Setup Instabilities in the Nearshore Hydrodynamics

The incoming wave field on a beach produces a setup on the mean water level elevation. In earlies studies, this setup had been hypothesized as an instability source in nearshore hydrodynamics capable of driving processes such as rip currents, horizontal eddies and rhythmic topography. An analytical study is presented in order to investigate if such a setup in isolation, that is, without longshore current and without wave-refraction, can cause the growth of infinitesimal disturbances capable of driving such processes. It is shown that the setup can not be considered as an instability source since the equation governing the disturbances is found to be a wave-type equation. However, the phase speed differs slightly from the classical value gD, and becomes anisotropic. This can be interpreted as an effect of the radiation stresses of short incoming waves on long wave propagation.     

Godunov‐type adaptive grid model of wave–current interaction at cuspate beaches

This paper presents a second‐order accurate Godunov‐type numerical scheme for depth‐ and period‐averaged wave–current interaction. A flux Jacobian is derived for the wave conservation equations and its eigensystem determined, enabling Roe's approximate Riemann solver to be used to evaluate convective fluxes. Dynamically adaptive quadtree grids are used to focus on local hydrodynamic features, where sharp gradients occur in the flow variables. Adaptation criteria based on depth‐averaged vorticity, wave‐height gradient, wave steepness and the magnitude of velocity gradients are found to produce accurate solutions for nearshore circulation at a half‐sinusoidal beach. However, the simultaneous combination of two or more separate criteria produces numerical instability and interference unless all criteria are satisfied for mesh depletion. Simulations of wave–current interaction at a multi‐cusped beach match laboratory data from the United Kingdom Coastal Research Facility (UKCRF). A parameter study demonstrates the sensitivity of nearshore flow patterns to changes in relative cusp height, angle of wave incidence, bed roughness, offshore wave height and assumed turbulent eddy viscosity. Only a small deviation from normal wave incidence is required to initiate a meandering longshore current. Nearshore circulation patterns are highly dependent on the offshore wave height. Reduction of the assumed eddy viscosity parameter causes the primary circulation cells for normally incident waves to increase in strength whilst producing rip‐like currents cutting diagonally across the surf zone. Copyright © 2004 John Wiley & Sons, Ltd.     

撞击杆中应变波激发的磁流

应变波通过铁磁性杆件时，由于压磁效应，会同时在杆件中激发出不均匀的磁流。利用实验和磁流理论的分析，证实了撞击杆中磁流的存在。     

A theoretical analysis of the electromigration-induced void morphological evolution under high current density

In this work,analysis of electromigration-induced void morphological evolution in solder interconnects is per-formed based on mass diffusion theory. The analysis is conducted for three typical experimentally observed void shapes: circular, ellipse, and cardioid. Void morphological evolution is governed by the competition between the electric field and surface capillary force.In the developed model,both the electric field and capillary force on the void's surface are solved analytically.Based on the mass conversation princi-ple,the normal velocity on the void surface during diffusion is obtained. The void morphological evolution behavior is investigated, and a physical model is developed to predict void collapse to a crack or to split into sub-voids under elec-tric current.It is noted that when the electric current is being applied from the horizontal direction, a circular void may either move stably along the electric current direction or col-lapse to a finger shape,depending on the relative magnitude of the electric current and surface capillary force.However, the elliptical-shaped void will elongate along the electric cur-rent direction and finally collapse to the finger shape.On the other hand,the cardioid-shaped void could bifurcate into two sub-voids when the electric current reaches a critical value. The theoretical predictions agree well with the experimental observations.     

波作用量守恒原理在波流相互作用中的应用

波和流相遇后，经过相互作用产生稳定的波流场．基于波作用量守恒原理，本文提出了解决Ｓｔｏｋｅｓ波与指数剖面流同向或反向相遇过程中波传播特性改变问题的五阶理论，给出了这种相互作用前后波参数间的关系和相应流场的变化，并与线性结果进行了比较．结果表明，当波流同向时，波变平缓；当波流反向时，波明显变陡．随着波流强度的增加，非线性的影响变得越来越明显，尤其当波同反向流相遇时，波幅显著增大，这时必须用非线性理论来考虑波流相互作用的影响     

Flume testing of passively adaptive composite tidal turbine blades under combined wave and current loading

The tidal energy industry is progressing rapidly, but there are still barriers to overcome to realise the commercial potential of this sector. Large magnitude and highly variable loads caused by waves acting on the turbine are of particular concern. Composite blades with in-built bend-twist elastic response may reduce these peak loads, by passively feathering with increasing thrust. This could decrease capital costs by lowering the design loads, and improve robustness through the mitigation of pitch mechanisms. In this study, the previous research is extended to examine the performance of bend-twist blades in combined wave–current flow, which will frequently be encountered in the field. A scaled 3 bladed turbine was tested in the flume at IFREMER with bend-twist composite blades and equivalent rigid blades, sequentially under current and co-directional wave–current cases. In agreement with previous research, when the turbine was operating in current alone at higher tip speed ratios the bend-twist blades reduced the mean thrust and power compared to the rigid blades. Under the specific wave–current condition tested the average loads were similar on both blade sets. Nevertheless, the bend-twist blades substantially reduced the magnitudes of the average thrust and torque fluctuations per wave cycle, by up to 10% and 14% respectively.     

Direct energy conversion from water waves into electromagnetic oscillations

Summary A new converter, transferring energy from water waves into electromagnetic oscillations, i.e. into an alternating electric current, is proposed. In this converter, an oscillating circuit is combined with a plate capacitor immersed into water. The water waves propagate through the capacitor, thus periodically changing its capacity. In the vicinity of the resonance, under certain conditions to be examined in the paper, stable parametrically excited vibrations are generated in the circuit. In order to show that these conditions can be met in practice, the design of a test converter is outlined, and numerically verified.     

On the front condition for compositional gravity currents

A two-dimensional potential flow is employed to derive the front condition of the gravity current. The derivation starts from the balance between the static pressure of the gravity current and the form drag imposed on the gravity current by the ambient fluid. After employing Bernoullis equation along the interface of the gravity current near the head, we end up with a front condition that is in better agreement with experiment than previous theoretical models. This condition is a function of the density ratio between current and ambient fluids, which was different from previous theoretical models, while it has been widely used in experimental studies. The present front condition suggests that the form drag may account for a significant part of the resistance force applied on the current head.     

Phase velocity effects of the wave interaction with exponentially sheared current

The nonlinear interaction between the unidirectional bichromatic wave-train and exponentially sheared current in water of an infinite depth is investigated. The model is based on the vorticity transport equation and the exact free surface conditions, without any assumptions for the existence of small physical parameters. Earlier works of the wave–current interaction were mainly restricted to either current acted on the monochromatic wave or irregular waves limited to irrotational current. Different from these previous works, no constraint is made in our model for amplitudes of the primary wave, and the current owns an exponential type profile along the vertical line. To ensure that the effect of vorticity on the phase velocity is consistent with earlier derivation, the case of a small amplitude wave traveling on the exponentially sheared current is examined firstly. Then the effect of nonlinearity on the phase velocity of primary waves in a bichromatic wave-train is considered. Accurate high-order approximations of the phase velocity are obtained under consideration of both the nonlinear wave self–self and mutual interactions. Finally, the combined effect of vorticity and nonlinearity on the phase velocity is investigated through the case of a bichromatic wave-train propagating on an exponentially sheared current. It is found that the characteristic current slope determines the effect of vorticity on the phase velocity caused by nonlinear wave self–self and mutual interactions, and the surface current strength may amplify/reduce this effect.     

Interaction of an internal gravity current with a submerged circular cylinder

The problem of hydrodynamic loads due to the interaction of a gravity current propagating over a bottom channel with a submerged circular cylinder is studied experimentally. It was shown that in the examined range of parameters, the hydrodynamic loads are simulated after Froude. The hydrodynamic loads are maximal if the cylinder lies on the bottom, and they decrease rapidly with increase in the distance from the cylinder to the channel bottom. The effects of mixing and entrainment on the nature of the hydrodynamic loads are considered.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 2, pp. 81–90, March–April, 2005.     

载流圆锥薄壳的磁弹性应力与变形分析

在建立旋转壳体的非线性磁弹性运动方程的基础上,研究了电磁场和机械载荷联合作用下载流圆锥薄壳的磁弹性效应．通过算例,得到了载流圆锥薄壳的位移及应力与通电电流强度之间的关系．解决了圆锥薄壳顶点处的奇异性问题,给出了轴对称条件下的数值解．计算结果表明：改变通电电流强度,可以改变载流圆锥薄壳的应力与变形状态,达到控制圆锥薄壳的受力与变形的目的．     

Surf Zone Currents

A large fraction of the water-wave energy incident on beaches is dissipated as the waves break and travel towards the shore through the surf zone. However, the momentum associated with the incident waves is not destroyed and drives other motions within the surf zone. An analysis is given of the unsteady and irregular currents that may occur in the surf zone. The forcing due to a discrete group of waves, and the vorticity of the surf-zone currents are the major topics. In particular, the almost two-dimensional nature of the flow implies that significant eddies are likely to be generated. There is evidence that eddies arise from long-shore currents, and the combination of eddies of opposite sign gives rip currents. It is noted that generation of vorticity by non-uniformities in bores may be a useful way of considering the forcing of surf zone motion. Expressions for the rate of increase in the circulation about material circuits and the vorticity generated at bores are derived. Received 30 January 1997 and accepted 16 May 1997     

带有半埋藏空间裂纹的Cr12构件电磁热止裂分析

对带有半埋藏环形裂纹的Cr12标准拉伸试件进行了电磁热止裂研究.通过ZL-2型放电装置实现了半埋藏空间裂纹的止裂实验,采用微机控制电子万能试验机完成了止裂前后试件拉伸性能测试,同时计算了电磁热止裂前后应力强度因子的变化;建立了超强脉冲电流放电瞬间,环形裂纹尖端附近电流绕流、温度场和热应力场的数值分析模型,通过热-电耦合和热-机械耦合两个过程得到了脉冲放电瞬间拉伸试件中的电流矢量场、温度场和热应力场.研究表明:脉冲电流放电瞬间,围绕裂纹尖端金属熔化,钝化了裂尖,并围绕裂尖形成了热压应力场,得到了超细化的金属组织,止裂后抗拉强度得到了提高,达到了止裂的目的.     

Temperature field at time of pulse current discharge in metal structure with elliptical embedding crack

Theoretical analysis is made on the temperature field at the time of pulse current discharge in a metal structure with an elliptical embedding crack. In finding the temperature field, analogy between the current flow through an elliptical embedding crack and the fluid flow through a barrier is made based on the similarity principle. Boundary conditions derived from this theory are introduced so that the distribution of current density and the temperature field expressions can be obtained. The study provides a theoretic basis to the applications of stopping spatial crack with electromagnetic heating.     

连续弯道水流紊动特性试验研究

采用电磁流速仪, 在接近天然河道形态的连续弯道中, 对沿程断面上各点的脉动流速进行量测.根据测得的数据分析得到了紊动能谱、紊动强 度、紊动切应力、紊动能、紊动参数等的全断面分布.研究结果表明水流紊动主要集中在10 Hz以下,不同环流形式与各种紊动量之间会互相影响,边壁处纵向紊动强烈、横向和垂向紊动较弱.     

A coupled-mode model for water wave scattering by vertically sheared currents in variable bathymetry regions

A coupled-mode model is developed for treating the wave–current–seabed interaction problem, with application to wave scattering by non-homogeneous, sheared current with linear vertical velocity profile, over general bottom topography. The wave potential is represented by a series of local vertical modes containing the propagating and evanescent modes, plus additional terms accounting for the satisfaction of the boundary conditions. Using the above representation, in conjunction with a variational principle, a coupled system of differential equations on the horizontal plane is derived, with respect to the unknown modal amplitudes. In the case of small-amplitude waves, a linearized version of the above coupled-mode system is obtained, extending previous analysis by Belibassakis et al. (2011) to the propagation of water waves over variable bathymetry regions in the presence of vertically sheared currents. Keeping only the propagating mode in the vertical expansion of the wave potential, the present system reduces to a one-equation model, that is shown to extend known mild-slope mild vertical shear equation concerning wave–current interaction over slowly varying topography. After additional simplifications, the latter model is shown to be compatible with the extended mild-slope mild-shear equation by Touboul et al. (2016). Results are presented for various representative test cases demonstrating the usefulness of the present coupled mode system and the importance of various terms in the modal expansion, and compared against experimental data collected in wave flume validating the present method. The analytical structure of the present system facilitates extensions to model non-linear effects and applications concerning wave scattering by inhomogeneous currents in coastal regions with general 3D bottom topography.     

潮流能开发现状、发展趋势及面临的力学问题1）

随着化石能源大规模应用导致的环境污染问题日益严重,世界各国越来越关注清洁可再生能源的开发和利用.潮流能属于海洋可再生能源,清洁无污染且资源丰富,具有良好的开发前景.21世纪以来,潮流能开发在世界范围内取得较大进步,装机规模呈上升趋势,多种潮流能转换装置进入示范研究或准商业化阶段.在此方面,人们将面临如何利用研制生产的各类转换装置提高对潮流能的利用效率这一关键力学问题.通过调研和分析,详细梳理了我国潮流能资源状况、国内外开发利用最新状况、潮流能开发利用关键技术以及潮流能利用前景,最后结合我国研究现状给出技术发展建议.主要内容包括：潮流能的生成、特点及分布,主要介绍潮流的生成、潮流能的特点、潮流能资源及分布等;潮流能开发利用发展现状,主要介绍潮流能开发利用技术研发进程、国外发展现状、国内发展现状等;潮流能开发利用关键技术,主要介绍能量捕获与转换、传动系统与发电机、潮流能发电阵列布置、海上电网构建、支撑结构等;潮流能开发利用技术研究展望,主要介绍国外潮流能技术研究的重点、我国潮流能技术研究的重点等;我国潮流能开发利用的建议;最后是前景展望.     

导电薄板内电流密度分布与反平面剪切的比拟

定量分析电流密度在含裂纹载流薄板内的分布是当前利用电流热效应止裂技术中一个首先要解决的问题．由于裂纹的存在,电流密度在裂尖形成带奇异性分布的高度密集．现有的分析方法往往比较复杂或局限于特殊布置形式的裂纹．通过电流密度分布与弹性力学里反平面剪切问题的比拟,把分析含裂纹载流薄板内电流密度的分布等效于考虑相应的III型裂纹问题,并比照III型裂纹的应力强度因子来定义电流密度因子．而对于裂纹问题的处理可采用分布位错法这一断裂力学里便利有效的分析手段．由给出的算例可见,所提出的比拟解法可以方便精确地求解电流密度在裂尖附近的奇异分布,并有助于对这一奇异性在概念上的直观理解．     

Nonlinear stability analysis of a clamped rod carrying electric current

This paper is devoted to the analysis of the nonlinear stability of a clamped rod carrying electric current in the magnetic field which is produced by the current flowing in a pair of inifinitely long parallel rigid wires. The natural state of the rod is in the plane of the wires and is equidistant from them. Firstly under the assumption of spatial deformation, the governing equations of the problem are derived, and the linearized problem and critical currents are discussed. Secondly, it is proved that the buckled states of the rod are always in planes. Finally, the global responses of the bifurcation problem of the rod are computed numerically and the distributions of the deflections, axial forces and bending moments are obtained. The results show that the buckled states of the rod may be either supercritical or subcritical, depending on the distancz between the rod and the wires. Furthermore, it is found that there exists a limit point on the branch solution of the supercritical buckled state. This is distinctively different from the buckled state of the elastic compressive rods.Project supported by the Foundation of the Natural Science of China and Gansu Province     

求解予裂试件实时裂纹长度的一种新的直接法

本文提出的估算予裂试件实时裂纹长度的直接法，只需单试件的Ｐ－Ｖ记录和对应静裂纹试件塑性位移有限元解，这就避免了在规则化方法中由于几何函数不确定性对计算实时裂纹长度的影响。这一新的直接法的基础是推广的Ｇａｒｗｏｏｄ—Ｅｒｎｓｔ假设，即予裂试件与对应静裂纹试件在载荷、位移分别相等时其实时裂纹长度也相等。本文用实验验证这一假设，并在此基础上提出推论：这两类试件在Ｐ、Ｖ（和α）分别相等时其弹性和塑性位移分量也分别相等。按照这一推论可对予裂试件实时裂纹长度进行估算。文中的五个算例表明，这一直接法与卸载柔度法符合较好。