Membrane finite element method for simulating fluid flow in porous medium

A new membrane finite element method for modeling fluid flow in a porous medium is presented in order to quickly and accurately simulate the geo-membrane fabric used in civil engineering. It is based on discontinuous finite element theory, and can be easily coupled with the normal Galerkin finite element method. Based on the saturated seepage equation, the element coefficient matrix of the membrane element method is derived, and a geometric transform relation for the membrane element between a global coordinate system and a local coordinate system is obtained. A method for the determination of the fluid flux conductivity of the membrane element is presented. This method provides a basis for determining discontinuous parameters in discontinuous finite element theory. An anti-seepage problem regarding the foundation of a building is analyzed by coupling the membrane finite element method with the normal Galerkin finite element method. The analysis results demonstrate the utility and superiority of the membrane finite element method in fluid flow analysis of a porous medium.

Design and application of structural health monitoring system in long-span cable-membrane structure

Cable-membrane structures have small rigidity and are highly sensitive to wind. Structural health monitoring is necessary to ensure the serviceability and safety of the structure. In this research, the design method of a structural health monitoring system is using the characteristics of a cable-membrane structure. Taking the Yueyang Sanhe Airport Terminal as an example, a finite element model is established to determine the critical structural components. Next, the engineering requirements and the framework of the monitoring system are studied based on the results of numerical analysis. The specific implementation of the structural health monitoring is then carried out, which includes sensor selection, installation and wiring. The proposed framework is successfully applied to the monitoring system for the Yueyang Airport terminal building, and the synchronous acquisition of fiber Bragg grating and acceleration sensor signals is implemented in an innovative way. The successful implementation and operation of structural health monitoring will help to guarantee the safety of the cablemembrane structure during its service life.

Patterned catalyst layer boosts the performance of proton exchange membrane fuel cells by optimizing water management

Mass transport is crucial to the performance of proton exchange membrane fuel cells,especially at high current densities.Generally,the oxygen and the generated water share same transmission medium but move towards opposite direction,which leads to serious mass transfer problems.Herein,a series of patterned catalyst layer were prepared with a simple one-step impressing method using nylon sieves as templates.With grooves 100μm in width and 8μm in depth on the surface of cathode catalyst layer,the maximum power density of fuel cell increases by 10%without any additional durability loss while maintaining a similar electrochemical surface area.The concentration contours calculated by finite element analysis reveal that the grooves built on the surface of catalyst layer serve to accumulate the water nearby while oxygen tends to transfer through relatively convex region,which results from capillary pressure difference caused by the pore structure difference between the two regions.The separation of oxidant gas and generated water avoids mass confliction thus boosts mass transport efficiency.

Method to analyze wrinkled membranes with zero shear modulus and equivalent stiffness

To solve the problems of divergence,low accuracy and project application of membrane wrinkling analysis,an analysis method of zero shear modulus and equivalent stiffness was proposed.This method is an improvement to the previous method (Method I) of local coordinate transposition and stiffness equivalence.The new method is derived and the feasibility is theoretically proved.A small-scale membrane structure is analyzed by the two methods,and the results show that the computational efficiency of the new method (Method II) is approximately 23 times that of Method I.When Method II is applied to a large-scale membrane stadium structure,it is found that this new method can quickly make the second principal stress of one way wrinkled elements zero,and make the two principal stresses of two-way wrinkled elements zero as well.It could attain the correct load responses right after the appearance of wrinkled elements,which indicates that Method II can be applied to wrinkling analysis of large-scale membrane structures.

Analysis on nonlinear wind-induced dynamic response of membrane roofs with aerodynamic effects

Based on the characteristics of membrane structures and the air influence factors,this paper presented a method to simulate the air aerodynamic force effects including the added air mass,the acoustic radiation damping and the pneumatic stiffness.The infinite air was modeled using the acoustic fluid element of commercial FE software and the finite element membrane roof models were coupled with fluid models.A comparison between the results obtained by FE computation and those obtained by the vibration experiment for a cable-membrane verified the validity of the method.Furthermore,applying the method to a flat membrane roof structure and using its wind tunnel test results,the analysis of nonlinear wind-induced dynamic responses for such geometrically nonlinear roofs,including the roof-air coupled model was performed.The result shows that the air has large influence on vibrating membrane roofs according to results of comparing the nodal time-history displacements,accelerations and stress of the two different cases.Meantime,numerical studies show that the method developed can successfully solve the nonlinear wind-induced dynamic response of the membrane roof with aerodynamic effects.

Prediction of wrinkles in the membrane

This paper presents an analytical approach for predicting the detailed out-of-plane wrinkle deformation that formed in the membrane. The analytical wrinkle model is based on the assumption that the membrane is able to resist small compressive stress once it has wrinkled. This model is developed for the cases of the rectangular membrane subjected to pure shear and local tension by using the equilibrium equation of the membrane in the deformed configuration. Predictions from this model are compared with the finite element simulation based on the nonlinear buckling finite element method and the results are found to be accurate.

骨架膜装配式节点设计与承载力分析

提出的骨架膜结构装配式节点具有构造简单、预制程度高、便于现场装配、可以调节消减加工误差、适应各种形状的骨架膜结构、传力清晰等特点。基于Hertz接触理论分析了该节点的关键承载结构——销轴连接的应力分布形式并与有限元分析相互印证。基于有限元分析得出了该节点中杆件与耳板最佳的连接方式,对该节点的三维模型进行简化并建立有限元模型,计算了该节点的极限承载能力,得到其破坏形式并与同规格的空间相贯焊接钢管节点进行对比。结果表明:该装配式节点的失效由下弦杆控制,满足“强节点,弱杆件”的设计要求;该装配式节点的极限承载能力为设计荷载的2.95倍,与同规格的空间相贯焊接钢管节点几乎相同,有充足的安全余量,可以更加灵活地应用于各种建筑结构中。

ANALYSIS OF A MIXED FINITE ELEMENT METHOD FOR A PHASE FIELD BENDING ELASTICITY MODEL OF VESICLE MEMBRANE DEFORMATION

In this paper, we study numerical approximations of a recently proposed phase field model for the vesicle membrane deformation governed by the variation of the elastic bending energy. To overcome the challenges of high order nonlinear differential systems and the nonlinear constraints associated with the problem, we present the phase field bending elasticity model in a nested saddle point formulation. A mixed finite element method is then employed to compute the equilibrium configuration of a vesicle membrane with prescribed volume and surface area. Coupling the approximation results for a related linearized problem and the general theory of Brezzi-Rappaz-Raviart, optimal order error estimates for the finite element approximations of the phase field model are obtained. Numerical results are provided to substantiate the derived estimates.

鼓膜穿孔导致听力下降的有限元模型分析

目的探讨鼓膜穿孔导致听力下降的机制。方法通过有限元分析法(finite element analysis,FEA)构建全耳有限元模型并验证;参照实际鼓膜穿孔病例,创建个性化的鼓膜穿孔有限元模型,模拟鼓膜穿孔导致的听力损失;使用基底膜的位移响应与基线的差异来模拟听力损失情况,分析中耳各个组分对听力损失的贡献程度,探讨鼓膜穿孔导致听力下降的机制。结果如果将圆窗膜和中耳腔的耦合去掉,将导致低频约40 dB的听力下降,而高频段基本与基线情况下鼓膜穿孔所导致的听力损失持平;去除鼓膜内侧面和中耳腔气体的耦合,可部分减轻低频听力损失,而高频听力损失加重;切断外耳道气体和中耳腔气体之间的连续性,低频听力损失加重;但是去除圆窗膜和中耳腔之间耦合关系并且去除中耳腔气体和鼓膜内侧面的连接后,原先低频的40 dB听力损失将会恢复到10 dB左右;而去除中耳腔气体和听骨链之间的耦合,对听力损失无明显影响。结论鼓膜穿孔导致听力下降的主要机制是鼓膜穿孔本身所导致的传声效能降低,以及鼓膜内外声压差的降低;圆窗膜处声压的增加可以部分弥补鼓膜穿孔所导致的听力损失。

Cell membrane tensile strain under cyclic compression:A viscoelastic myoblast finite element model

Cyclic mechanical stimulation could lead to subsequent biomechanical and biological effects on cells.Viscoelastic cells could deform or show energy dissipation with hysteresis behavior in response to external cyclic compression.The aim of this study was to investigate the effect of cyclic compression on a single viscoelastic myoblast through a confocal–based cell–specific finite element model,including cell membrane tensile strain and damaged elements.Sinusoidal compression was applied to the apical surface of the myoblast(cell membrane)with compressive stress of 500
500 Pa(with stress offset at 500 Pa and amplitude of 500 Pa)at 0 Hz(static compression of 500 Pa),0.25 Hz,0.5 Hz,0.75 Hz,1 Hz,5 Hz,and 10 Hz.Results showed that the ratio of average tensile strain integral in all cell membrane elements over a certain period of time(T)to that duration(T)(MAS index)decreased under cyclic compression compared to that of static compression in the short term(within 4 s).Furthermore,compared to static compression,the percentage of damaged elements of cell membrane under cyclic compression decreased assuming a 3%cell membrane tensile strain damage threshold.The optimal cyclic compression frequency 0.25 Hz led to the largest difference of MAS index under cyclic compression and static compression.These results may provide support for the application of cyclic compressive stimulation in the prevention of cell damage.

用有限元模拟方法研究阶层式多孔薄膜几何参数对水通量的影响

通过系统改变阶层式多孔薄膜大孔和小孔的孔径及孔隙率等几何参数,用有限元模拟方法研究薄膜通量与相关几何参数之间线性或非线性的定量关系.模拟结果表明,在弯曲孔道基体中添加球形空穴形成阶层式多孔结构后,可使薄膜水通量提升约为原来的171%.对于弯曲孔道,单纯增加其数量可使水通量线性提升,单纯增大其孔径可使水通量以指数函数形式提升.对于球形空穴,单纯增加其数量或单纯增大其孔径,水通量均以指数函数形式提升.此外,球形空穴的水通量提升作用还依赖于它与弯曲孔道基体网格的相对尺寸,通过调节制备条件可提升分离膜材料的性能.

基于在线监测数据的水库围坝渗流安全分析

以济南市济阳区稍门平原水库西坝为例,通过对测压管水位在线监测数据整理分析,采用有限元法对大坝复合土工膜反演等效渗透系数,重新复核大坝渗流安全。结果表明:测压管水位与库水位变化趋势一致,正相关性较好,实测数据可靠性高;坝体渗流正常稳定,无明显趋势性变化;各工况条件下的坝基水力比降、逸出点比降均小于允许比降,大坝渗流稳定性满足要求。

单层拉索式ETFE膜结构制作安装关键技术

结合实际工程应用,介绍单层拉索式ETFE膜结构安装技术特点与难点及其安装关键技术。采用膜结构逆向建模、双轴试验、现场组合安装、膜结构张拉等技术,实现了单层透明ETFE膜结构精准安装。实践表明,该技术节约了劳动力,缩短了工期,安装质量高,施工过程安全可靠,经济效果显著。

索膜反射面可展开天线的形态设计

为研究金属反射网和索网的张紧状态对可展开天线形面精度的影响,考虑支撑桁架变形对索膜反射面可展开天线进行形态设计研究。金属反射网可看作一类薄膜结构,由于金属反射网和索网均需施加预张力来达到一定的刚度,因此首先基于索膜结构力密度方程进行形态设计,使其在自身预张力和外载作用下达到满足要求的形状和精度;索膜结构的张拉作用会导致桁架变形,造成再平衡后索膜反射面形面精度的降低和张力分布不合理,因此,需要建立天线整体的非线性有限元模型;对天线模型进行迭代修正,最后得到满足要求的张力设计变量和天线构型。最后通过相应算例,验证了方法的正确性。

某充气膜结构承载能力分析及评估

以某实际工程为例,介绍了某充气膜结构的检查要点和承载能力评估方法。现场对充气膜结构的控制点位、充气系统、膜面性能、金属构件及连接节点等进行检查,并采用有限元软件3D3S对该充气膜进行建模计算,结果表明:该充气膜结构承载能力能满足规范要求,但是索与钢筋混凝土支承构件的连接构造不满足规范要求,应进行处理。本案例可为其它类似工程提供参考。

基于ANSYS的罐式集装箱结构性能分析

罐式集装箱在众多工程行业都有着广泛应用。以某罐式集装箱作为研究对象,利用三维建模软件搭建该罐式集装箱的整体模型,并对其主要分析结构进行简化,再将其导入到有限元分析软件ANSYS中基于已确定的典型工况对其进行结构静力学分析。基于ANSYS的分析结果,得出了相应的等效应力云图,验证了罐式集装箱结构在设计上的安全性与可靠性。

LOCKING-FREE DEGENERATED ISOPARAMETRIC SHELL ELEMENT

An 8-noded locking-free degenerated isoparametric shell element is presented. A revised interpolation for shear strain terms was constructed in natural co-ordinate system such that all necessary modes (translation, rotation and constant curvature) are preserved, which can be used to eliminate shear locking. A revised interpolation for membrane strains was produced in the local Cartesian co-ordinate system to overcome membrane locking behavior. The new 8-noded element has the proper rank, with the requisite number of zero eigenvalues each associated with a rigid mode. The element does not exhibit membrane or shear locking for large span-thickness ratio. The element does not form element mechanisms or extra spurious zero energy modes. Therefore, it can be used for both thin and thick shells.

基于流固耦合的骨架膜结构优化

为了实现骨架膜结构的优化设计,基于ANSYS平台考虑了膜结构在风荷载作用下的结构变形对建筑表面风压分布的影响,采用双向流固弱耦合的方法对某低矮类膜建筑在不同风向角下的定常风场特性与其围护骨架膜结构的风致结构受力进行了研究,并以此为依据采用响应面法对骨架膜结构中各杆件的截面尺寸进行了优化.结果表明:仿真所得建筑表面风压分布与同类模型风压实测数据、风洞试验数据以及国家相应规范吻合较好;不同风向角下骨架膜结构的受力状态不同,其中90°风向为该结构的最不利工况;采用响应面法得到了该骨架膜结构中各杆件的截面尺寸对结构受力的影响关系,并将该结构整体质量降低了14.73%.

气-膜耦合作用对充气薄膜管动力特性的影响

充气薄膜管属于柔性结构,荷载作用下产生的变形,会引起其内压改变,进而导致外围薄膜刚度的变化,对其变形产生重要影响,表现出内充气体压力与外围薄膜变形相互耦合的特点。该文采用有限元方法分析气-膜耦合作用对充气薄膜管动力特性的影响及其随影响因素的变化规律。通过将内充气体看作小扰动线性势流以考虑气-膜耦合作用以及内充气体附加质量的影响;通过建立内充气体的三种等效模型,分别将内充气体作用等效为外围薄膜静力边界条件、考虑内充气体附加质量影响的静力边界条件以及小扰动线性势流体,并将其相应的有限元分析结果进行对比,研究气-膜耦合作用和内充气体附加质量对充气薄膜管自振特性的影响及其随初始内压、长细比、膜厚以及端部约束类型的变化规律。研究结果表明:气-膜耦合作用以及内充气体的附加质量对低阶自振模态没有明显影响;气-膜耦合作用对自振频率有较显著的影响作用,而内充气体附加质量的影响则较小;随初始内压和长细比的增加,气-膜耦合作用对频率的影响体现出因阶次不同而不同的变化规律;气-膜耦合作用对频率的影响随膜厚的增加而降低,随约束程度的减弱而增强。该文的研究成果揭示了气-膜耦合作用对充气薄膜管自振特性的影响规律,有助于深入认识充气薄膜管的动力行为,确保其设计计算的合理性和可靠性。

气承式膜结构基础受力特征仿真分析

当前对于气承式膜结构的基础受力特征的研究较为匮乏,相关规范中对基础受力分析鲜有提及。影响气承式膜结构基础受力的主要因素包括结构内压、布索方式、外荷载等,实际工程中往往采用有限元分析方法对气膜基础受力情况进行研究。基于岱海大跨度矩形气承式膜结构的缩尺模型,以结构内压为参数对斜向索网及纵横向拉索布置的气承式膜结构基础所受上拔力进行ANSYS有限元分析。得到基础所受上拔力的分布及变化情况,总结相关规律,为实际工程应用提供参考。结果表明:基础所受上拔力随结构内压的增大而增大;斜向索网布置下基础受力分布较为均匀,基础在边中点位置受到最大上拔力;纵横向拉索布置下基础在布索位置受到较大上拔力,且此类布索方式较斜向索网布置基础受到的上拔力更大;两种布索方式下基础所受上拔力随内压增大而接近线性增大,且上拔力较大的位置其增幅也更大。