Unsteady bio-fluid dynamics in flying and swimming

Flying and swimming in nature present sophisticated and exciting ventures in biomimetics, which seeks sustainable solutions and solves practical problems by emulating nature's time-tested patterns, functions, and strategies. Bio-fluids in insect and bird flight, as well as in fish swimming are highly dynamic and unsteady; however, they have been studied mostly with a focus on the phenomena associated with a body or wings moving in a steady flow. Characterized by unsteady wing flapping and body undulation, fluid-structure interactions, flexible wings and bodies, turbulent environments, and complex maneuver, bio-fluid dynamics normally have challenges associated with low Reynolds number regime and high unsteadiness in modeling and analysis of flow physics. In this article, we review and highlight recent advances in unsteady bio-fluid dynamics in terms of leading-edge vortices, passive mechanisms in flexible wings and hinges, flapping flight in unsteady environments, and micro-structured aerodynamics in flapping flight, as well as undulatory swimming, flapping-fin hydrodynamics, body–fin interac-tion, C-start and maneuvering, swimming in turbulence,collective swimming, and micro-structured hydrodynamics in swimming. We further give a perspective outlook on future challenges and tasks of several key issues of the field.     

Elastic Waves in Bodies with Initial (Residual) Stresses

An analysis is made of the results obtained in the three-dimensional linearized theory of elastic waves propagating in initially stressed solids. Consideration is given to surface waves along planar and curvilinear boundaries and interfaces, waves in layers and cylinders, waves in composite materials, waves in hydroelastic systems, and dynamic problems for moving loads. The results were obtained in exact formulations.     

Prefacein：in dedication to Professors Yong Jin and Zhiqing Yu

This special issue of China PARTICUOLOGY is dedicated to Professors Yong Jin and Zhiqing Yu of Tsinghua University, China, to celebrate over five decades of their careers in chemical engineering research and education. Prof. Jin graduated in 1959 from Ural Polytechnic College in Svivdelovsk in the former USSR and then completed his post-graduate studies in Tianjin University in China in 1960. Thereafter, he taught at the Chinese University of Science and Technology for 13 years before joining Tsinghua University in 1973. Prof. Yu graduated in 1956 from Tianjin University in China and joined Tsinghua University thereafter. Between 1957 and 1959, Prof. Yu undertook more studies in Moscow Mendeleev Institute of Chemical Technology in the former USSR. During the 1970s, Professors Jin and Yu jointly created the Reaction Engineering Program in the Department of Chemical Engineering at Tsinghua University, which later became one of the largest fluidization research groups in the world, known as FLOTU - Fluidization Laboratory of Tsinghua University.     

Long-term trend in aerosol optical depth from 1980 to 2001 in north China

Using the Total Ozone Mapping Spectrometer (TOMS) monthly aerosol optical depth (AOD) at 500 nm data from 1980 to 2001 in north China, the spatial and temporal variations of AOD were examined. Seasonal AODs in Taldimakan Desert were 0.69 and 0.44 in spring and summer,respectively, which were mainly due to frequent occurrences of dust events in this region. Dust activities in spring also led to high aerosol loading in Gobi Desert and in northeast China where spring AODs were 0.33 and 0.29, respectively. Heavily impacted by events such as volcano eruption, forest fires and extraordinary dust storms, AODs showed large inter-annual variations. A decreasing tendency in AOD was observed in north China during 1980-1991, though a reverse tendency was revealed during 1997-2001, especially for spring AOD in northeast China. Further study is required to figure out how much human activities have contributed to the AOD tendency in north China.     

Application of ultrasonic-pulse-spectroscopy measurements to experimental stress analysis

We apply the technique of ultrasonic pulse spectroscopy to measure the interference effects between two shear waves propagating in specimens loaded in uniaxial compression. We show that the power spectrum of an echo containing both fast and slow components of a shear wave will exhibit periodic minima. The periodicity exhibited in the spectrum is 1/Δτ, where Δτ is the difference in arrival time between the fast and slow waves. A change in the state of stress which produces a change in the two shear velocities results in a stress-dependent change in wave-arrival times. Because of this velocity change, the frequency at which a particular minimum occurs in the spectrum changes, and this can be used to indicate the state of stress in the material. Our results indicate that, if the spectrum minima frequencies could be resolved to within 10 kHz, the principal-stress differences within 36 psi (0.251 MPa) could be measured in specimens of aluminum 1 in. (2.54 cm) thick. Inherent in analyzing and measuring echo-interference effects is a single-echo requirement. Thus, transducer coupling effects are minimized and measurements in highly attenuating materials or at high frequencies in normal attenuating materials are possible. This technique shows considerable promise as a means of measuring and monitoring the applied stresses in materials.     

Multiscale lattice Boltzmann-finite element modelling of chloride diffusivity in cementitious materials. Part II: Simulation results and validation

Chloride diffusivity in cementitious materials depends on the underlying microstructure and environmental conditions. The algorithms and implementation of the multiscale lattice Boltzmann-finite element scheme for prediction of chloride diffusivity in cementitious materials was described in detail in Part I (Zhang et al., 2013). Based on the obtained microstructures and the developed multiscale modelling scheme, chloride diffusivity in cementitious materials at the micro- and meso-scales, i.e. cement paste, mortar and concrete, are estimated and presented in Part II. The influences of w/c ratio, age, chloride binding, degree of water saturation, interfacial transition zone (ITZ) and aggregate content on chloride diffusivity are investigated in a quantitative manner. The simulations are validated with experimental data obtained from literature. The results indicate that the simulated chloride diffusivity in cementitious materials at each scale shows a good agreement with experimental data. In addition, the chloride binding, degree of water saturation, ITZ and aggregate content play significant roles in the chloride diffusivity in cementitious materials. The estimated chloride diffusivity in cementitious materials in this study accounting for the evolution of microstructure and environmental conditions can be directly used as input for the service life prediction of reinforced concrete structures.     

混凝土破坏理论研究进展

本文综述了混凝土的材料性质、破坏特点及机理,回顾了断裂力学与损伤力学在混凝土材料破坏研究中的应用,对当前混凝土破坏研究的主要问题和今后方向进行了讨论与展望。      

双柱单锥型液-液旋流管内流场的激光诊断

应用激光测速仪，对一种双柱单锥型液 液旋流管内的流动结构，进行了全场范围内的多工况流动诊断研究．揭示出其切向速度由内旋流区和外旋流区构成，其中内旋流区中的速度分布符合准强制涡关系，外旋流区中的速度分布符合准自由涡关系；轴向速度由上行流动区和下行流动区构成，两者之间在直管段以零速点作分界，在锥体段则以零速区作过渡并伴随有一定的回流出现，且该过渡区或回流区的大小随锥体截面的收缩而减小，直到进入直管段后消失；各湍流量的分布以管芯处最大向外逐渐减小，沿轴向是直管段中的湍流度大于锥体段中的湍流度，而且湍流度在旋流管内具有各向异性的特性．     

关于渗流中流线不封闭的特性和条件

本文对于流体在多孔介质中流动的特性进行理论研究和数值计算，提出两个关于渗流中流线不封闭的特性和条件，得到了在一般工程实际情况中的多孔介质区域内部不存在封闭流线的结论。本文以突变截面圆管中不可压缩渗流为算例，利用半人工瞬变方法进行数值计算，得到流体在充满多孔介质的突扩截面圆管和突缩截面圆管中流动时关于速度分布和压力分布的结果。由此表明，在突变截面附近的渗流区域中不存在回流和分离流，也不存在封闭的流线。渗流的这些流动特性不同于在无多孔介质的空间区域中的流动特性。     

Phenomenology and physical origin of shear localization and shear banding in complex fluids

We review and compare the phenomenological aspects and physical origin of shear localization and shear banding in various material types, namely emulsions, suspensions, colloids, granular materials, and micellar systems. It appears that shear banding, which must be distinguished from the simple effect of coexisting static-flowing regions in yield stress fluids, occurs in the form of a progressive evolution of the local viscosity toward two significantly different values in two adjoining regions of the fluids in which the stress takes slightly different values. This suggests that from a global point of view, shear banding in these systems has a common physical origin: Two physical phenomena (for example, in colloids, destructuration due to flow and restructuration due to aging) are in competition, and depending on the flow conditions, one of them becomes dominant and makes the system evolve in a specific direction.     

Instabilities in viscoelastic flows

Viscoelastic instabilities are of practical importance, and are the subject of growing interest. Reviewed here are the fresh developments as well as earlier work in this area, organized into the following categories: instabilities in Taylor-Couette flows, instabilities in cone-and-plate and plate-and-plate flows, instabilities in parallel shear flows, extrudate distortions and fracture, instabilities in shear flows with interfaces, instabilities in extensional flows, instabilities in multidimensional flows, and thermohydrodynamic instabilities. Emphasized in the review are comparisons between theory and experiment and suggested directions for future work.     

黄土构造节理研究及其应用

作者发现大量不同力学性质的黄土构造节理后，识别了它们的区域性、系统性特征；认定其两组扭裂面代表新构造应力场最大剪切应力方向，利用黄土构造节理系及由其控制的土层沟槽网络恢复了相应地区Q3-4构造应力场；提出构造节理是黄土区地下水运移的主要通道和赋存的场所；发现构造节理是黄土区地裂缝、滑坡、崩塌和水土流失等地质灾害的构造基础之一，证实构造节理是黄土地层的软弱面，风化和继之而来的应力侵蚀就由此开始，进而逐步塑造黄土碟、穴、井、桥、柱、墙、沟等潜蚀地貌和部分侵蚀地貌。研究黄土构造节理对恢复新构造应力场、帮助预测地裂乃至地震活动、控制水土流失和滑塌灾害、进行工程乃至区域稳定性评价、重新认识黄土潜蚀地貌发育规律、指导干旱半干旱黄土区找水，既有理论意义又有实际意义。     

Parameter identification and synchronization of uncertain general complex networks via adaptive-impulsive control

Multifractal theory has been widely used in kinds of field. In this paper, methods were proposed to study in the power-law auto-correlation and cross-correlation of power operating data based on multifractal detrended analysis (MF-DFA) and multifractal detrended cross-correlation analysis (MF-DXA). We find that both the price and the load time series in California power market and PJM power market exhibit long-term correlation. And the cross-correlation behaviors of the two series in each power market and between the two markets are also analyzed by the method MF-DXA after testing the existence of the cross-correlation of the above power operating data. However, there are some differences in the cross-correlation behaviors between the two markets. It shows that the cross-correlation of the price and the load is significant in every time periods in California 1999 power market, but in the year of 2000 in the same region market, the cross-correlation is insignificant in most time periods. Meanwhile, we conclude that cross-correlation is weaker in the California market than in the PJM market by studying the two consecutive years of the California 1999–2000 and PJM 2001–2002 power markets. We also discuss how the time intervals affect the cross-correlation exponents of the power operating data based on time-delay MF-DXA. An interesting finding is that the biggest cross-correlation exponent of the two series appeared in about 12 days time delay for the PJM 2001 power market and strongest cross-correlation in the California 1999 power market is found in lots of cyclical time intervals.     

Surface integrity of inconel-718 nickel-base superalloy using controlled and natural contact length tools. Part II: Unlubricated

The surface integrity of inconel-718 nickel-base superalloy was investigated using orthogonal cutting at various cutting speeds, depths of cut and chip-tool contact lengths under unlubricated conditions. The experimental work involved the determination of residual stress, plastic strain and microhardness distribution in the surface region and the examination of the surface and subsurface using scanning electron and optical microscope. The results are interpreted in terms of the variation in shear-plane length and consequently the variation in tool forces with the cutting conditions. The results are compared with similar results obtained under lubricated conditions. It is found that the lubricant is effective at low cutting speeds in reducing the tool forces that led to lower hardness and plastic strain in the surface region. In general, the severity of surface damage in terms of intensity and total area affected was decreased with the application of a lubricant. Both residual stresses and plastic strains decreased and the quality of the machined surface improved with an increase in cutting speed, a decrease in depth of cut and with tools having controlled chip-tool contact lengths.     

统计细观损伤力学和损伤演化诱致突变（Ⅱ）

材料的损伤和破坏是固体力学中一个十分复杂的基本问题，近年来在这方面的研究从不同的角度取得一系列新的进展。本文较系统地介绍了统计细观损伤力学和损伤演化诱致突变理论，这是把力学与统计物理学及非线性科学结合起来研究材料损伤和破坏问题的一种探索。     

Forced capillary-gravity wave motion of two-layer fluid in three-dimensions

A class of problems associated with forced capillary-gravity wave motion in a channel are analyzed in the presence of surface and interfacial tensions in a two-layer fluid in both the cases of finite and infinite water depths. The two and three-dimensional Green functions associated with the capillary-gravity wave problems in the presence of surface and interfacial tensions are derived using the fundamental source potentials. Using the two-dimensional Green function along with Green’s second identity, the expansion formulae for the velocity potentials associated with the capillary-gravity wavemaker problems in two-dimensions are obtained. The two-dimensional results are generalized to derive the expansion formulae for the velocity potentials associated with the forced capillary-gravity wave motion in the presence of surface and interfacial tensions in three-dimensions. Certain characteristics of the eigen-system associated with the expansion formulae are derived. The velocity potentials associated with the free oscillation of capillary-gravity waves in a closed basin and semi-infinite open channel in the presence of surface and interfacial tensions are obtained. The utility of the forced motion in a channel is demonstrated by analyzing the capillary-gravity wave reflection by a wall in a channel in the presence of surface and interfacial tensions. Long wave equations associated with capillary-gravity wave motion in the presence of surface and interfacial tensions are derived under shallow water approximation and the associated dispersion relation are obtained. Various expansion formulae and Green functions derived in the present study will be useful for analyzing a large class of physical problems in ocean engineering and mathematical physics.     

Mathematical simulation of hydrodynamic generators of oscillations

Unsteady turbulent swirled water flow in a channel in the presence of cavitation is calculated. It is shown that in the near-axial channel zone a fluctuating vapor region, or cavity, arises and variations in the cavity shape and dimensions lead to the onset of undamped pressure fluctuations. The amplitude-frequency characteristics of the oscillations are obtained in different channel cross-sections. The amplitude maximum position in the channel expansion zone is in agreement with the available experimental data. The dynamics of toroidal vortices formed in the hydrodynamic generator channel and in the expansion zone at its exit are established.     

Nonlinear guided bulk waves in heterogeneous elastic structural elements

Guided nonlinear bulk strain waves are considered in heterogeneous composite materials, containing macro- or micro(nano)inclusions as the main sources of nonuniformity. Theory is based on the combination of the finite deformation and the 5-constant (Murnaghan) nonlinear elasticity, that leads under several assumptions to the only nonlinear doubly dispersive equation with variable coefficients (DDE) for a component of longitudinal strain in rod and in shell. Numerical simulations performed and experimental data are discussed, and lead to conclusions concerning dramatic influence of heterogeneity in dynamic behaviour of strain solitons in solids. We have shown how different smooth variations in elasticity may lead to either amplification or decay of strain solitons in both polymer rod and shell. These date can be useful in the NDT problems, and in problems of the solids integrity under intense elastic pulse loading.     

Structure and stability of a laminar diffusion flame in a compressible,three-dimensional mixing layer

In this paper we consider the structure and stability of a three-dimensional, finite rate, reacting compressible mixing layer lying between two streams of reactants with different freestream speeds and temperatures. Using both numerical calculations and asymptotic analyses, the structure of the ignition and diffusion flame regimes was studied; in particular the effect of crossflow on the structure. The results of both approaches are in good agreement. In the stability analysis the general case of three-dimensional disturbances was studied. It was shown that certain general results, in particular the circle theorem, could be easily extended to three-dimensional disturbances in a compressible fluid with crossflow. The introduction of a chemical reaction, in the form of a flame sheet, was found to have complex effects on flow stability. These included the appearance of additional neutral modes compared with the nonreacting case and substantial changes in growth rates with heat release, the skew angle of the mean flow, and the direction of propagation of the disturbance wave. These results are discussed in detail.The first author was supported, in part, by AFOSR Contract 91-0180, and in part by the National Aeronautics and Space Administration under NASA Contract No. NASI-19480 while in residence at the Institute for Computer Applications in Science and Engineering, NASA Langley Research Center, Hampton, VA 23681-0001, U.S.A. The second author was supported, in part, by AFOSR Contract 91-0250, and in part by the National Aeronautics and Space Administration under NASA Contract No. NASI-19480 while in residence at the Institute for Computer Applications in Science and Engineering, NASA Langley Research Center, Hampton, VA 23681-0001, U.S.A.     

爆炸对自然磁场干扰机理

爆炸产生的电磁场对爆炸测试有较明显的干扰, 一些含金属炸药产生的电磁干扰甚至可以使设备失效, 为了给出爆炸产生电磁效应的理论分析方法和特性, 针对常规炸药爆炸过程中观察到的自然磁场扰动现象进行理论机理和数值模拟研究. 采用热平衡电离理论描述爆炸过程中产生的电离气体, 结合爆炸问题的数值模拟方法, 获得爆炸场中电导率的时空分布, 计算磁场扩散率; 基于非理想全磁流体力学方程求解一定磁扩散率下等离子体高速运动引起的自然磁场扰动, 实现爆炸产生磁场效应的二维模拟; 对比炸药不同起爆条件对产生磁场扰动的影响, 结果表明: 炸药起爆参数对磁场扰动有很大的影响, 这种影响源自于电导率的巨大差异, 虽然爆轰产物的高速运动有引起磁场大幅度扰动的能力, 然而只有具有一定电导率的爆轰气体才能冻结磁场扰动. 数值模拟结果与文献中的结果进行了对比, 模拟得到的由电磁波引起的磁场扰动在时间尺度上与文献结果符合较好, 从一定程度上证明了数值模拟方法的可靠性. 本文的数值模拟结果指出炸药几何构型不对称的时候, 在自然磁场取不同方向时将会产生不同的磁场扰动强度, 而这是目前该领域中尚且无人关注和讨论过的问题.