Wave propagation in an elastic tube: a numerical study

In this paper, a fluid–wall interaction model, called the elastic tube model, is introduced to investigate wave propagation in an elastic tube and the effects of different parameters. The unsteady flow was assumed to be laminar, Newtonian and incompressible, and the vessel wall to be linear-elastic, isotropic and incompressible. A fluid–wall interaction scheme is constructed using a finite element method. The results demonstrate that the elastic tube plays an important role in wave propagation. It is shown that there is a time delay between the velocity waveforms at two different locations and that the peak velocity increases while the low velocity decreases in the elastic tube model, contrary to the rigid tube model where velocity waveforms overlap each other. Compared with the elastic tube model, the increase of the wall thickness makes wave propagation faster and the time delay cannot be observed clearly, however, the velocity amplitude is reduced slightly due to the decrease of the internal radius. The fluid–wall interaction model simulates wave propagation successfully and can be extended to study other mechanical properties considering complicated geometrical and material factors.     

横向冲击载荷下不同约束泡沫铝夹芯圆管的动态响应

以泡沫铝夹芯圆管为研究对象,采用数值模拟研究了横向冲击载荷作用下4种不同约束夹芯圆管的动态响应。研究了夹芯圆管的几何参数、冲击速度和芯层泡沫铝相对密度对夹芯圆管力学行为的影响,对比分析了不同约束条件对泡沫铝夹芯圆管变形模式和吸能性能的影响。结果表明:随着内管直径的减小、冲击速度的增大和芯层泡沫铝相对密度的增大,泡沫铝夹芯圆管的比吸能增大;内管壁厚的增加使无约束和倾斜约束下夹芯圆管的比吸能增大,对侧壁约束和组合约束下夹芯圆管的吸能影响不明显;采用一定的外部约束条件是提高泡沫铝夹芯圆管吸能性能的一种可行的方法。     

沿海软粘土取土质量的对比分析

本文在简要介绍固定活塞式薄壁取土器取土技术基础上，详尽分析了在连云港、赤湾港和广深路所进行的软粘土取土技术对比试验结果。试验结果表明，土样扰动对于室内所测定的力学性指标影响极大，但对物理性指标影响不大。研究结果还有力地证明，只要操作正确，采用薄壁取土器能取得高质量土样，而采用敞口式厚壁取土器取出的土样扰动较大。     

环空流道液流失速水击特性及其成因的研究

环空流道与圆管的结构差异,使得其失速水击特性及其成因亦有不同。为了对其进行分析研究,利用PIV对套管环空和圆管流水击流场进行拍摄,并通过Tecplot显示流场,提取轴向速度和径向速度、等速度线等参数加以分析;同时采用高精度智能动态压力传感器采集了套管环空内、外壁及内管内壁压力,对环空中水击特性进行了进一步研究。结论是:水击发生时环空断面外壁面水击压力大于内壁面水击压力;同初速情况下环空液流水击压力大于圆管流水击压力;环空中水击压力衰减速度快于圆管中;涡流是水击压力衰减以及速度变化的主要因素;水击压力振荡变化主要是压力涡流引起的断面能量的相互转化形成的。     

Bioluminescence-based imaging technique for pressure measurement in water

The dinoflagellate Pyrocystis lunula emits light in response to water motion. We developed a new imaging technique for measuring pressure using plankton that emits light in response to mechanical stimulation. The bioluminescence emitted by P. lunula was used to measure impact water pressure produced using weight-drop tests. The maximum mean luminescence intensity correlated with the maximum impact pressure that the cells receive when the circadian and diurnal biological rhythms are appropriately controlled. Thus, with appropriate calibration of experimentally determined parameters, the dynamic impact pressure can be estimated by measuring the cell-flash distribution. Statistical features of the evolution of flash intensity and the probability distribution during the impacting event, which are described by both biological and mechanical response parameters, are also discussed in this paper. The practical applicability of this bioluminescence imaging technique is examined through a water drop test. The maximum dynamic pressure, occurring at the impact of a water jet against a wall, was estimated from the flash intensity of the dinoflagellate.     

Effect of elastic wall motion on oscillatory flow of micropolar fluid in an annular tube

Summary Oscillatory flow of a micropolar fluid in an annular tube is investigated. The outer wall of the tube is taken to be elastic and the variation in the diameter of the elastic wall due to pulsatile nature of pressure gradient is assumed to be small. The wall motion is governed by a tube law. The nonlinear equations governing the fluid flow and the tube law are solved using perturbation analysis. The steady-streaming phenomenon due to the interaction of convected inertia with viscous effects is studied. The analysis, is carried out for zero mean flow rate. It presents the effects of the elastic nature of the wall combined with micropolar fluid parameters on the mean pressure gradient and wall shear stress for different catheter sizes and frequency parameters. It is found that the effect of micropolarity is of considerable importance for small steady-streaming Reynolds number. Also, it is observed that the relationship between mean pressure gradient and the flow rate depends on the amplitude of the diameter variation, flow rate waveforms and the phase difference between them.     

Effect of wall conditions on DDT in hydrogen–oxygen mixtures

Detonation in ducts is usually studied assuming adiabatic walls because of the high kinetic energy due to the incoming flow being supersonic. In the present work, numerical simulations of deflagration-to-detonation transition (DDT) using a detailed chemical reaction model are performed under adiabatic and isothermal boundary conditions in a tube with no-slip walls. The results show a local explosion driving DDT, which occurs near the tube wall in the case of an adiabatic wall, but close to the flame front in the case of an isothermal wall. Furthermore, to examine the effects of a turbulent boundary layer, a simulation using the Baldwin–Lomax turbulence model is carried out. In the case of the isothermal wall, there is again a local explosion near the tube wall, which leads to detonation. In summary, the present study confirms that the boundary conditions affect the transition to detonation and that the boundary layer is a key component of DDT.     

平面应力下薄壁管材连续矫直压弯量力学模型与数值解法

薄壁管材在连续矫直过程中,各矫直辊组的压弯量作为核心工艺参数直接决定了薄壁管材的矫直精度。而目前现场仍沿用经验图表结合人工经验和反复试矫对其进行估定,亟待建立针对性的压弯量数学模型以指导生产。为此从薄壁管材的结构特点和矫直辊系组成出发,构建了针对压弯量计算的简化悬臂结构模型,基于相关假设和弹塑性相关理论,分别确定了平面应力状态下弹性区和弹塑性区管材横截面的弯矩模型,运用虚功原理建立了矫直辊压弯量力学模型,并给出了数值计算方法,完成了程序开发。经有限元动态仿真试验证明了模型的正确性和适用性,通过对典型管材数据的计算绘制了一系列工艺参数曲线,得到管材轴线弯曲半径和压弯量随管材直径、壁厚和屈服极限的变化关系,为现场压弯量的调整提供理论依据。     

Evaporation of laminar,falling liquid film on a horizontal cylinder

An analytical study of falling film evaporation on a horizontal tube is accomplished for the case of constant heat flux condition imposed at the wall. The process of evaporation from the liquid film interface is conjugated to thermal conduction in the wall of the tube around its periphery. In addition, the flow of the film is considered to be thermally developing throughout its travel around the tube. Influence of various governing parameters on local and average heat transfer coefficient is presented. The theoretical investigation is compared with the available experimental data. The agreement is found to be satisfactory.     

Effect of void fraction and friction factor models on the prediction of pressure drop of R134a during downward condensation in a vertical tube

The theoretical flow models of homogeneous and separated flow are applied to in-tube condensation to predict the pressure drop characteristics of R134a. The homogeneous flow model is modified by ten different dynamic viscosity correlations and various alternative correlations of total, frictional and momentum pressure drops to take account of the partial condensation inside the tube. Numerical analyses were performed to determine the average and local homogeneous wall shear stresses and friction factors by means of a CFD program. The equivalent Reynolds number model is modified by six different two-phase friction factors to determine the total condensation pressure drop in the separated flow model. The refrigerant side total pressure drops, frictional pressure drops, friction factors and wall shear stresses are determined within a ±30% error band. The importance of using the alternative total, momentum and frictional pressure drop correlations for the homogeneous flow model is also shown.     

High-Velocity Laminar and Turbulent Flow in Porous Media

We model high-velocity flow in porous media with the multiple scale homogenization technique and basic fluid mechanics. Momentum and mechanical energy theorems are derived. In idealized porous media inviscid irrotational flow in the pores and wall boundary layers give a pressure loss with a power of 3/2 in average velocity. This model has support from flow in simple model media. In complex media the flow separates from the solid surface. Pressure loss effects of flow separation, wall and free shear layers, pressure drag, flow tube velocity and developing flow are discussed by using phenomenological arguments. We propose that the square pressure loss in the laminar Forchheimer equation is caused by development of strong localized dissipation zones around flow separation, that is, in the viscous boundary layer in triple decks. For turbulent flow, the resulting pressure loss due to average dissipation is a power 2 term in velocity.     

Onset of flow instability in a heated capillary tube

We study the stability of flow in a heated capillary tube with an evaporating meniscus. The behavior of the vapor/liquid system, which undergoes small perturbations, is analyzed by linear approximation, in the frame of a one-dimensional model of capillary flow, with a distinct interface. The effect of the physical properties of both phases, the wall heat flux and the capillary sizes, on the flow stability is studied. The velocity, pressure and temperature oscillations in a capillary tube with a constant wall heat flux or a constant wall temperature are determined. A scenario of a possible process at small and moderate Peclet numbers corresponding to the flow in capillaries is considered. The boundaries of stability, subdividing the domains of stable and unstable flows, are outlined, and the values of geometrical and operating parameters corresponding to the transition from stable to unstable flow are estimated. It is shown that the stable capillary flow occurs at relatively small wall heat fluxes, whereas at high ones, the flow is unstable, with continuously growing velocity, pressure and temperature oscillations.     

机抖激光陀螺敏感轴动态偏移误差参数估计与补偿

针对振动环境下机抖激光陀螺敏感轴产生动态偏移造成惯导系统精度下降的问题,从理论上推导了机抖激光陀螺敏感轴动态偏移误差模型,并结合工程实际建立了简化的误差模型;在此简化误差模型基础上,推导了陀螺敏感轴动态偏移造成的等效陀螺漂移与比力、角速度的耦合关系;将机抖激光陀螺敏感轴动态偏移误差归结为9个待辨识参数,针对该模型中的待辨识参数设计了标定方法,并给出了标定实验设计原则;以姿态误差为观测量进行振动实验对待辨识参数进行估计,振动实验结果表明,在10 min线振动时间内,机抖激光陀螺敏感轴动态偏移误差补偿后,捷联惯导系统纯惯导速度误差减小30%以上。     

Simultaneous heat and mass transfer inside a vertical tube in evaporating a heated falling alcohols liquid film into a stream of dry air

A numerical study of the evaporation in mixed convection of a pure alcohol liquid film: ethanol and methanol was investigated. It is a turbulent liquid film falling on the internal face of a vertical tube. A laminar flow of dry air enters the vertical tube at constant temperature in the downward direction. The wall of the tube is subjected to a constant and uniform heat flux. The model solves the coupled parabolic governing equations in both phases including turbulent liquid film together with the boundary and interfacial conditions. The systems of equations obtained by using an implicit finite difference method are solved by TDMA method. A Van Driest model is adopted to simulate the turbulent liquid film flow. The influence of the inlet liquid flow, Reynolds number in the gas flow and the wall heat flux on the intensity of heat and mass transfers are examined. A comparison between the results obtained for studied alcohols and water in the same conditions is made.     

聚氯乙烯弹性体静动态力学性能及本构模型

为揭示聚氯乙烯弹性体在静、动态载荷下的力学性能,采用万能材料试验机和改进的分离式霍普金森压杆实验装置获得了材料在应变率为0.001、0.01、0.1、1 510、2 260和3 000 s?1下的应力应变曲线,并以屈服强度为整形器优选参数,对比了紫铜、铜版纸和铅等3种整形器材料的整形效果。使用修正的ZWT非线性黏弹性本构模型描述聚氯乙烯弹性体在静、动态载荷下的力学性能。结果表明:聚氯乙烯弹性体在静态载荷下具有应变率效应和显著的超弹性特性,动态载荷下表现出较明显的应变率效应和较强的抗变形能力,且静动态载荷下的力学行为受应变历史影响较大。3种整形器材料中铜版纸的整形效果最好。修正后的ZWT非线性黏弹性本构模型能够得到统一参数的本构表达式,且各应变率下的拟合结果与实验结果具有较好的一致性。     

风力机叶片翼型动态试验技术研究

风力机叶片动态振荡过程往往伴随着俯仰和横摆同时进行, 以前对许多动态问题不清楚的阶段, 工程上不惜以增加叶片重量为代价而采用偏安全的设计, 通常忽略横摆振荡的影响; 大型风力机设计对获取翼型更加全面、准确的动态载荷提出了更高要求, 研究横摆振荡对翼型动态气动特性的影响规律具有重要意义. 本文首次开展翼型横摆振荡动态风洞试验研究, 采用“电子凸轮”技术代替机械凸轮实现了振荡频率和振荡角度的无级变化, 基于设计的电子外触发装置实现了对动态流场的实时测量, 实现了风洞来流、模型角位移和动态压力数据的同步采集, 分别开展了翼型静态测压、俯仰/横摆动态测压、粒子图像测速和荧光丝线等试验研究, 试验结果准度较高、规律合理; 分析了动态试验洞壁干扰影响机制. 研究表明, 横摆振荡翼型的气动曲线也存在明显迟滞效应; 随着振荡频率升高, 翼型俯仰和横摆振荡下的气动迟滞性均增强; 翼型俯仰振荡正行程的动态失速涡破裂有所延迟; 洞壁与模型端部交界处的强三维效应对翼型压力分布影响较大; 建立的横摆振荡试验技术可为风力机动态掠效应的研究提供技术支撑.      

Mechanics of formation and rupture of human aneurysm

The mechanical response of the human arterial wall under the combined loading of inflation, axial extension, and torsion is examined within the framework of the large deformation hyper-elastic theory. The probability of the aneurysm formation is explained with the instability theory of structure, and the probability of its rupture is explained with the strength theory of material. Taking account of the residual stress and the smooth muscle activities, a two layer thick-walled circular cylindrical tube model with fiber-reinforced composite-based incompressible anisotropic hyper-elastic materials is employed to model the mechanical behavior of the arterial wall. The deformation curves and the stress distributions of the arterial wall are given under normal and abnormal conditions. The results of the deformation and the structure instability analysis show that the model can describe the uniform inflation deformation of the arterial wall under normal conditions, as well as formation and growth of an aneurysm under abnormal conditions such as the decreased stiffness of the elastic and collagen fibers. From the analysis of the stresses and the material strength, the rupture of an aneurysm may also be described by this model if the wall stress is larger than its strength.     

基于ANN-GA协同寻优的动态拉伸试样尺寸优化方法

材料动态拉伸力学性能测试中,动态拉伸试样的几何尺寸对测试结果的准确性与有效性有着较大影响。为对动态拉伸试样的结构进行优化设计,以使其在动态拉伸过程中更好地满足一维应力与变形均匀等基本假设。首先,建立了量化的试样测量准确度指标,即应力平衡达到时间、变形均匀度、非轴向应力相对水平、过渡段相对变形。然后,对试样结构参数进行正交试验设计,通过数值模拟的方法得到了关于试样尺寸与测量准确度指标的正交试验数据库,并对正交试验数据库进行多目标正交试验矩阵分析,得到了试样结构参数对各测量准确度指标影响的主次顺序和规律。最后,以正交试验数据库为训练集,采用人工神经网络（artificial neural network, ANN）协同遗传算法（genetic algorithm, GA）的全局寻优方法对试样的结构尺寸进行优化设计,得到了试样的最优结构尺寸,并对最优尺寸的有效性进行了验证。结果表明,优化后的试样结构在材料动态拉伸力学性能测试精度上的表现明显得以提升。因此,采用ANN-GA协同优化的方法对动态拉伸试样的结构进行优化具有可行性和有效性。