Physical Modeling of Current-Induced Seabed Scour around a Vibrating Submarine Pipeline

Most of the existing researches either focus on vortex-induced-vibrations (VIV) of a pipeline near a rigid boundary, or on seabed scour around a fixed pipeline. In the fields, pipeline vibration and seabed scour are actually always coupled. Based on the similarity analysis, a series of tests were conducted with a hydro-elastic facility to investigate the influence of pipe vibration on the local scour and the effects of scour process on the pipeline dynamic responses. Experimental results indicate that, there exist two phases in the process of sand scouring around the pipeline with small embedment, i.e. Phase I: scour beneath pipe without VIV, and Phase II: scour with VIV of pipe. It is also found that the gap-to-diameter ratio (e/D) has much effect upon the scour depth for the fixed pipes. For a given value of e/D, the vibrating pipes with close proximity to seabed may induce a deeper scour hole than the fixed ones. Within the examined gap-to-diameter ratio range (425 < e/D < 0.75), the influences of gap-to-diameter ratio on the maximum values of scour-depth for the case of vibrating pipes are not as much as those for the case of fixed pipes.

Characteristics of Wake Flow in Long Bluff Bodies

Vortex shedding is the main characteristics of bluff bodies, which will cause the bluff bodies to vibrate and sometimes result in the structures failure. In this paper the wake flow characteristics of 21 bluff bodies with rectangular, rounded and angular profiles and the length-to-width ratio in the range of 4~12 were deeply studied by Micro ADV. Two parameters, namely the relative intensity of the load due to Karman vortices and the large scale vortex intensity, were introduced to measure the wake flow intensity. Generally, the values of these parameters for different bluff bodies are consistent with each other. The experiment results showed that the key factor affecting the wake flow characteristics is the bluff edge, especially the leading edge geometry. The wake flow in bluff bodies with rounded edge profiles has more regular vortices and becomes more periodic than that in bluff bodies with rectangular ones. A bluff body with angular edged profile was witnessed to have not only small wake loading but small hydraulic resistance also.

Information Preservation Modeling of Rayleigh-Benard Transition from Thermal Conduction to Convection

Onset and evolution of the Rayleigh-Benard (R-B) convection are investigated using the Information Preservation (IP) method. The information velocity and temperature are updated using the Octant Flux Splitting (OFS) model developed by Masters & Ye based on the Maxwell transport equation suggested by Sun & Boyd. Statistical noise inherent in particle approaches such as the direct simulation Monte Carlo (DSMC) method is effectively reduced by the IP method, and therefore the evolutions from an initial quiescent fluid to a final steady state are shown clearly. An interesting phenomenon is observed: when the Rayleigh number (Ra) exceeds its critical value, there exists an obvious incubation stage. During the incubation stage, the vortex structure clearly appears and evolves, whereas the Nusselt number (Nu) of the lower plate is close to unity. After the incubation stage, the vortex velocity and Nu rapidly increase, and the flow field quickly reaches a steady, convective state. A relation of Nu to Ra given by IP agrees with those given by DSMC, the classical theory and experimental data.

Experimental Study on Flow Induced Vibration of a Cylinder with Two Degrees of Freedom Near a Rigid Wall

The flow-induced vibration of a cylinder with two degrees of freedom near a rigid wall under the action of steady flow is investigated experimentally. The vibration amplitude and frequency of the cylinder and the vortex shedding frequency at the wake flow region of the cylinder are measured. The influence of gap-to-diameter ratio upon the amplitude response is analyzed. The experimental results indicate that when the reduced velocity (Vr) is in the range of 1.2 < Vr < 2.6, only streamwise vibration with small amplitude occurs, whose frequency is quite close to its natural frequency in the still water. When the reduced velocity Vr > 3.4, both the streamwise and transverse vibrations of the cylinder occur. In this range, the amplitudes of transverse vibration are much larger than those of streamwise vibrations, and the amplitudes of the streamwise vibration also get larger than those at the range of 1.2 < Vr < 2.6. At the range of Vr > 3.4, the frequency of streamwise vibration undergoes a jump at certain values of Vr, at which the streamwise vibrating frequency is twice as much as the transverse one. However, when the streamwise vibration does not experience a jump, its frequency is the same as that of the transverse vibration. The maximum values of second streamwise and transverse vibration amplitudes increase with increasing gap-to-diameter ratios.

Microstructure and mechanical properties of copper subjected to high pressure torsion

Experiments were conducted on copper subjected to High Pressure Torsion to investigate the evolution of microstructure and microhardness with shear strain, gamma. Observations have been carried out in the longitudinal section for a proper demonstration of the structure morphology. An elongated dislocation cell/subgrain structure was observed at relatively low strain level. With increasing strain, the elongated subgrains transformed into elongated grains and finally into equiaxed grains with high angle grain boundaries. Measurements showed the hardness increases with increasing gamma then tends to saturations when gamma >5. The variation tendency of microhardness with gamma can be simulated by Voce-type equation.

Study on fatigue crack nucleation of electrodeposited nanocrystalline nickel

The mechanism of fatigue crack nucleation for nanocrystalline (nc) nickel was experimentally investigated in this paper. The samples of electrodeposited ne nickel were loaded cyclically by using a three point bending instrument at first. Then, atomic force microscopy (AFM) was used to scanning the sample surface after fatigue testing. The results indicated that, after fatigue testing, there are vortex-like cells with an average size of 108nm appeared along the crack on nc nickel sample. And, the roughness of sample surface increased with the maximum stress at the surface.

Dislocations and twins in nanocrystalline Ni after severe plastic deformation: the effects of grain size

Deformation microstructures have been investigated in nanocrystalline (nc) Ni with grain sizes in the 50-100 nm range. It was found that deformation twinning started to occur in grains of similar to 90 nm, and its propensity increased with decreasing grain size. In most of the nc grains dislocations were observed as well, in the form of individual dislocations and dipoles. It is concluded that dislocation-mediated plasticity dominates for grain sizes in the upper half, i.e. 50-100 nm, of the nanocrystalline regime. (C) 2007 Published by Elsevier B.V.

钱学森技术科学思想与力学

本书收录会议论文60篇。

　　本书内容大体分为三类。一类着重介绍了钱学森先生的风范，他的爱国主义，他的勤奋好学，他的治学精神和治学态度，他在学术上的重大贡献以及他的科学思想。第二类是介绍在他的领导或影响下，五十年来我国力学研究取得的成就及进一步的进展。第三类涉及我国力学界正在开展的其他方面的学术研究进展。我们的共同目的是继承优良传统，开拓我国力学在新世纪的发展。

　　读者对象：力学、航空航天、机械等领域的工程技术人员，研究人员。

谈镐生文集

本文集收入了著名力学家、应用数学家、中国科学院院士谈镐生先生在流体力学、稀薄气体动力学和应用数学研究领域的论文和研究报告26篇，谈镐生先生倡导和支持力学基础研究的有关文章和论述21篇，谈先生的学术活动和生活图片多幅，以及谈镐生先生生平，最后附有谈先生生平年表。

 

 

粘弹性材料界面裂纹的瞬态响应和散射研究

本文研究粘弹性材料界面裂纹对冲击载荷的瞬态响应和对广义平面波的稳态散射。相对于已有广泛研究的弹性材料裂纹瞬态响应和稳态散射问题，本文的研究有三个突出特点：1）粘弹性材料；2）界面裂纹；3）广义平面波入射。粘弹性材料界面裂纹对冲击载荷的瞬态响应和对广义平面波的散射尚无开展研究，本文在弹性材料相应问题的研究基础上，首先开展了这一问题的研究。对于冲击载荷下粘弹性界面裂纹的瞬态响应问题，利用Laplace积分变换方法，将粘弹性材料卷积型本构方程转化为Laplace变换域内的代数型本构方程，从而可以在Laplace变换域内象处理弹性材料的冲击响应一样，将相应的混合边值问题归结为关于裂纹张开位移COD的对偶积分方程，并进一步引入裂纹位错密度函数CDD （Crack Dislocation Density），将对偶积分方程化成关于CDD的奇异积分方程（SIE）。用数值方法求解奇异积分方程得到变换域内的动应力强度因子数值解，最后利用Laplace积分逆变换数值方法得到时间域内的动应力强度因子的时间响应。理论分析考虑了两种裂纹模型，即Griffith界面裂纹和柱面圆弧型界面裂纹。考虑的载荷包括反平面冲击载荷和平面冲击载荷。对于平面冲击载荷，通过对裂尖应力场的奇性分析，首次发现粘弹性界面裂纹裂尖动应力场奇性指数不是常数0.5，而是与震荡指数一样依赖材料参数。针对反平面冲击载荷给出了一个算例，计算了裂尖动应力强度因子的时间响应，并与弹性材料的结果作了比较，发现粘弹性效应的影响不仅使过冲击峰值降低，而且使峰值点后移。粘性效应较大时，过冲击现象甚至不出现。关于粘弹性界面裂纹对广东省义平面波的散射问题，首先研究广义平面波在无裂纹存在的理想界面的反射和透射，再研究由于界面裂纹的存在而产生的附加散射场。利用粘弹性材料的复模量理论，可将粘弹性材料的卷积型相构方程化成频率域内的代数型本构方程。类似弹性平面波的处理，在频率域内将问题最终归结为关于裂纹位错密度CDD的奇异积分方程。数值方法求解奇异积分方程即可得到频率域内的散射场，并进而得到裂尖动应力强度因子和远场位移型函数和散射截面。理论分析考虑了两种裂纹模型：Griffith界面裂纹和柱面圆弧型界面裂纹。研究的入射波有广义的SH波和P波。对于广义平面P波入射的情况，通过对裂尖应力场的奇性分析，同样发现粘弹性界面裂纹裂尖动应力场奇性指数不地常数0.5，而是与震荡指数一样依赖于材料参数。对柱面裂纹散射远场的渐近分析，发现远场位移和应力除含有几何衰减因子外，都含有一个材料衰减速因子。散射截面由于材料衰减因子的存在也成为依赖散射半径的量。为了使散射截面仍有意义，文中提出一种修正办法。对Griffith界面裂纹，给出了一个广义平面SH波入射的算例；对柱面界面裂纹，给出了一个广义平面P波入射的算例。计算了不同入射角和入射频率下裂纹的张开位移和动就应力强度因子，并分析了其依赖关系。求解奇异积分方程的数值方法和Laplace积分逆变换数值方法是本文的基本数值方法。本文对这两种方法作了大量的调研和系统的研究。在对比分析的基础上，对现有的各种方法从原理，适用范围，计数效率，优势及特点进行了归纳总结。并尝试了奇异积分方程的最新数值方法--分片连续函数法，证实了其适用性和方便性．

基于跨尺度模型的界面研究

探索和建立不同尺度理论之间的关联模式是科学研究的重要课题，本文基于跨尺度模型着重探讨了金属陶瓷界面的凝聚能和原子结构问题。本文遵循原始Peierls-Nabarro模型的基本思想，提出了一种处理一维界面失配位错组的新方法。在这个推广的Peierls-Nabarro模型中，本文得到了一个简单而且准确的解析解，此解反映了失配位错的核结构、能量与失配度、剪切模量之间的依赖关系。当界面剪切模量较强而失配度较小时，界面的结构可以用一组奇导师Volterra位错来描述，这与一些原子模拟结果一致。采用这一简单的模型，引入第一原理计算得到的数据，此模型可以估算金属陶瓷界面的凝聚能。一维界面失配位错组的Peierls-Nabarro模型还被解析推广描述一大类较宽的位错。在模型中我们引进了一个参数a，通过控制参数a，我们可以系统地改变失配位错芯的宽度、剪切应力的分布和弹性恢复力。随着a增加，位错宽度增加，同时弹性恢复力和失配位错应力的幅度减少。当界面剪切模量强和失配度小时，失配位错的宽度近似线性反比于弹性恢复力的幅度大小。同时当界面剪切模量和失配度固定时，失配能、弹性能和总的界面能随a的增加而减少。界面能和恢复力律形式密切相关，当界面剪切模量弱和失配度大时，这种依赖关系更强。考虑到界面常常是在晶格两个方向都有失配，本文还引进了描述界面周期失配位错的二维广义Peierls-Nabarro模型，使得我们能够定量地研究界面的结构和能量。文中定量分析了广义堆垛能γ面对界面失配位错的结构和能量的影响，分析了位错网中两种位错组的相互作用。当界面剪切模量τ_0变大和失配度f变小时，随着位错核区占整个界面的比重下降，γ面的形状对界面能量和结构影响减弱，结果两种位错组之间的相互作用也减弱。此外γ面的变化还有可能导致位错网结构的转变，也就是导致界面结构的转变。应用此模型，本文还研究了金属－陶瓷Ag/MgO(100)界面，给出了界面的能量和原子结构。文中得出结论：在Ag/MgO(100)界面将会形成{1/2<110>; <110>}类型的位错网。此外由于界面失配位错的形成，Ag/MgO(100)界面凝聚能的理论值900mJ/m~2将减少214mJ/m~2，最终成为686mJ/m~2。基于第一原理赝势平面波的总能计算，文中给出了金属陶瓷Al/MgO(100)界面弛豫和未弛豫时的广义堆垛能面。然后结合第三章发展的广义二维Peierls-Nabarro模型，详细研究了金属陶瓷Al/MgO(100)界面的原子结构和界面能。文中得出的“在Al/MgO(100)界面将会形成{1/2<110>; <110>}类型位错网”的推论，证实了Vellinga等的猜测；文中还预测了凝聚能的理论是在600mJ/m~2（未弛豫情形）和670mJ/m~2（弛豫情形）之间。这个应用表明此方法能够容易地建立连续介质理论和第一原理计算之间的联系，实现理论上的跨尺度。本文最后提出了一种得到界面原子有效对势的反演方法。通过反演金属－MgO陶瓷界面的第一原理计算的凝聚能曲线，我们得到了一些金属原子和陶瓷离子之间的对势，此对势反映了金属陶瓷键合的特性。本文的反演方法提供了通过第一原理计算数据来拟合界面原子对势的一种可行性途径。这种方法可归结为第一类尺度关联理论，即单向的跨尺度关联模式。

均匀来流绕旋转及旋转振荡圆柱绕流研究

该文通过数值方法求解二维不可压Navier-Stokes方程，对均匀来流中静止、旋转和旋转振荡圆柱绕流进行了系统的数值模拟.该文采用有限体积法对控制方程进行离散，选用元结构化四边形网格剖分计算区域，关于速度-压力耦合的处理使用了SIMPLEC方法.经过了大量的数值模拟，分盺166L鸬玫搅苏饧钢秩屏鞯氖的Ｄ饨峁?该文重点是用快速傅里叶变换（FFT）方法对旋转振荡圆柱绕流中的频率耦合现象进行研究，并分析在不同频率耦合作用下涡形成、发展和脱落的规律.

Plastic Deformation Of Nanocrystalline Nickel

A high-resolution electron microscopy study has uncovered the plastic behavior of accommodating large strains in nanocrystalline (NC) Ni subject to cold rolling at liquid nitrogen temperature. The activation of grain-boundary-mediated-plasticity is evidenced in NC-Ni, including twinning and formation of stacking fault via partial dislocation slips from the grain boundary. The formation and storage of 60A degrees full dislocations are observed inside NC-grains. The grain/twin boundaries act as the barriers of dislocation slips, leading to dislocation pile-up, severe lattice distortion, and formation of sub-grain boundary. The vicinity of grain/twin boundary is where defects preferentially accumulate and likely the favorable place for onset of plastic deformation. The present results indicate the heterogeneous and multiple natures of accommodating plastic strains in NC-grains.

A smoothing technique for discrete delta functions with application to immersed boundary method in moving boundary simulations.

The effects of complex boundary conditions on flows are represented by a volume force in the immersed boundary methods. The problem with this representation is that the volume force exhibits non-physical oscillations in moving boundary simulations. A smoothing technique for discrete delta functions has been developed in this paper to suppress the non-physical oscillations in the volume forces. We have found that the non-physical oscillations are mainly due to the fact that the derivatives of the regular discrete delta functions do not satisfy certain moment conditions. It has been shown that the smoothed discrete delta functions constructed in this paper have one-order higher derivative than the regular ones. Moreover, not only the smoothed discrete delta functions satisfy the first two discrete moment conditions, but also their derivatives satisfy one-order higher moment condition than the regular ones. The smoothed discrete delta functions are tested by three test cases: a one-dimensional heat equation with a moving singular force, a two-dimensional flow past an oscillating cylinder, and the vortex-induced vibration of a cylinder. The numerical examples in these cases demonstrate that the smoothed discrete delta functions can effectively suppress the non-physical oscillations in the volume forces and improve the accuracy of the immersed boundary method with direct forcing in moving boundary simulations.

Effects of geometric shape on the hydrodynamics of a self-propelled flapping foil

The hydrodynamics of a free flapping foil is studied numerically. The foil undergoes a forced vertical oscillation and is free to move horizontally. The effect of chord-thickness ratio is investigated by varying this parameter while fixing other ones such as the Reynolds number, the density ratio, and the flapping amplitude. Three different flow regimes have been identified when we increase the chord-thickness ratio, i.e., left-right symmetry, back-and-forth chaotic motion, and unidirectional motion with staggered vortex street. It is observed that the chord-thickness ratio can affect the symmetry-breaking bifurcation, the arrangement of vortices in the wake, and the terminal velocity of the foil. The similarity in the symmetry-breaking bifurcation of the present problem to that of a flapping body under constraint is discussed. A comparison between the dynamic behaviors of an elliptic foil and a rectangular foil at various chord-thickness ratios is also presented.