关的流体力模型.用建立起的流体力模型对管束结构流弹失稳临界流速进行了预测,结果较好.这种以管束结构流致振动数值仿真为基础,结合给定的函数形式建立起的流体力模型,能反映管束结构和流体相互作用过程中的主要特征,该模型对流弹失稳临界流速有一定的预测能力.     

三维横向流体诱发直管振动的数值模拟

基于有限体积法和有限元法,结合动网格控制技术,建立了横向流体作用下三维弹性直管流致振动计算的数值模型,实现了计算结构动力学与计算流体力学之间的联合仿真．首先,通过对刚性管的静止绕流计算,研究了网格离散方式和不同湍流模型对圆柱类结构静止绕流流场特征的影响和预测能力,得到了适用于双向耦合分析的CFD模型;其次,利用基于双向流固耦合方法的流致振动模型,计算并分析了流体力与结构位移间的相位关系,指出流体力与位移间的相位差是由流体力引起的,同时对双向耦合和单向耦合进行了比较分析;最后通过对直管流致振动的数值计算,联合管表面压力、尾流区时均速度、分离角等时均量,分析了尾流区的流场特征.     

圆筒流体域内管束振动与碰撞接触的流固耦合动力学方法研究

在海洋平台、核电站、石油石化等工业中,海洋从式井组隔水管、换热器管束等在流体作用下,可能诱导管束振动过大,引起管束之间碰撞,从而导致管束失效.这是一种典型的管束振动与碰撞接触的流固耦合动力学问题.但管束与流体耦合动力学及其碰撞接触的研究成果还未见报道.该文针对管束振动与碰撞接触非线性动力学问题,以三维管束及圆筒流体域内的非定常流体为研究对象,不仅考虑流固耦合界面位移、速度协调以及载荷平衡条件,还考虑管束接触边界以及流体域耦合边界拓扑结构的改变,建立了圆筒流体域内管束振动与碰撞力学模型及算法.算例结果表明,在圆筒流体域内,弹性管束发生接触碰撞时,流体压力在管束接触点处相等,且流体在管束侧流面流动较快.     

血液流动模拟方法对比分析

对现有主要血液流动模拟方法进行了对比研究.以单个直血管为研究对象,分别用牛顿单相流模型、非牛顿单相流模型、液固两相流剪切稀化模型、液固两相流颗粒动力学模型和血液两相流修正模型计算了血液流场,然后对比了五种模型模拟得到的血液动力学参数:血液流速、红细胞体积分数、流体粘度和壁面剪切应力.结果表明:血液组成和粘度方程的选择对血液速度场计算结果有明显影响;流体本构方程和升力公式对壁面剪切应力计算结果均有明显影响;液固两相流颗粒动力学模型计算得到的血液粘度远偏离正常的血液粘度,模型不适用于模拟稳态血液流动;血液两相流修正模型可以较准确模拟红细胞在血管内的径向分布及其影响.研究结果可为今后相关研究中血液模拟方法的选择提供指导.     

微曲输流管道振动固有频率分析与仿真北大核心CSCD

首次建立了基于Timoshenko梁理论的微曲输流管道横向振动的动力学模型,并分析了流体流动影响下微曲管道横向自由振动的固有特征.采用广义Hamilton原理,导出了考虑流体影响的微曲管道横向振动的控制方程,通过Galerkin截断对控制方程离散化,再由广义本征值问题得到管道横向振动的固有频率,并研究了液体流速和弯曲幅度对管道横向固有振动特征的影响.发展了基于等效刚度和等效阻尼方法的考虑流体影响的微曲管道振动分析的有限元仿真计算方法,并通过有限元软件实现数值仿真,验证了Galerkin截断的分析结果以及所建立的Timoshenko微曲管道动力学模型的有效性.研究表明,流体的流速以及管道的弯曲幅度对管道横向振动固有频率均有显著影响.     

非局部因子和表面效应对微纳米材料振动特性的影响

基于非局部理论和表面效应模型,导出表面吸附物对微纳米材料的动力学方程,研究非局部因子和表面能对微纳米传感器振动特性的影响．结果显示,非局部因子、表面能、吸附物种类、附加刚度和基底种类对微纳米结构的振动特性有重要影响．     

基于厚板拉伸振动精确化方程求解动应力集中问题

过去,对拉伸平板考虑应力集中的工程设计多借鉴弹性力学平面问题分析求解结果,例如弹性力学Kirsch问题的解或弹性动力学平面问题的解．基于厚板拉伸振动精确化方程,对含圆孔平板中弹性波散射与动应力集中问题进行了研究．研究结果表明:1) 两种模型得到的开孔附近的应力是不同的;2) 当入射波波数变大或者说入射波频率变高时,动应力集中系数最大值趋于单位1．含孔平板拉伸振动的动应力集中系数最大值达到3.30,以及基于弹性动力学平面问题模型得到的结果为2.77．对数值计算结果做了分析讨论, 可以看到,当孔径厚度比是a/h=0.10,基于平板拉伸振动精确化方程得到的动应力集中系数可以达到最大值,超出基于弹性动力学平面问题所得到结果的19%．分析方法和数值计算结果可望能在工程平板结构的动力学分析和强度设计中得到应用．     

计算流体力学在大型风力机空气动力学的应用进展北大核心CSCD

针对大型风力机设计中的关键空气动力学问题,比较系统地介绍了计算流体力学(computational fluid dynamics,CFD)方法的主要应用,特别是在大型风力机翼型气动分析、风力机流动的数值模拟、风轮空气动力特性的数值计算以及大型风力机叶片的多目标气动优化设计方面的进展.基于CFD方法分别实现了风力机翼型与叶片二维/三维气动特性的准确预测,风力机尾流场涡系结构的准确捕捉;并结合多目标遗传算法对1.5 MW风力机叶片进行了优化,获得了具有高风能利用效率的叶片方案.     

求解流固耦合问题的一种四步分裂有限元算法

基于arbitrary Lagrangian Eulerian (ALE) 有限元方法,发展了一种求解流固耦合问题的弱耦合算法．将半隐式四步分裂有限元格式推广至求解ALE描述下的Navier-Stokes（N-S）方程,并在动量方程中引入迎风流线（streamline upwind/Petrov-Galerkin, SUPG）稳定项以消除对流引发的速度场数值振荡;采用Newmark-β法对结构方程进行时间离散;运用经典的Galerkin有限元法求解修正的Laplace方程以实现网格更新,每个计算步施加网格总变形量防止结构长时间、大位移运动时的网格质量恶化．运用上述算法对弹性支撑刚性圆柱体的流致振动问题进行了数值模拟,计算结果与已有结果相吻合,初步验证了该算法的正确性和有效性．     

有限流体域内不同位置处的柔性管涡致振动机理研究

针对有限流体域内柔性管的涡致振动问题,将柔性管离散为若干空间梁单元,流体域采用实体单元离散,建立了有限流体域内柔性管系统的流固耦合模型及数值计算方法,设计并加工圆柱流体域内柔性管振动专用实验装置,采用GWT-2B双轴加速度传感器对柔性管的振动进行监测,并与数值模拟结果进行对比,吻合程度较好.基于该文的模型和方法,对圆柱流体域内不同位置处柔性管的涡致振动机理进行研究.结果表明,柔性管偏离入口流速位置的角度越大,越容易发生流体弹性不稳定性,柔性管的振动愈剧烈;而正对入口流速的位置,不易发生流体弹性不稳定性,柔性管振动减弱.     

结构振动特性在线识别的理论模型和计算方法

环境自然激励作用下的大型结构动力特性在线识别方法受到广泛的关注,这个方法仅仅利用结构自然响应的被测试数据,识别结构动力特性．Ibrahim方法和ARMAV方法是基本的识别方法．该文研究了受随机激励作用动力学模型,给出了有别于传统谐波恢复的子空间分解识别方法．数值仿真结果表明,该方法对结构振动特性的识别具有较好的鲁棒性和较高的计算精度．     

Eigenfrequencies of a tube bundle immersed in a fluid

In this paper we study a simplified version of a mathematical model that describes the eigenfrequencies and eigenmotions of a coupled system consisting of a set of tubes (or a tube bundle) immersed in an incompressible perfect fluid. The fluid is assumed to be contained in a rectangular cavity, and the tubes are assumed to be identical, and periodically distributed in the cavity. The mathematical model that governs this physical problem is an elliptic differential eigenvalue problem consisting of the Laplace equation with a nonlocal boundary condition on the holes, and a homogeneous Neumann boundary condition on the boundary of the cavity. In the simplified model that we study in this paper, the Neumann condition is replaced by a periodic boundary condition. Our goal in studying this simple version is to derive some basic properties of the problem that could serve as a guide to envisage similar properties for the original model. In practical situations, this kind of problem needs to be solved for tube bundles containing a very large number of tubes. Then the numerical analysis of these problems is in practice very expensive. Several approaches to overcome this difficulty have been proposed in the last years using homogenization techniques. Alternatively, we propose in this paper an approach that consists in obtaining an explicit decomposition of the problem into a finite family of subproblems, which can be easily solved numerically. Our study is based on a generalized notion of periodic function, and on a decomposition theorem for periodic functions that we introduce in the paper. Our results rely on the theory of almost periodic functions, and they provide a simple numerical method for obtaining approximations of all the eigenvalues of the problem for any number of tubes in the cavity. We also discuss a numerical example.     

Method for constructing an external polyhedral estimate of the trajectory tube for a nonlinear dynamic system

A numerical method is proposed for constructing an external polyhedral estimate for the trajectory tube of a nonlinear dynamic system described by a differential inclusion. The method is based on the approximation of cross sections of the trajectory tube (reachable sets) for an auxiliary system described by the convex hull of the graph of the differential inclusion. It produces polyhedral estimates suitable for the direct study of tubes via computer visualization and for the solution of more general problems.

     

Analysis of flow characteristics of granular material unloaded on nonlinear vibration inclined platform

In this study, the repeated discontinuous friction between granular material and contact platform and structural nonlinearity of inclined vibration platform giving rise to the vibration flow-aiding unloading is a complicated process, which has significant effects on the dynamic behaviors and flow characteristics of granular material. A simplified mathematical model of the inclined vibration platform and granular material is deduced by mechanical properties. Based on the equations of motion and a good degree of accuracy and applicability of the process with calculated data reported in the literature, the approximate analytical solution and flow properties are investigated by using the modified incremental harmonic balance method and numerical integration method. Moreover, the influences of friction coefficient, excitation amplitude, nonlinear stiffness and inclined angle on the complicated dynamic behaviors are explored and discussed. It is shown that the different motion paths of granular material on inclined vibration platform are observed depending on the different parameters. The increasing friction coefficient has complicated effects on the nonlinear dynamic behaviors of the granular material. The excitation amplitude and nonlinear stiffness can effectively control the flow characteristics of granular material at low excitation frequency but the inclined angle presents opposite property. The research may contribute to improve unloading efficiency, predict the motion state of granule and provide a theoretic foundation for further design the unloading system.     

Prediction of risers’ tubes temperature in water tube boilers

A dynamic model is developed which enables the prediction of risers’ tubes temperature of water tube boilers under various operating conditions. The model is composed of fluid dynamics model representing the fluid flow in the drum-downcomer-riser loop and a dynamic thermal model of the riser’s temperature. The model gives a detailed account of the two-phase heat transfer process which takes place between the risers’ inner walls and the water–steam mixture flow inside the tubes. The model is used to simulate various operational scenarios of water tube boilers. Results of the simulation provide insight into the dynamic interactions of the boiler’s main variables including the drum pressure, water volume, steam quality and risers’ temperature. Such a model is useful in checking operational scenarios before their actual plant implementation, can be a basis for developing boiler start up procedures and online temperature predictions to prevent eminent tube overheating.     

Influence of induced magnetic field on peristaltic flow in an annulus

This paper looks at the influence of the induced magnetic field on peristaltic transport through a uniform infinite annulus filled with an incompressible viscous and Newtonian fluid. The present theoretical model may be considered as mathematical representation to the movement of conductive physiological fluids in the presence of the endoscope tube (or catheter tube). The inner tube is uniform, rigid, while the outer tube has a sinusoidal wave traveling down its wall. The flow analysis has been developed for low Reynolds number and long wave length approximation. Exact solutions have been established for the axial velocity, stream function, axial induced magnetic field, current distribution and the magnetic force function. The effects of pertinent parameters on the pressure rise and frictional forces on the inner and outer tubes are investigated by means of numerical integrations, also we study the effect of these parameters on the pressure gradient, axial induced magnetic field and current distribution. The phenomena of trapping is further discussed.     

基于导纳方法的大型空间结构被动减振研究

以导纳方法研究空间桁架或框架结构的振动,建立了适合分析各种边界条件下均匀梁的机械导纳的通用计算方法,然后通过导纳函数求解各结构单元传递或损耗的功率,通过对损耗功率较大的单元来增加结构的阻尼特征。根据导纳分析结果,可以寻找阻尼单元的最优位置,对结构进行局修改。