苏门答腊岛西北海域大型海底滑坡过程反分析

针对苏门答腊岛西北海域大型海底滑坡,建立基于欧拉-欧拉多相流理论的滑坡模型,对滑坡体动态运动过程开展模拟反分析。通过与BING程序和解析模型结果对比,考虑土体分段软化后的多相流模型能够较好的重现该滑坡体的运动过程,能获得与实际海底滑坡相近的沉积形态和滑动距离。分析结果表明滑动首先从坡角较大处开始,并逐渐推动中、前部坡度较小处土体滑动;滑坡体整体启动后沿斜坡逐渐加速,当滑动至斜坡底部时端部峰值速度达到最大值43 m/s,之后逐渐减速并在深海平原处停止运动;土体扰动以及混水后土体强度降低是海底滑坡长距离运移的根本原因,同时滑动过程中滑坡体端部发生的滑水效应,减小了海床摩擦力的影响,增加了滑坡体的运动距离。     

车用交流发电机气动噪声数值分析

以某型车用交流发电机（也称爪极发电机）为研究对象,采用计算流体力学技术对交流发电机的空气动力学特征进行了三维非定常数值模拟,应用滑移网格技术和大涡模拟方法对交流发电机进行气动噪声特性研究。得到大涡模拟在交流发电机噪声数值预测方面其主要阶次和对应的幅值与试验对比有很好的一致性;前后扇叶为该型交流发电机的气动噪声声源;第6、8、10、12和18等阶次为该型交流发电机的主要气动噪声成分。在数值模拟基础上,以低噪声、高流量为优化目标,对交流发电机前端盖径向栅格分布角度进行气动噪声优化设计及降噪研究。得到前端盖径向栅格倾斜40°分布角度时交流发电机远场气动噪声最低、质量流量最大。文中所得研究成果可为车用交流发电机的气动性能和高转速下噪声的改进提供一种切实可行的参考依据。     

Mixing at the front of a gravity current

Summary The mixing at the front of a gravity current such as a saline flow or a turbidity current depends on two main processes.The main process is the formation of Kelvin Helmholtz billows. These form at the frontal interface where both velocity and density gradients exist between the two fluids.The Kelvin-Helmholtz billows may be distorted and broken up by an second instability caused by the over-running of less dense fluid beneath the nose. In a shifting pattern of lobes and clefts, much of the flow is diverted into the clefts and can cause lineations in deposits. The form of the head is also modified by ambient head- or tail-flows, which modify the velocity gradient and hence the form of the billows.The initial effect of external turbulence is to increase the mixing rate. In a strong turbulent field a sharp front can be dissipated; the advance is then only by turbulent diffusion, a much slower rate of distribution of the dense fluid.One important frontal feature occurs only in three dimensional gravity currents. In the initial stages of a radial divergent flow the rapid stretching of the leading edge results in the formation of a large roll which can extend down to the ground.With 8 Figures     

A two-layer model for a non-Newtonian gravity current subjected to wind shear

Summary.  A theoretical model is developed for the gravity current resulting from the sudden release of a fixed volume of fluid of non-Newtonian power law rheology on top of a slightly denser Newtonian fluid layer in the presence of wind stress. The model incorporates the flow of both layers and accounts for the effects of inertial and viscous forces, and is suited for moderate Reynolds number flows. The governing equations are obtained by depth-averaging the unsteady equations of motion in accordance with the von Kármán's momentum integral method, and constitute a hyperbolic system of four equations for the flow rates and thicknesses of the fluid layers. Results are obtained by a well established numerical scheme for systems of nonlinear hyperbolic equations. For a particular case analytical results are obtained by employing an asymptotic matching approach. Good agreement is obtained between the numerical and analytical results. The effects of the thickness of the ambient layer, wind stress, Reynolds numbers, and rheology on the gravity current are discussed. Received July 22, 2002; revised November 27, 2002 Published online: May 8, 2003     

Simulation of front evolving liquid film flowing down an inclined plate using level set method

In this paper, the projection/level set method is used to simulate the liquid film flow down on an inclined plate for both Newtonian and non-Newtonian fluids. Special numerical treatments are designed to cope with the viscous terms. The simulation is carried out for different inclined plate angles ranging from 20 to 90. The effects of the inclined angle, surface tension, and shear thinning on the flow are discussed. It is found that surface tension reduces the contact angle, increases the film thickness, and promotes a fuller flow front. The effect of shear thinning is to increase the velocity near the wall, make the contact angle larger, and reduce the film thickness. It is also found that the variation of rheological properties and surface tension may produce a complex flow behaviour due to the swollen flow front.The authors are grateful to Dr. M. Sussman (Florida State University) and X-J Fan (Institute of High Performance Computing) for their helpful discussions.     

Unsteady flow in flexible tubes: A modulated simple wave

The theory of the modulated simple wave is used to describe high frequency flow in a flexible tube. The natural length and time scales are motivated by considering the linearised system and the propagation of an acceleration front in the nonlinear system. A small amplitude, time periodic flow and an arbitrary amplitude pulse down the tube are described.     

On the nature of the Vorob’evs effect in pulse breakdown of solid dielectrics

The Vorob’evs effect consists in certain features of the discharge observed when a solid dielectric in contact with two rodlike electrodes is placed in a liquid dielectric medium and a voltage pulse with increasing front is applied to the electrodes. When the pulse front slope is small, the discharge develops in the liquid over the solid dielectric surface; whereas the discharge at a sufficiently large slope of the pulse front penetrates into the solid and produces its fracture with cleavage of the surface fragments. In order to explain this phenomenon, it is suggested that, at a sufficiently high voltage buildup rate, a displacement current that is related to the motion of the surface discharge plasma passes through a microprotrusion occurring on the electrode surface at the contact site and causes the electric explosion of this microprotrusion. The metal plasma jet generated as a result of this explosion penetrates into the solid dielectric and forms a discharge channel in depth of this material. The surface discharge plasma formed at a small slope of the voltage pulse front closes the electrode circuit, thus preventing the discharge penetration in depth of the solid.     

Intensity of stress singularity for the circumferential V‐shape corner front of a three‐dimensional diamond‐like defect

Three‐dimensional diamond‐like defects with circumferential V‐shape corner fronts are often contained in engineering materials. In this paper, generalized stress intensity factors are calculated for this type of defect using a modified advanced finite element method. A super corner front element model in the global coordinates is established to capture the stress singularities along the circumferential corner front. Three‐dimensional numerical series eigen‐solutions in the element have been transformed from asymptotic expressions in the local curvilinear coordinates. The element is suitable for a sharp V‐shape corner with arbitrary opening and inclination angle. Singular stress fields near various shapes of diamond‐like defects are systematically investigated. The interaction of an embedded defect with free surface or another identical defect is also investigated. The numerical results can be used as stress intensity parameters to predict fatigue strength at circumferential corner front of a diamond‐like defect.     

叶栅内非定常不可压流动的数值模拟

基于SMAC(SimplifiedMarkerandCell)方法推导出直接求解二维非定常、不可压N-S方程的隐式数值方法。求解的基本方程是任意曲线坐标系中以逆变速度为变量的N-S方程和椭圆型的压力Poisson方程。采用该方法,对二维叶栅非定常分离流场进行了数值模拟,叶栅表面压力的计算结果与试验结果相比比较吻合,从而验证了这种方法的可靠性。同时对叶栅非定常流场的流场结构和流动机理做了初步的探讨。在均匀来流和定常边界条件下,叶栅内部流动表现出强烈的非定常性;在小冲角和高雷诺数时,叶栅尾部产生类似卡门涡街的周期性流动。     

The M-integral for calculating intensity factors of an impermeable crack in a piezoelectric material

In this study, a conservative integral is derived for calculating the intensity factors associated with piezoelectric material for an impermeable crack. This is an extension of the M-integral or interaction energy integral for mode separation in mechanical problems. In addition, the method of displacement extrapolation is extended for this application as a check on results obtained with the conservative integral. Poling is assumed parallel, perpendicular and at an arbitrary angle with respect to the crack plane, as well as parallel to the crack front. In the latter case, a three-dimensional treatment is required for the conservative integral which is beyond the scope of this investigation. The asymptotic fields are obtained; these include stress, electric, displacement and electric flux density fields which are used as auxiliary solutions for the M-integral.Several benchmark problems are examined to demonstrate the accuracy of the methods. Numerical difficulties encountered resulting from multiplication of large and small numbers were solved by normalizing the variables. Since an analytical solution exists, a finite length crack in an infinite body is also considered. Finally, a four point bend specimen subjected to both an applied load and an electric field is presented for a crack parallel, perpendicular and at an angle to the poling direction. It is seen that neglecting the piezoelectric effect in calculating stress intensity factors may lead to errors.     

Eddy-Current Coil Interaction With A Perfectly Conducting Wedge of Arbitrary Angle

In this article, a closed-form expression for the impedance of a tangential eddy-current coil in the presence of an infinite conducting wedge of arbitrary angle is derived. The truncated eigenfunction expansion (TREE) solution given here is valid in the quasi-static frequency regime. The theory was validated via comparison to an independent analytical expression for the impedance change of a horizontal coil over a conducting half-space due to Burke. We present results for three geometries: a conducting quarter-space, a conducting wedge of angle 225 degrees, and a semi-infinite conducting sheet. Our theory predicts a measurable change in the tangent coil reactance in the presence of all three geometries.     

四步法三维编织预制件的渗透率

采用径向法测量了三维编织预制件的渗透率, 利用修正注入半径的循环方法, 基于Visual C + + 计算了渗透率的大小, 主渗透率方向分两步确定: 一是通过选择阈值的图像处理运算提取流动前沿; 二是采用Matlab中的非线性最小二乘法将所得轮廓进行椭圆拟合, 并计算该椭圆长轴方向与编织方向的夹角。结果表明: 编织参数影响渗透率, 当编织角一定时, 随着纤维体积含量的增加, 主渗透率降低; 当纤维体积含量一定时, 随着编织角的增加, 主渗透率降低, 但主渗透率的方向不随编织角的变化而改变, 始终保持与编织轴向一致。      

Construction of 3D Polymer Brushes by Dip‐Pen Nanodisplacement Lithography: Understanding the Molecular Displacement for Ultrafine and High‐Speed Patterning

Dip‐pen nanodisplacement lithography (DNL) is a versatile scanning probe‐based technique that can be employed for fabricating ultrafine 3D polymer brushes under ambient conditions. Many fundamental studies and applications require the large‐area fabrication of 3D structures. However, the fabrication throughput and uniformity are still far from satisfactory. In this work, the molecular displacement mechanism of DNL is elucidated by systematically investigating the synergistic effect of z extension and contact time. The in‐depth understanding of molecular displacement results in the successful achievement of ultrafine control of 3D structures and high‐speed patterning at the same time. Remarkably, one can prepare arbitrary 3D polymer brushes on a large area (1.3 mm × 1.3 mm), with <5% vertical and lateral size variations, and a patterning speed as much as 200‐fold faster than the current state‐of‐the‐art.     

Effects of a Small Inclination Misalignment in Talbot Interferometry by use of Gratings with Arbitrary Line Orientation. I. Theoretical Analysis

We discuss the effects of a general small inclination misalignment, which is formed by rotation of the beam-splitter grating around an axis that is laid on the grating plane and that has an arbitrary angle with respect to the line direction of the grating, between the two grating planes on the moiré fringes in Talbot interferometry. It is shown that the small inclination angle has a significant influence on measurement results based on Talbot interferometry because both the period and the slope of the moiré fringes are sensitive to the angle, especially when the rotation axis is nearly parallel to the lines of the grating. Simple and practical detection methods for the small inclination angle are proposed, and the effects of the inclination angle on the contrast in the moiré fringes are also briefly discussed.     

Radiation of water waves from an oscillating source located at a slope of angle π/4

An analytical investigation is performed of the linear radiation problem for water waves generated by an oscillating normal velocity distribution along a sloping beach with slope angle /4. The distribution of normal velocity is arbitrary, and it oscillates with a given frequency. The solution is expressed in terms of the Green function which represents a source of unit oscillatory flux located at an arbitrary position along the slope. At infinity the radiation condition is applied to determine the outgoing wave. As a simple example of integrating the Green function, the reflection of an incoming sinusoidal wave is calculated.     

Comparison of measured and computed Strehl ratios for light propagated through a channel flow of a He-N(2) mixing layer at high Reynolds numbers

A lateral shearing interferometer was used to measure the slope of perturbed wave fronts after they propagated through a He-N(2) mixing layer in a rectangular channel. Slope measurements were used to reconstruct the phase of the turbulence-corrupted wave front. The random phase fluctuations induced by the mixing layer were captured in a large ensemble of wave-front measurements. Phase structure functions, computed from the reconstructed phase surfaces, were stationary in first increments. A five-thirds power law is shown to fit streamwise and cross-stream slices of the structure function, analogous to the Kolmogorov model for isotropic turbulence, which describes the structure function with a single parameter. Strehl ratios were computed from the phase structure functions and compared with a measured experiment obtained from simultaneous point-spread function measurements. Two additional Strehl ratios were calculated by using classical estimates that assume statistical isotropy throughout the flow. The isotropic models are a reasonable estimate of the optical degradation only within a few centimeters of the initial mixing, where the Reynolds number is low. At higher Reynolds numbers, Strehl ratios calculated from the structure functions match the experiment much better than Strehl ratio calculations that assume isotropic flow.     

Holographic optical receiver front end for wireless infrared indoor communications

Multispot diffuse configuration (MSDC) for indoor wireless optical communications, utilizing multibeam transmitter and angle diversity detection, is one of the most promising ways of achieving high capacities for use in high-bandwidth islands such as classrooms, hotel lobbies, shopping malls, and train stations. Typically, the optical front end of the receiver consists of an optical concentrator to increase the received optical signal power and an optical bandpass filter to reject the ambient light. Using the unique properties of holographic optical elements (HOE), we propose a novel design for the receiver optical subsystem used in MSDC. With a holographic curved mirror as an optical front end, the receiver would achieve more than an 10-dB improvement in the electrical signal-to-noise ratio compared with a bare photodetector. Features such as multifunctionality of the HOE and the receiver's small size, light weight, and low cost make the receiver front end a promising candidate for a user's portable equipment in broadband indoor wireless multimedia access.     

Strain energy release rate calculation for a moving delamination front of arbitrary shape based on the virtual crack closure technique. Part I: Formulation and validation

This paper proposes a simple, efficient algorithm to trace a moving delamination front with an arbitrary and changing shape so that delamination growth can be analyzed by using stationary meshes. Based on the algorithm, a delamination front can be defined by two vectors that pass through any point on the front. The normal vector and the tangent vector for the local coordinate system can then be obtained based on the two delamination front vectors. An important feature of this approach is that it does not require the use of meshes that are orthogonal to the delaminations front. Therefore, the approach avoids adaptive re-meshing techniques that may create a large computational burden in delamination growth analysis. An interface element that can trace the instantaneous delamination front, determine the local coordinate system, approximate strain energy release rate components and apply fracture mechanics criteria has been developed and implemented into ABAQUS^® with its user-defined element (UEL) feature. In this Part I of a two-part paper, the approach and its implementation are described and validated by comparison to results from existing cases having analytical solutions or other established FEA predictions.     

Elastic–plastic fracture behavior of a radial matrix crack interacting with a circle inclusion with generalized Irwin corrections

A generalized Irwin plastic zone model is proposed to investigate the interaction problem for a circular inclusion embedded in an elastic–plastic matrix that contains a radial crack, oriented at an arbitrary angle from a remote load. The distributed dislocation technology is applied to formulate the current problem. The effective stress intensity factors, the plastic zone size, and the crack tip opening displacement are evaluated by solving the formulated singular integral equations. In the numerical examples, the inclusion is taken as a void and a rigid body. The effects of the crack angle and the inclusion–crack distance (the distance from the inclusion center to the crack center) on the effective stress intensity factors, the plastic zone size, and the crack tip opening displacement are discussed in detail. Numerical results show that if the crack angle is not large, the values of the plastic zone size and the crack tip opening displacement are less than the corresponding values in the homogenous case when the inclusion is a rigid body; when the inclusion is a void, these values are larger than the corresponding values in the homogenous case.