HYPERSINGULAR INTEGRAL EQUATIONS FOR PERIODIC ARRAYS OF PLANAR CRACKS IN A PERIODICALLY-LAYERED ANISOTROPIC ELASTIC SPACE UNDER ANTIPLANE SHEAR STRESS

Hypersingular integral equations are derived for the problem of determining the antiplane shear stress around periodic arrays of planar cracks in a periodically-layered anisotropic elastic space. The unknown functions are directly related to the jump in the displacements across opposite crack faces. Once the integral equations are solved, crack parameters of interest, such as the clack tip stress intensity factors, may be readily computed.For some specific examples of the problem, the integral equations are solved numerically by using a collocation technique, in order to compute the relevant stress intensity factors.

Estimation of Bed Shear Stresses in the Pearl River Estuary

Mean and fluctuating velocities were measured by use of a pulse coherent acoustic Doppler profiler （PC-ADP） and an acoustic Doppler velocimeter in the tidal bottom boundary layer of the Pearl River Estuary. The bed shear stresses were estimated by four different methods： log profile （LP）, eddy correlation （EC）, turbulent kinetic energy （TKE）, and inertial dissipation （ID）. The results show that （a） all four methods for estimating bed stresses have advantages and disadvantages, and they should be applied simultaneously to obtain reliable frictional velocity and to identify potential sources of errors; （b） the LP method was found to be the most suitable to estimate the bed stresses in non-stratified, quasi-steady, and homogeneous flows; and （c） in the estuary where the semi-diurnal tidal current is dominant, bed shear stresses exhibit a strong quarter-diurnal variation.

New interpretation of specific sign of Reynolds stress in the boundary layer on a flat plate

The specific sign of Reynolds stress in the boundary layer on a flat plate at zero incidence is newly interpreted in present paper based on the theory of vortex-induced vortex. It avoids some problems appeared in a traditional explanation, on the basis of relationship between mean and fluctuating flows due to the transport of momentum. Through the analysis of local flow field in the immediate neighborhood of wall, the characteristics of Reynolds stress are identified through introducing turbulence-induced small-scale streamwise eddies above the flat plate. The positive Reynolds stress is theoretically verified. And such new interpretation illustrates that the generation of Reynolds stress, as well as fluctuating velocity, is intrinsically independent of the mean flow. But its specific sign would be determined by the mean flow due to the inertial forces. Other features,such as the intensity relationship among three components of fluctuating velocity, are also presented.

Optimum Design for Shrink-fit Multi-layer Vessels under Ultrahigh Pressure Using Different Materials

Multi-layer pressure vessels are widely used in every field of high pressure technology.For the purpose of enhancing a vessels＇ load bearing capacity,a beneficial process like shrink-fit is usually employed.However,few documents on optimum design for multi-layer shrink-fit vessels made of different strength materials can be found,available data are mainly on two-layer vessels.In this paper,an optimum design approach is developed for shrink-fit multi-layer vessels under ultrahigh pressure by using different materials.Maximum shear stress theory is applied as design criteria.The inner and outer radii of a multi-layer vessel,as well as the material of each layer,are assumed to be known.The optimization mathematical model is,thereby,built.Lagrange multipliers method is required to obtain the optimal design formula of wall ratio（ratio of outer to inner radii） of each layer,from which the optimum formulas of shrinkage pressure and radial interference are derived with the superposition principle employed.These formulas are applicable for the optimization design of all multi-layer vessels made of different materials,or same materials.The formulas of the limit working pressure and the contact pressure show that the optimum wall ratio of each layer and limit working pressure are only related to all selected material strength and unrelated to the position of the layer placement in the vessel.However,shrinkage pressure is related to the position of the layer placement in the vessel.Optimization design of an open ended shrink-fit three-layer vessel using different materials and comparisons proved that the optimized multi-layer vessels have outstanding characteristics of small radial interference and are easier for assembly.When the stress of each layer is distributed more evenly and appropriately,the load bearing capability and safety of vessels are enhanced.Therefore,this design is material-saving and cost-effective,and has prospect of engineering application.

Wave Reflection at the Boundary Layer and Initial Factors of Atherosclerosis

The aim is to study the blood flow and vessel wall viscoelastic alterations at the boundary layer. Methods and Results: In 12 healthy men (18 - 52 years of age) at the different sites of the aorta peak velocity, net flow, flow acceleration has been investigated by Magnetic Resonance Angiography. In the aortic arch in the end systole blood flow separates into the opposite directed streams resulting in the wave superposition. At the outer curvature of the isthmus, flow acceleration in the initial diastole is 6.26 times higher than that in systole. Net flow from systole to diastole increases 2.5 ± 0.5 folds. From the end systole to the initial diastole there is a plateau on the net flow graph. At the outer curvature of isthmus, group wave at the boundary reflection, changes in phase at 180o. Herewith, flow wave oscillation frequency at the outer curvature is two times higher (2.5 Hz) than that at the inner (1.25 Hz). Conclusion: During the heart cycle, blood motion at the boundary layer, forms the surface wave and facilitates the blood structural rearrangement and flow. At the end systole, at the outer curvature of the isthmus, pulse pressure at the reflection is in the resonance with the end systolic pressure drop. Amplitude of the wall stress increases. Forming standing wave leads to the dissipation of the wall mechanical energy. Here, in the initial diastole, group wave, due to the wave reflection and frequency dispersion, facilitates the structural rearrangement/denudation of the vessel wall. By the removing resonance oscillation during the end systole/initial diastole between the heart and vessel wall, atherosclerosis can be avoided.

Study on the double-logarithmic profile of tidal flow velocity in the near-bed layers

Tidal current velocity profile in the near-bed layers has been widely studied. The results showed that velocity profile in the near-bed layer obviously departure from the traditional logarithmic profile, due to the acceleration or deceleration. Although the logarithmic linear profile can reduce the rate of deviation from this, only it is a lower-order approximate solution. In this paper, considering the unsteady and non-linear features of tidal motion, the double logarithmic profile near-bed layers in estuarine and coastal waters is established on the assumption that the turbulent shear stress along the water depth was parabolic distribution, and on the basis of Prandtl＇s mixing length theory and yon Karman＇s self-similar theory. Having been verified the data observed at the West Solent in the south of England, and comparison of the logarithmic linear profile, it found that the double logarithmic profile is more precious than the latter. At last, the discussed results showed that： （1） The parabolic distribution of the tidal shear stresses verified good by the field data and experimental data, can be better reflected the basic features of the tidal shear stress deviating from linear distribution that is downward when to accelerate, upward when to decelerate. （2） The traditional logarithmic velocity profile is the zero-order approximation solution of the double logarithmic profile, the logarithmic linear profile is the first order, and the logarithmic parabolic profile is the second order. （3） Ignoring the conditions of diffusion and convection in the tida movement, the double logarithmic profile can reflect the tidal properties of acceleration or deceleration, so that the calculation of the friction velocity and roughness length are more reasonable. When the acceleration or the deceleration is about zero, the double logarithmic profile becomes the logarithmic profile.

A NEW MEASURE FOR DIRECT MEASUREMENT OF THE BED SHEAR STRESS OF WAVE BOUNDARY LAYER IN WAVE FLUME

In this article, a shear plate was mounted on the bottom in a wave flume and direct measurements of the smooth and rough bed shear stress under regular and irregular waves were conducted with the horizontal force exerted on the shear plates by the bottom shear stress in the wave boundary layer. Under immobile bed condition, grains of sand were glued uniformly and tightly onto the shear plate, being prevented from motion with the fluid flow and generation of sand ripples. The distribution of the bottom mean shear stress varying with time was measured by examining the interaction between the shear plate and shear transducers. The relation between the force measured by the shear transducers and its voltage is a linear one. Simultaneous measurements of the bottom velocity were carried out by an Acoustic Doppler Velocimeter （ADV）, while the whole process was completely controlled by computers, bottom shear stress and velocity were synchronously measured. Based on the experimental results, it can be concluded that （1） the friction coefficient groews considerably with the increase of the Reynolds number, （2） the shear stress is a function varying with time and linearly proportional to the velocity. Compared with theoretical results and previous experimental data, it is shown that the experimental method is feasible and effective, A further study on the bed shear stress under regular or irregular waves can be carried out. And applicability to the laboratory studies on the initiation of sediments and the measurement of the shear stress after sediment imigration.

波、流共存时的水体挟沙力

从边界层理论建立了床面剪切力 ,进一步以能量理论导出了波、流共存时的水体挟沙力公式 ,并利用现场资料得出有关系数。

波、流共存时的床面剪切力和泥沙运动

利用边界层理论 ,导出了床面剪切力、综合摩阻系数、综合速度和综合摩阻速度 ,建立了波浪、水流共同作用下的泥沙起动和水体挟沙力公式。

设防水层的混凝土桥桥面铺装结构剪应力计算与分析

应用有限元法 ,对设防水层的水泥混凝土桥桥面沥青铺装结构进行剪应力计算 ,分析了防水层的厚度、模量、泊松比、沥青混凝土铺装层厚度和模量等参数对结构层剪应力的影响 ,认为层间最大剪应力主要取决于面层厚度和防水层模量 ,相同的防水层模量通过增加面层厚度是降低层间剪应力的最有效手段。在计算与分析的基础上 。

冻黏土与结构接触界面层单剪力学特性试验

为了解在荷载作用下，接触界面层发生的力学响应区别于冻土和结构材料本身力学特性，使用新研制的冻土与结构接触界面层力学试验仪，比较系统地研究人工冻黏土与粗糙钢板接触界面层单调剪切力学特性。分别从宏观和细观2个角度出发，分析冻土界面层的基本力学特性和受力变形机理。结果表明：随着法向荷载增大，接触界面的峰值剪应力、稳定剪应力、初始剪切劲度也增大，接触界面的抗剪强度与法向应力呈线性关系，可用摩尔-库仑准则来描述；冻土颗粒的剪切位移随冻土深度增大而减小，同一冻土深度内，剪切位移随法向应力的增大而增大，不同粗糙度的结构接触界面层变形特性基本相似，试验条件范围内接触界面层厚度为0.5～3.5 mm；界面层变形可分为冻土与结构接触界面上滑移变形和受结构约束的冻土剪切变形两部分；结构面粗糙度、冻土温度和法向应力等因素对接触界面层的力学特性具有显著影响。

波、流共存时的床面剪切力

利用边界层理论 ,导出了波、流共存时的床面剪切力 ,得出综合摩阻系数。

偏心环空层流顶替滞留层边界位置研究

水泥浆顶替过程中环空边界处的泥浆所受的剪应力最大,随着顶替界面从环空最大流速位置处逐渐向两边扩展,泥浆层各处所受的剪应力逐渐减小,直到环空边界处的剪应力等于泥浆的动切力,顶替界面不再扩展,形成稳定的顶替边界。根据水泥浆顶替泥浆过程中顶替界面达到稳定的平衡条件,建立了偏心环空不同周向角下套管和井壁处的泥浆滞留层边界位置计算模型,通过计算表明:随着套管偏心度的增大,宽间隙处的剖面顶替效率变化不大,而窄间隙处的剖面顶替效率却急剧下降,易形成泥浆窜槽;泥浆滞留层首先在井壁处出现,井壁处的泥浆滞留层厚度和范围要明显高于套管处,严重影响第二胶结面固井质量,应重点加强第二胶结面顶替效率和固井质量技术的研究。

水泥-乳化沥青混凝土在桥面铺装结构中的力学行为

为了减少桥面铺装在运营中的各类病害,将水泥-乳化沥青混凝土应用于桥面铺装的下层,并取代防水粘结层,对其适用性进行了研究.采用有限元力学分析方法,通过变换结构层的厚度来模拟该桥面结构层的力学行为.结果表明:当增大水泥-乳化沥青结构层的厚度时,各层之间的剪应力均减小,与桥面板之间的剪应力减小幅度较为明显,该层本身所承受的压应力也随之减小,当该层的厚度增加到3 cm以上时,上下层之间的剪应力下降幅度开始减缓.通过对新型桥面铺装结构力学行为特点进行研究,表明该材料适用于中、小型混凝土桥.

随机波浪作用下底部层流边界层剪应力谱的研究

本文在随机波浪底部剪应力的研究中引入了波浪谱的概念． 根据浅水波的特点， 将波浪谱作为输入谱，通过传递函数确定作为输出谱的随机波浪作用下的底部剪应力谱． 本研究中研制了一套底部剪应力测量仪． 对于层流边界层得到了底部剪应力谱的理论解，

波、流共同作用下的泥沙起动

对波、流共存时的床面剪切力及摩阻系数等作了探讨,根据泥沙起动试验,建立了泥沙起动Shields曲线,导出了波、流共同作用下层流和紊流时的泥沙起动公式和起动波高、起动水深。

平板及减阻沟槽表面雷诺切应力的实验研究

利用二维LDV系统对零压力梯度下光滑面和减阻沟槽面湍流边界层流场进行对比测量。着重考察了相同流动条件下光滑面、沟槽面的槽及峰上部区域雷诺切应力的分布特性,并利用象限分析法研究了沟槽面对湍流边界层相干结构的影响。研究发现:减阻沟槽面槽峰增强了雷诺切应力水平,使其上部区域下扫事件的平均强度增强;而槽部则抑制了雷诺切应力水平,削弱了其上部区域下扫事件的强度;沟槽上部区域雷诺切应力水平的降低幅度高于槽峰上的增加幅度。由此可以认为:沟槽的抑制作用抵消了沟槽峰部的增强效果,使得近壁区内动量交换水平减弱,从而,减阻沟槽面的雷诺切应力在总体上表现为减小。

波浪作用下的床面摩阻系数

波浪作用下床面摩阻系数对于波浪变形和床面剪应力的准确计算,乃至海岸泥沙运动的合理描述具有重要意义。在收集大量波浪边界层和床面剪应力测量数据的基础上,建立了波浪作用下光滑和粗糙紊流边界层床面摩阻系数的显式计算公式,并进一步建立了可涵盖波浪边界层所有流态的摩阻系数显式计算公式,该公式具有不需迭代、不需判断流态、应用方便的特点。

单层涂层最佳厚度的有限元分析

采用有限元数值计算的方法对TiN、TiC、Si3 N4及SiC四种材料的硬涂层体系表面下的应力分布进行了模拟分析。结果表明 ,对于TiN材料 ,当涂层厚度与接触半宽之比t/a <0 .3时 ,表面下的最大剪应力分布对提高轴承疲劳寿命较为有利 ;t/a =0 .5时 ,涂层系统的摩擦力学性能最差。涂层厚度较薄时 ,位于赫兹接触中心附近的涂层表层上的最大剪应力要远大于基体内的最大静态剪应力 ;当t/a >0 .1时 ,涂层表面上的最大剪应力对提高轴承疲劳寿命较为有利 ;涂层材料与基体材料的弹性模量之比小于 2 .0时 。

低速流体边界层分离检测试验

结合边界层分离点附近的切应力变化规律,对边界层分离特性进行研究,确定边界层分离点的判定依据。采用分立元件通过表面贴装工艺来实现柔性微型热膜传感器阵列的集成,并研究该传感器阵列中敏感元件、柔性衬底的设计及传感器的排布,解决柔性传感器阵列对微细加工工艺要求较高的难题。根据边界层分离点判定依据,在低速风洞试验中对传感器阵列的性能和传感器阵列输出信号采用统计量算法和傅里叶变换功率谱算法进行处理判断,并与计算流体力学仿真结果对比,结果表明制造的微型热敏传感器阵列能够实现对流体边界层分离位置的在线测量。