Measurements in the vicinity of a stagnation point

This paper presents measurements of a plane jet impinging onto a normal flat plate placed up to five jet widths from the jet outlet. The small spacing ensured that the stagnation streamline remained in the potential core of the jet. The plate shear stress distribution compared well to that from an analytical solution for the laminar development of the plate boundary layer whose external velocity was determined from the measured pressure. By comparing the shear stress measured under the present low level of free stream turbulence (0.35%) at the jet exit with that of Tu and Wood [Exp. Thermal Fluid Sci. 13 (1996) 364–373] made at about 4%, it is concluded that the turbulence level at the nozzle exit has only a second-order influence on the surface shear stress around the stagnation point. Some spanwise non-uniformity was observed in the plate shear stress, but this was confined largely to the transition region. The mean velocity, Reynolds stresses, and fluctuating pressure were measured along the stagnation streamline using a fast-response pressure probe. A significant increase in the streamwise normal stress and the mean square of the pressure fluctuations occurred before they were eventually attenuated by the plate. This increase occurred in the region where the streamwise velocity was decreasing close to the plate causing extra energy production through the normal stresses. Spectra of the velocity and pressure fluctuations showed that the increase in level was mainly due to the low frequency motion, whereas the subsequent decrease occurred at higher frequencies.     

Numerical investigation of plate edge and slot size effects in low yield stress measurements with a slotted plate device

There is a need for accurate yield stress measurements, especially in the case of low yield stress complex materials such as biological samples. This task cannot be accomplished with conventional rotational rheometers due to significant wall slip effects and the necessity to operate the device at very low shear rates, often beyond the limit that such rheometers can achieve. In this paper, we focused on the slotted plate method proposed recently for low yield stress measurements. Using computational fluid dynamics, we studied the effects of plate geometry on the measurement accuracy of the slotted plate method. Results of this study indicate that both wall slip effects and pressure drag force can be substantially reduced by adopting a thin plate with sharp front and rear edges, high slot area ratio, and large number of slots. If the plate has 30° triangular edges, a slot area ratio of 80%, and 12 slots, the slotted plate method overpredicts the yield stress of a 0.09 wt.% Carbopol dispersion (yield stress of 9.17 Pa) by only 8.4% under no-slip conditions and underpredicts the yield stress by 12.3% under free-slip conditions. Similar results were obtained for human saliva characterized by a very low yield stress (0.073 Pa).     

Image analysis of oil film interferometry — a method of measuring wall shear stress distributions

A method is described of measuring a wall shear stress distribution that varies in the direction of the flow. Variations in the height of a very thin oil film moving under the boundary layer generate interference fringes, which are recorded and digitised using image processing equipment.The evolution of the film surface in space and time can be reconstructed from the interference fringe patterns and used to calculate the shear stress field. This reconstruction is achieved by comparing the picture data with images that were calculated for prescribed heights that are adjusted iteratively, until the calculated intensities match the data.The method is applied to a flow approaching a step, and the results are compared with pulsed-wire measurements.     

A nonintrusive laser interferometer method for measurement of skin friction

A method is described for monitoring the changing thickness of a thin oil film subject to an aerodynamic shear stress using two focused laser beams. The measurement is then simply analyzed in terms of the surface skin friction of the flow. The analysis includes the effects of arbitrarily large pressure and skinfriction gradients, gravity, and time-varying oil temperature. It may also be applied to three-dimensional flows with unknown direction. Applications are presented for a variety of flows including two-dimensional flows, three-dimensional swirling flows, separated flows, supersonic high-Reynolds-number flows, and delta-wing vortical flows.     

Mixed-mode failure of thin films using laser-generated shear waves

A new test method is developed for studying mixed-mode interfacial failure of thin films using laser generated stress waves. Guided by recent parametric studies of laser-induced tensile spallation, we successfully extend this technique to achieve mixed-mode loading conditions. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin-film surface. A shear wave is obtained with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions reveals significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading.     

A reflection-type oil-film skin-friction meter

Tanner has shown analytically that the slope characteristics of a thin oil-film lying in contact with a surface on which an aerodynamic boundary-layer is developing are explicitly related to the local skin-friction. Unlike many conventional methods of measuring skin-friction, the oil-film method is absolute in nature, requires no calibration and in principle, can be universally applied. In all existing forms of the meter, however, interferometry is used to make measurements of the oil film. In the present contribution, the technique has been simplified by completely eliminating interferometry. This has been achieved by making direct and dynamic measurements of the oil-film slope by directing a reflected beam of light off the top of the oil film to a small position sensing photodiode. The raw data now is very different from that in the interferometric method and consequently, new methods of determining skin-friction from measured oil-film slope histories have been developed. The reflection method has been verified in incompressible flat plate turbulent boundary-layers. The standard of deviation of the measurements is about 10% of the mean. The present version of the meter is compact and simple.     

An experimental test of equations predicting the load bearing capacity of squeeze films of elastico-viscous liquid

A form of squeeze film apparatus was recently described in which the movement of one plate towards the other was simulated by the continuous volume generation of liquid over the plate area. The liquid exuded from a large number of holes in the lower plate surface and formed a “continous flow” version of squeeze film apparatus with no moving parts [1]. A later paper gave derivations of equations from which squeeze film load bearing capacity could be evaluated, taking into account viscous, inertial and normal stress effects in the liquid film [2].In order to find the total load in a squeeze film system, it was necessary to obtain the relationship between the first normal stress difference and shear rate for the liquid in use, using an experimental method. At high shear rates, the jet thrust method provided these data [3,4] and from them the load bearing capacity of squeeze films of hot, polymer-thickened oil were predicted [2].A more complete test of the method is possible with a highly elastic liquid because considerable load enhancement due to extra stress is present at moderate deformation rates in squeeze film systems [1,5,6,7]. Thus a 0.1 per cent aqueous polyacrylamide solution gives well-defined load enhancement and (quite independently) the jet thrust method gives the relationship between normal stress and shear rate from which predictions of load enhancement may be made. Furthermore, convergent nozzles may be used in the jet thrust apparatus [3] to measure the stress development in an elastic liquid which is being simulateneously sheared and stretched, a situation which more closely resembles the squeeze film case than that of steady shear.     

Ordinary state-based peridynamics for plastic deformation according to von Mises yield criteria with isotropic hardening

This study presents the ordinary state-based peridynamic constitutive relations for plastic deformation based on von Mises yield criteria with isotropic hardening. The peridynamic force density–stretch relations concerning elastic deformation are augmented with increments of force density and stretch for plastic deformation. The expressions for the yield function and the rule of incremental plastic stretch are derived in terms of the horizon, force density, shear modulus, and hardening parameter of the material. The yield surface is constructed based on the relationship between the effective stress and equivalent plastic stretch. The validity of peridynamic predictions is established by considering benchmark solutions concerning a plate under tension, a plate with a hole and a crack also under tension.     

基于油膜干涉法的表面摩擦应力测量技术研究

本文应用油膜干涉法及速度型方法在平板模型上进行低速湍流的表面摩擦阻力测量,探索了油膜法测量表面摩擦阻力的实验技术,分析了不同实验方式对测量结果的影响以及实验误差产生的原因.研究表明,模型安装方式、油膜布置方式以及重力作用的影响在油膜干涉法中基本都可以忽略,在精度要求不高的情况下,不需要对结果进行修正.通过采用热膜法和油...     

Tensile and mixed-mode strength of a thin film-substrate interface under laser induced pulse loading

Laser induced stress waves are used to characterize intrinsic interfacial strength of thin films under both tensile and mixed-mode conditions. A short-duration compressive pulse induced by pulsed-laser ablation of a sacrificial layer on one side of a substrate is allowed to impinge upon a thin test film on the opposite surface. Laser-interferometric measurements of test film displacement enable calculation of the stresses generated at the interface. The tensile stress at the onset of failure is taken to be the intrinsic tensile strength of the interface. Fused-silica substrates, with their negative nonlinear elasticity, cause the compressive stress wave generated by the pulse laser to evolve a decompression shock, critical for generation of the fast fall times needed for significant loading of surface film interfaces. By allowing the stress pulse to mode convert as it reflects from an oblique surface, a high amplitude shear wave with rapid fall time is generated and used to realize mixed-mode loading of thin film interfaces. We report intrinsic strengths of an aluminum/fused silica interface under both tensile and mixed-mode conditions. The failure mechanism under mixed-mode loading differs significantly from that observed under pure tensile loading, resulting in a higher interfacial strength for the mixed-mode case. Inferred strengths are found to be independent, as they should be, of experimental parameters.     

An inclined wall jet: Mean flow characteristics and effects of acoustic excitation

 The mean velocity field of a 30° inclined wall jet has been investigated using both hot-wire and laser Doppler anemometry (LDA). Provided that the nozzle aspect ratio is greater than 30 and the inclined wall angle (β) is less than 50°, LDA measurements for various β show that the reattachment length is independent of the nozzle aspect ratio and the nozzle exit Reynolds number (in the range 6670–13,340). There is general agreement between the reattachment lengths determined by LDA and those determined using wall surface oil film visualisation technique. The role of coherent structures arising from initial instabilities of a 30° wall jet has been explored by hot-wire spectra measurements. Results indicate that the fundamental vortex roll-up frequency in both the inner and outer shear layer corresponds to a Strouhal number (based on nozzle exit momentum thickness and velocity) of 0.012. The spatial development of instabilities in the jet has been studied by introducing acoustic excitation at a frequency corresponding to the shear layer mode. The formation of the fundamental and its first subharmonic has been identified in the outer shear layer. However, the development of the first subharmonic in the inner shear layer has been severely suppressed. Distributions of mean velocities, turbulence intensities and Reynolds shear stress indicate that controlled acoustic excitation enhances the development of instabilities and promotes jet reattachment to the wall, resulting in a substantially reduced recirculation flow region. Received: 24 November 1998/Accepted: 24 August 1999     

层状横观各向同性地基上异性矩形薄板的弯曲解析解

选用更具广泛性的层状横观各向同性弹性地基模型,来分析四边自由各向异性矩形地基板的弯曲解析解。先基于直角坐标下横观各向同性体的静力胡海昌通解,借助双重傅里叶变换及矩阵传递法,获得层状横观各向同性地基的静力位移场和应力场;然后将异性薄板的弯曲控制方程,与基于层状横观各向同性弹性地基的位移解建立的板与地基变形协调方程相结合,先按对称性分解,再用三角级数法,得出层状横观各向同性弹性地基上四边自由各向异性矩形薄板的弯曲解析解,包括地基反力、板的挠度及板的内力的解析表达式。克服了数值法的弊端,取消了对地基反力的假设,且避免了矩阵指数函数的计算;同时考虑了地基的层状性及板和地基的各向异性,从而得到板的内力及地基反力更切实际的分布规律。算例结果与文献的有限元结果吻合良好,证明本文方法是切实可行的。     

Laminar free-convective heat transfer from a vertical isothermal plate to water at low temperatures with variable physical properties

All studies concerning laminar free convection along a vertical isothermal plate in water at low temperatures have been conducted assuming constant dynamic viscosity and thermal conductivity both taken at ambient or film temperature. In this study the problem has been treated taking into account the temperature dependence of all water physical properties. The results are obtained with the numerical solution of the boundary layer equations. The variation of μ and k with temperature has a small influence on wall heat transfer but a strong influence on wall shear stress. These quantities show a significant reduction at density extremum.     

高压微间隙下4010航空油的润滑特性研究

在自制的新型膜厚测量仪上,测量4010航空油在不同接触压力、温度和卷吸速度下的干涉图像,分析接触区的润滑特性。结果表明：在低温高速区主要表现为弹流润滑,中心膜厚与接触压力呈负相关;而在低温低速、高温区主要表现为薄膜润滑,中心膜厚受接触压力的影响较小。在弹流润滑区内高接触压力下油膜形状呈平坦状分布,而薄膜润滑区内油膜形状总体上比较平滑。随着载荷的增加,弹流润滑区内由Hamrock-Dowson理论算得的膜厚值和实测值逐渐偏离,理论公式中卷吸速度和载荷的指数需要调整;而薄膜润滑区的膜厚值基本上保持平稳。     

基于不同板理论的嵌于柔性层中薄膜的皱曲分析

论文对于柔性层-薄膜-柔性层三层结构系统,基于经典板理论、一阶剪切变形理论和高阶剪切变形理论,分别推导给出薄膜皱曲的控制方程。对于两个柔性层,则把它们处理成具有有限厚度的平面应变弹性体。针对上下柔性层固支边界或自由边界条件,利用线性扰动动方法得到柔性层对薄膜的横向压力差,最终获得确定薄膜具有周期性正弦型皱曲的临界载荷方程。同时,对结构系统建立了有限元数值模型,通过模拟结果与三种板理论的结果进行了比较,验证了理论解的精确性和适用性。最后,进行了参数和极限情况分析,阐述了上下层边界条件、薄膜和柔性层的材料和几何参数对临界值的影响。     

地震边坡失稳机理及稳定性分析

通过对汶川地震边坡调研,发现大量边坡破坏形式为坡顶拉裂、下部坡体剪切破坏。本文利用有限差分程序,从边坡土体的应力状态出发,通过监测边坡土体的状态、位移和剪应变增量变化等,分析了岩土体在静力、横向地震和耦合地震作用下的破坏过程,发现坡顶附近发生张破坏,以下部位发生剪切破坏,而非传统的地质工程观点——地震边坡破坏主要是地震惯性力造成的剪切破坏。并提出一种新方法——关键点相对位移法,来判断边坡的动力稳定性,数值模拟结果与已有研究成果及震后灾害调查结果具有良好的一致性。     

A nth-order shear deformation theory for the free vibration analysis on the isotropic plates

In the present paper, a nth-order shear deformation theory is used to perform the free vibration analysis of the isotropic plates. The present nth-order shear deformation theory satisfies the zero transverse shear stress boundary conditions on the top and bottom surface of the plate. Reddy??s third order theory can be considered as a special case of present nth-order theory (n=3). The governing equations and boundary conditions are derived by the principle of virtual work. The governing differential equations of the isotropic plates are solved by the meshless radial point collocation method based on the thin plate spline radial basis function. The effectiveness of the present theory is demonstrated by applying it to free vibration problem of the square and circular isotropic plate.     

A film-based wall shear stress sensor for wall-bounded turbulent flows

In wall-bounded turbulent flows, determination of wall shear stress is an important task. The main objective of the present work is to develop a sensor which is capable of measuring surface shear stress over an extended region applicable to wall-bounded turbulent flows. This sensor, as a direct method for measuring wall shear stress, consists of mounting a thin flexible film on the solid surface. The sensor is made of a homogeneous, isotropic, and incompressible material. The geometry and mechanical properties of the film are measured, and particles with the nominal size of 11 μm in diameter are embedded on the film’s surface to act as markers. An optical technique is used to measure the film deformation caused by the flow. The film has typically deflection of less than 2% of the material thickness under maximum loading. The sensor sensitivity can be adjusted by changing the thickness of the layer or the shear modulus of the film’s material. The paper reports the sensor fabrication, static and dynamic calibration procedure, and its application to a fully developed turbulent channel flow at Reynolds numbers in the range of 90,000–130,000 based on the bulk velocity and channel full height. The results are compared to alternative wall shear stress measurement methods.     

Mixed-mode interfacial adhesive strength of a thin film on an anisotropic substrate

The mixed-mode interfacial adhesion strength between a gold (Au) thin film and an anisotropic passivated silicon (Si) substrate is measured using laser-induced stress wave loading. Test specimens are prepared by bonding a fused silica (FS) prism to the back side of a 〈1 0 0〉 Si substrate with a thin silicon nitride (Si_xN_y) passivation layer deposited on the top surface. A high-amplitude stress wave is developed by pulsed laser ablation of a sacrificial absorbing layer on one of the lateral surfaces of the FS prism. Due to the negative non-linear elastic properties of the FS, the compressive stress wave evolves into a decompression shock with fast fall time. Careful selection of the incident angle between the pulse and the FS/Si interface generates a mode-converted shear wave in refraction, subjecting the Si_xN_y/Au thin film interface to dynamic mixed-mode loading, sufficient to cause interfacial fracture. A detailed analysis of the anisotropic wave propagation combined with interferometric measurements of surface displacements enables calculation of the interfacial stresses developed under mixed-mode loading. The mixed-mode interfacial strength is compared to the interfacial strength measured under purely tensile loading.     

二甲基—γ—全氟辛酰氧丙基硅烷自组装膜的制备及其摩擦学性能研究

利用分子自组装技术 ,用含有全氟烷基的氯硅烷作为前驱体 ,在活化玻璃表面制备了二甲基 -γ-全氟辛酰氧丙基硅烷单分子膜 ;用 X射线光电子能谱仪对组装膜表面的几种特征元素及其化学环境进行了表征 ;采用接触角测定仪测定了蒸馏水在自组装薄膜表面的接触角 ,在动静摩擦磨损试验机上评价了薄膜同 GCr1 5钢球对摩时的摩擦磨损性能 .结果表明 :所制备的自组装膜的表面自由能很低 ,具有很好的疏水 -疏油性 ,其对水的接触角高达 1 1 0°;二甲基 -γ-全氟辛酰氧丙基硅烷组装膜可以降低基片的摩擦系数 ,而且在较低负荷下具有很好的耐磨性