一种新型的细胞骨架力学模型

确定细胞骨架的结构和力学特性,建立和完善细胞骨架力学模型,是研究细胞在外界机械刺激下的复杂力学响应的关键。本文基于柔性结构设计中的找形分析,提出了一种新型的细胞骨架力学模型——找形模型。找形模型依靠随机理论生成模型中的单元,利用找形分析确定细胞模型的最终形状,模型更加接近真实的细胞骨架结构。与经典的细胞骨架力学模型(如泡沫模型、张力整合模型、索网模型等)相比,找形模型反映了细胞骨架结构的多样性和复杂性,符合细胞处于预应力状态的试验观测;找形模型计算出的细胞弹性模量为103Pa数量级,与大多数的细胞试验结果相符;另外,找形模型还可以分析细胞骨架组成成分的含量、几何尺寸和力学属性对细胞刚度的影响。     

植物细胞力学模型研究进展

植物细胞在结构上具有特殊性, 即细胞壁和胞内物质在力学性质上差异很大. 因此其力学模型的研究具有特殊意义. 植物细胞力学模型是植物组织力学行为的研究基础, 是连接宏观与微观生物力学发展的桥梁, 在农业和食品加工等领域中有重要的潜在应用价值. 本文就目前国际上在细胞力学模型研究中所采用的模型形式和理论分析与数值模拟等方法做较全面而简要的介绍, 并对本领域中存在的现实问题加以论述, 希望能够对我国在细胞力学领域的研究有所帮助.      

力敏感受体介导细胞功能调控的力学生物学研究

细胞膜是细胞与外部环境进行物质与能量交换的界面,是调节细胞正常生命活动的重要结构基础.细胞膜上力敏感受体可通过力学作用方式参与并影响细胞的力信号转导等功能.整合素和钙黏素是细胞膜上典型的力敏感受体,可介导细胞与细胞周围基质或邻近细胞发生力学作用,并将力学刺激信号转导为生化信号,进而激活细胞内一系列应答反应,最终影响细胞生长、分化、增殖、凋亡和迁移等功能.力敏感受体介导细胞功能调控研究已成为探索细胞主动响应外界复杂力学微环境的力学生物学机制的关键,为进一步深入认识生理和病理状态下细胞功能变化规律,为揭示疾病的发生、发展机制提供重要的力学生物学理论与实验依据.本文总结了力敏感受体介导细胞功能调控的国内外研究进展;介绍了黏附界面处典型力敏感受体的结构和功能;总结了这些力敏感受体参与的细胞力信号感知与响应的数理模型;概述了细胞通过力敏感受体进行力学信号转导的过程;介绍了黏附介导细胞功能调控的力学生物学过程和机制;简述了体外构建模拟细胞力学微环境中细胞-细胞外基质和细胞-细胞力学相互作用的技术;指出了力敏感受体介导细胞功能调控的力学生物学研究发展趋势和未来方向.     

新的复合材料格栅加筋板的平铺等效刚度法

针对薄壁复合材料格栅加筋结构的受力特点,在改进原有力学假设的基础之上,推导了一种新的平铺等效刚度计算方法,它充分考虑了筋条和面板之间的相互作用.通过格栅单元结构布局形式的参数化表示,建立了通用的力学分析模型;该模型可用于分析各种结构布局形式和面板铺层方式下的结构总体屈曲问题,故对于航空航天结构设计非常有用.结合Rayleigh-Ritz方法,推导出了求解格栅加筋板屈曲载荷的通用线性特征方程;最后,分析了多种类型格栅结构的算例,并与现有的各种方法进行了比较,结果更为精确,因而对格栅加筋结构的优化设计具有很好的应用价值.     

心肌细胞大变形黏弹性模型及在实验中的应用

在衡量单个细胞力学行为的研究中,越来越多地采用结合实验的数值模拟方法. 在连续介质力学框架下,发展了一种新的心肌细胞本构模型,并与微管吮吸实验结合,探讨了心肌细胞的力学特性. 本构模型是对普遍使用的仅能用于小变形分析的标准线性固体模型的一种扩展,它将超弹性性能引入到黏弹性模型中,用以描述细胞的大变形黏弹性效应. 基于改进的本构模型,对心肌细胞微管吮吸实验过程进行了有限元模拟,并将计算结果与实验结果以及经典理论解进行了对比. 结果显示发展的本构模型适合细胞大变形问题的有限元数值模拟.      

CD44-配体相互作用的生物力学与功能调控

作为一种广谱表达的细胞粘附分子, I型跨膜糖蛋白CD44(cluster of differentiation 44)参与细胞增殖、分化、迁移, 血管生成等生物学过程,对于介导细胞信号转导, 调节组织稳态等功能具有关键作用. 特别地,CD44-选择素、CD44 -透明质酸相互作用介导的细胞粘附动力学在经典炎症反应、肿瘤转移或组织特异的肝脏免疫中具有重要作用.该综述分别从细胞层次粘附动力学、二维与三维条件下的分子层次反应动力学、原子层次微观结构以及胞内信号转导通路等方面综述了CD44 -选择素、CD44 -透明质酸相互作用的研究进展及尚待回答的生物力学问题.力学、物理因素对生命活动的不可或缺性逐渐被研究者们接受,力学医学、力学免疫学、力学组学等新概念相继提出. 生理、病理条件下,CD44 -配体相互作用介导的细胞粘附必将受到血流剪切、基底硬度等力学、物理微环境的调控,但是其调控机制还远不清楚. 基于此,本文就CD44 -配体相互作用相关的未来研究方向做出展望, 主要包括:力学、物理因素如何调控CD44 -配体相互作用介导的细胞粘附动力学及其内在机制;CD44 -配体相互作用反应动力学的力学调控规律及结构基础是什么;以及力学作用下CD44 -配体相互作用原子层次的微观结构如何发生动态演化.本文可为深入理解CD44 -配体相互作用的生物学功能及其结构功能关系提供线索.      

基础振动的弹性半空间理论的若干基本问题

由于机器基础设计的需要,在二十年代后期,德国和苏联都开始组织力量开展块式基础振动的计算理论的研究.到了五十年代,美、英、日等国也开始重视这一领域的工作.六十年代中期以来,成果颇为显著.这些成果不仅应用于机器基础的设计,而且还构成了地震工程中一个崭新的课题--土壤与结构的相互作用--的重要部分.根据所采用的力学模型,各种计算理论可分为几种类型: ...     

黄瓜卷须II：显微组织与力学行为的实验模拟

 对卷须显微组织的研究表明,它的一侧存在一个双层凝胶状细胞的纤维带,当卷须外端拴住支撑物后,此纤维带出现不对称的木质化收缩,引起卷须自盘卷. 老卷须的弯曲刚度远大于扭转刚度,因此盘卷成的螺旋弹簧在拉伸时刚度增加,盘卷圈数增加. 卷须的力学行为可以由预应力双层复合条的力学模型来模拟,据此,北京航空航天大学材料力学实验室开发了一个选修教学实验.     

微流控通道内细胞及其初级纤毛的力传导行为

细胞处于复杂的生理环境之下,附着在细胞表面的初级纤毛被认为是重要的力学信号传感器,其与细胞的代谢、发育、分裂和增殖等生理活动密切相关.为了研究细胞及其初级纤毛在微流体环境下的力传导行为,本文建立了力-电协同驱动下的矩形微流控通道和含有多孔黏弹性属性的贴壁细胞有限元模型系统.考察了细胞的细胞质和细胞核在振荡层流下的应力、应变、孔隙压力和孔隙流速等力学信号响应,量化研究了初级纤毛作为细胞独特的力学感受器的生物力学行为. 结果表明:细胞在振荡层流下的力学响应表现出和外加力-电驱动载荷相同的震荡规律.渗透率是细胞多孔弹性力学行为的主要影响因素. 初级纤毛是细胞主要的力学感受器,细胞可以通过纤毛长度和直径调节其力学感受敏感性(应力影响区域),随着初级纤毛长度的增大, 其纤毛挠曲刚度减小, 但是敏感性增大.模型的建立为进一步研究微流体剪切作用下的细胞生长、分化等微观机理提供基础,同时也为检测细胞微结构器(纤毛等蛋白链)的力学性能提供了理论技术支持.      

软骨组织工程构建中的生物力学

关节软骨是关节表面具有弹性的承重组织, 其结构复杂, 由固体相和液体相组成. 固体相包括胶原纤维、蛋白多糖等, 属纤维增强型复合结构; 液体相包括水、电解质等.关节软骨提供了一个低磨损和低摩擦的光滑界面, 起缓冲振动和传递载荷等支撑作用. 由于膝关节承受的运动量大、应力高, 关节软骨损伤在临床上较为常见. 但软骨内没有血管, 代谢缓慢, 其损伤后难以实现自我修复. 组织工程从理论上建立了一种治疗软骨缺损的理想方法, 但尚未成为临床上常规的治疗选择. 如何获得结构和功能相匹配, 同时适用于临床治疗的工程软骨, 至今仍是亟需解决的问题.在体外构建功能化工程软骨, 关键在于运用生物反应器对组织施加合适的力学载荷: 首先保证工程软骨复合体内信号分子、营养和废物的有效运输; 其次对支架内种子细胞产生特定的力学刺激; 同时促进细胞外基质结构与功能的适应性发展.本文对力学载荷在软骨组织工程构建中的应用进展加以综述: 按照作用于组织层面的力学载荷传递所需的介质属性, 将其分为液体介导、固体介导和其他媒质介导三种类型, 重点关注不同载荷对工程软骨功能化构建的作用和效果; 分析讨论软骨组织工程构建中存在的关键生物力学问题; 总结和展望软骨组织工程未来的发展趋势.软骨组织工程体外培养需要考虑力学载荷和生化刺激的耦合作用; 在合适的生化条件下进行滚动、滑动和压缩复合加载, 将有利于工程软骨的体外功能化构建.      

Suspension-based rheological modeling of crystallizing polymer melts

The applicability of suspension models to polymer crystallization is discussed. Although direct numerical simulations of flowing particle-filled melts are useful for gaining understanding about the rheological phenomena involved, they are computationally expensive. A more coarse-grained suspension model, which can relate the parameters in a constitutive equation for the two-phase material to morphological features, such as the volume fractions of differently shaped crystallites and the rheological properties of both phases, will be more practical in numerical polymer processing simulations. General issues, concerning the modeling of linear and nonlinear viscoelastic phenomena induced by rigid and deformable particles, are discussed. A phenomenological extension of linear viscoelastic suspension models into the nonlinear regime is proposed. A number of linear viscoelastic models for deformable particles are discussed, focusing on their possibilities in the context of polymer crystallization. The predictions of the most suitable model are compared to direct numerical simulation results and experimental data.     

空间充气管展开动力学研究进展

最近空间充气展开结构引起了人们的广泛关注, 这是因为空间充气展开结构具有折叠体积小, 重量轻和展开可靠性高等优点, 于是一些空间计划也开始考虑利用充气展开结构的优势.然而, 在空间充气展开结构应用到太空之前还有一些重要的问题需解决, 其中之一就是控制和掌握充气展开结构的充气展开动力学特性.充气管用于驱动展开并支撑空间结构是一种重要的构件. 围绕充气管在展开过程中的动力学问题, 详细地综述了近20年来国际上的研究进展情况. 首先讨论了有关的充气展开模型, 包括非线性铰链模型、卷曲折叠管展开模型、控制体积模型、能量法模型和流-固耦合模型等.然后介绍了有限元模拟研究进展, 并分别评述了$Z$形、卷曲、多边形以及变直径伸缩等折叠管的充气展开实验研究. 文章最后指出了充气管的充气展开的理论、有限元模拟以及实验在今后值得关注的研究方向.     

大长细比导弹的气动弹性降阶模型

对于大长细比导弹,需要在设计阶段准确计算气动弹性/气动伺服弹性,但其复杂的气动力给计算带来困难,因此气动力降阶模型是突破大长细比导弹跨音速气动弹性分析与控制瓶颈的关键技术.虽然气动力模型降阶方法已在预测二维机翼结构的气动弹性方面取得重要进展,但几乎未见关于全机模型的气动力降阶模型研究报道.本文基于递归Wiener模型的气动力降阶方法,利用CFD计算的气动力作为模型辨识数据,用鲁棒子空间和Levenberg-Marquardt算法辨识降阶模型参数,建立了大长细比导弹气动力降阶模型.在此基础上与大长细比导弹有限元模型相结合,构造出气动弹性降阶模型,并在数值仿真中测试气动弹性降阶模型在不同马赫数下的适用性.数值仿真结果表明,该气动弹性降阶模型能够精确预测导弹模型在不同飞行条件下的非定常气动力和导弹模型的气动弹性频率响应特性.     

In-plane biaxial crushing of honeycombs—: Part II: Analysis

The in-plane biaxial crushing experiments on polycarbonate honeycomb presented in Part I are simulated using large scale finite element models. The models account for nonlinearities in geometry and due to contact while the polycarbonate is modeled as an elastic-powerlaw viscoplastic solid. Full-scale simulations of the uniaxial crushing of this honeycomb were shown in the past to reproduce experiments with accuracy. In biaxial crushing, it was not practical to model specimens the same size as those in the experiments due to computational limitations; instead, a smaller model with 10×11 cells was adopted. Results from simulations of seven of the crushing experiments in Part I with various biaxiality ratios are presented. Through parametric studies it is demonstrated that the size of the specimen and friction between the specimen and the loading surfaces affect the initial elastic parts of the stress–displacement responses and the onset of instability. By contrast, for average crushing strains higher than approximately 10%, their effect was relatively small and the calculated responses were in good agreement with the experimental ones. As a consequence, the energy absorption capacity was predicted to good accuracy for all biaxiality ratios. In addition, many of the modes of cell collapse seen in the experiment are reproduced in the simulations.     

New flow rules in elasto-viscoplastic constitutive models for spheroidal graphite cast-iron

A specific flow rules and the corresponding constitutive elasto-viscoplastic model combined with new experimental strategy are introduced in order to represent a spheroidal graphite cast-iron behaviour on a wide range of strain, strain rate and temperature. A “full model” is first proposed to correctly reproduce the alloy behaviour even for very small strain levels. A “light model” with a bit poorer experimental agreement but a simpler formulation is also proposed. These macroscopic models, whose equations are based on physical phenomena observed at the dislocation scale, are able to cope with the various load conditions tested – progressive straining and cyclic hardening tests – and to correctly describe anisothermal evolution. The accuracy of these two models and the experimental databases to which they are linked is estimated on different types of experimental tests and compared with the accuracy of more standard Chaboche-type constitutive models. Each test leads to the superiority of the “full model”, particularly for slow strain rates regimes. After developing a material user subroutine, FEM simulations are performed on Abaqus for a car engine exhaust manifold and confirm the good results obtained from the experimental basis. We obtain more accurate results than those given by more traditional laws. A very good correlation is observed between the simulations and the engine bench tests.     

Improved droplet breakup models for spray applications

The current study examines the performance of two zero-dimensional (0D) aerodynamically-induced breakup models, utilized for the prediction of droplet deformation during the breakup process in the bag, multi-mode and sheet-thinning regimes. The first model investigated is an improved version of the widely used Taylor analogy breakup (TAB) model, which compared to other models has the advantage of having an analytic solution. Following, a model based on the modified Navier–Stokes (M-NS) is examined. The parameters of both models are estimated based upon published experimental data for the bag breakup regime and CFD simulations with Diesel droplets performed as part of this work for the multi-mode and sheet-thinning regimes, for which there is a scarcity of experimental data. Both models show good accuracy in the prediction of the temporal evolution of droplet deformation in the three breakup regimes, compared to the experimental data and the CFD simulations. It is found that the best performance of the two is achieved with the M-NS model. Finally, a unified secondary breakup model is presented, which incorporates various models found in the literature, i.e. TAB, non-linear TAB (NLTAB), droplet deformation and breakup (DDB) and M-NS, into one equation using adjustable coefficients, allowing to switch among the different models.     

Micromechanical modelling of the static and cyclic loading of an Al 2124-SiC MMC

The objective of this study was to use micromechanical finite element models to simulate both the static and cyclic mechanical behaviour of a metal matrix composite: a forged Al 2124 alloy reinforced with 17% SiC particles, at two different temperatures: room temperature and 150°C. In the simulations, periodic unit cell models incorporating the explicit representation of the matrix and the reinforcing particles in both 2D and 3D, were used. Micromechanical models with both idealised and realistic reinforcing particle shapes and distributions were generated. The realistic particle shapes and distributions were inferred from experimental SEM micrographs. The pattern and intensity of the plastic deformation within the matrix was studied and the macroscale behaviour of the composite was inferred from average stress and strain values. In order to include the effects of residual stresses due to the processing of the material, a quenching simulation was performed, prior to the mechanical loading, and its effects on the macroscopic tensile behaviour of the MMC was assessed. The effects of removing the periodicity constraint on the models by using a cell embedding technique was investigated. In order to try and model the deformation behaviour of the matrix more accurately, crystal plasticity models, which included the explicit representation of individual grains were examined for different matrix grain morphologies. The results of the simulations were compared with experimental results for the MMC in terms of macroscopic tensile stress–strain curves. Finally, the effects of different matrix strain hardening models were examined in order to investigate the cyclic behaviour of the MMC.     

Specified discharge velocity models for numerical simulations of laminar vortex rings

We numerically and theoretically investigate the flow generated at the exit section of a piston/cylinder arrangement that is generally used in experiments to produce vortex rings. Accurate models for the velocity profile in this section (also called specified discharge velocity, SDV models) are necessary in (i) numerical simulations of laminar vortex rings that do not compute the flow inside the cylinder and (ii) in slug-models that provide a formula for the total circulation of the flow. Based on the theoretical and numerical analysis of the flow evolution in the entrance region of a pipe, we derive two new and easy to implement SDV models. A first model takes into account the unsteady evolution of the centerline velocity, while the second model also includes the time variation of the characteristics of the boundary layer at the exit plane of the vortex generator. The models are tested in axisymmetric direct numerical simulations of vortex rings. As distinguished from classical SDV model, the new models allow to accurately reproduce the characteristics of the flow. In particular, the time evolution of the total circulation is in good agreement with experimental results and previous numerical simulations including the vortex generator. The second model also provides a more realistic time evolution of the vortex ring circulation. Using the classical slug-model and the new correction for the centerline velocity, we finally derive a new and accurate analytical expression for the total circulation of the flow.     

Computational and experimental analysis of supersonic air ejector: Turbulence modeling and assessment of 3D effects

Numerical and experimental analyses are performed on a supersonic air ejector to evaluate the effectiveness of commonly-used computational techniques when predicting ejector flow characteristics. Three series of experimental curves at different operating conditions are compared with 2D and 3D simulations using RANS, steady, wall-resolved models. Four different turbulence models are tested: k–ε, k–ε realizable, k–ω SST, and the stress–ω Reynolds Stress Model. An extensive analysis is performed to interpret the differences between numerical and experimental results. The results show that while differences between turbulence models are typically small with respect to the prediction of global parameters such as ejector inlet mass flow rates and Mass Entrainment Ratio (MER), the k–ω SST model generally performs best whereas ε-based models are more accurate at low motive pressures. Good agreement is found across all 2D and 3D models at on-design conditions. However, prediction at off-design conditions is only acceptable with 3D models, making 3D simulations mandatory to correctly predict the critical pressure and achieve reasonable results at off-design conditions. This may partly depend on the specific geometry under consideration, which in the present study has a rectangular cross section with low aspect ratio.     

Slight asymmetry in the winding angles of reinforcing collagen can cause large shear stresses in arteries and even induce buckling

Many models of the mechanical response of arteries assume a reinforcement with two families of helically wound fibres of collagen of opposite pitch. Motivated by experimental observations, the consequences for the internal pressurisation of arteries of a slight asymmetry in the winding angles is investigated here. It is shown that a torsional shear stress is generated as a result of this flaw, with some common models of the mechanical response of arteries exhibiting significant shear stresses. If the shear stress is significant, then the corresponding model would not seem to be robust, given that an infinitesimal change in a model parameter results in a large change in system response, although it is also shown that there is a ‘magic-angle’ for fibre winding that eliminates torsional shear stress for many of the commonly used models. Finite Element simulations are used to further illustrate the main consequences of fibre asymmetry for some of the more common models of arterial response. If the fibre asymmetry is localised in a region, then simulations show that there is the possibility of significant bending of the artery centred in this region at physiological blood pressure.