基于颗粒流的新老混凝土加固构件轴压性能细观研究

考虑到混凝土是一种非均质材料，为研究新老混凝土界面黏结特性和破坏机理，利用PFC3D细观离散元软件对新老混凝土黏结试块双面剪切性能和增大截面法加固钢筋混凝土构件轴压力学性能进行数值研究。结果表明，对于双面剪切试验，新老混凝土剪切-位移数值曲线与试验曲线基本一致，模型接触断裂主要发生在新老混凝土界面处，裂缝数量随着模型达到峰值后急剧上升，在界面处形成贯通的断裂面；在轴压试验模拟中，轴压力、混凝土应变关系数值曲线与试验结果吻合较好，数值结果能较好地预测软化阶段，模型中接触断裂主要发生在柱中部及下部位置，与实际破坏模式相符。     

爆轰产物状态方程的水下爆炸反演理论研究

由于水下爆炸过程中爆轰产物的信息以水中压缩波形式向外传递，本文旨在探讨如何利用水下爆炸试验数据确定爆轰产物的状态方程。相较于常规圆筒试验，水下爆炸试验具有装置简单成本低、装药尺寸限制少、测定压力范围更广的特点，更适用于大药量或非理想炸药的现场测试。本文从水中冲击波轨迹和波后压力时程曲线出发，发展了由冲击波及其波后流场还原水气界面的逆特征线算法，以及根据水气界面确定爆轰产物状态方程的遗传算法。与水下爆炸正演结果对比，发现逆特征线法可以较准确地还原水气界面的位置和压力参数，有效压力下限可达2 MPa，远低于圆筒试验的测试下限0.1 GPa。根据水气界面的反演结果，遗传算法也能稳定地优化JWL方程参数，8种常用炸药的等熵衰减压力误差在爆压至0.01 GPa的区间内都小于3%。结果表明，利用本文的逆特征线算法和遗传算法，理论上可以反演出压力范围较宽、准确性较高的爆轰产物状态方程。     

界面端应力奇异性与复合材料界面剪切强度细观实验分散性分析

复合材料细观实验方法主要有纤维拔出、纤维压力、纤维段裂和微球脱粘实验等四种；但这四种试验得到的界面剪切强度结果存在很大的分散性。虽经三十余年的研究和改进，仍未能消除。为研究分散性产生的原因，本文以轴对称界面端应力奇异性分析为基础，推导出求解四种试件界面端的特征值的特征方程，并给出了特征值随Dundurs常数的变化情况，由此发现用相同的纤维和基体制作的四种试件在界面端存在奇异性不同的应力场，从而阐明了四种界面剪切强度试验结果巨大分散性的产生原因在于纤维和基体间界面处的应力奇异性。     

纤维─基体界面剪切强度与断裂韧性的表征和测定

根据单向复合材料纤维分布、剪切变形与裂纹扩展的细观分析,提出纤维一基体界面剪切强度和断裂韧性的简易细观表征与测定方法．     

单纤维段裂试验评述

单纤维段裂试验作为复合材料界面剪切强度的一种测试方法被沿用至今.但是, 这种方法的可信度已受到一些研究者的质疑.为了明确单纤维段裂试验的问题, 本文首先对试验技术、试验结果分析等方面作了概述, 并指出: 纤维段裂的饱和状态是单纤维段裂试验的终点标志,以及临界长度是由试验得到的唯一数据, 而这二点是这种试验方法独具的特点, 同时也是这种试验方法难以克服的缺陷.在单纤维段裂试验中, 按照纤维段界面端处的局部损伤模式, 有3种界面端应力奇异性分析的问题需要予以考虑:(1)纤维断裂, 基体没有开裂, 和界面没有脱粘;(2)纤维断裂, 基体开裂, 但界面没有脱粘;(3)纤维断裂, 界面脱粘, 基体已开裂或基体未开裂.在单纤维段裂试验的界面端应力奇异性分析的基础上, 本文对单纤维段裂试验的可靠性进行了研究.结论是: 任何纤维和基体组成的复合材料的单纤维段裂试验都存在界面端应力奇异性, 这就排除了用单纤维段裂试验测定界面剪切强度的可能性.      

基于WENO-THINC/WLIC模型的水气二相流数值模拟

水气二相流与诸多领域的实际工程问题密切相关. 对二相流运动进行高精度的数值模拟是计算流体力学研究的难点和热点. 针对开敞水域的自由表面流运动问题, 将水和空气均视为不可压缩流体, 采用五阶加权基本无震荡(weighted essentially non-oscillatory, WENO)格式求解描述流体运动的纳维斯托克斯(Navier-Stokes, NS)方程, 利用以加权线性界面算法改进的多维双曲正切函数界面捕捉法(tangent of hyperbola for interface capturing with weighed line interface calculation, THINC/WLIC)追踪水气界面, 建立WENO-THINC/WLIC水气二相流运动数值模型. 模型采用分步计算法离散求解控制方程, 通过压力投影法求解压强场, 并利用三阶总变差递减(total variation diminishing, TVD)龙格库塔(Runge-Kutta, RK)法对时间项进行离散求解. 通过对环境速度场下Zalesak's disk和shearing vortex界面运动问题, 线性液舱晃荡问题以及溃坝问题的模拟结果与理论分析或试验结果的比较, 对所建立的水气二相流数值模型的适用性及模拟精度进行了验证. 结果表明, 本模型的模拟结果与物理模型或理论分析结果吻合良好, 能较为准确地再现不可压缩水气二相流运动现象. 鉴于WENO格式和THINC法本身在算法及应用等方面仍在不断改进, 本研究提出的WENO-THINC耦合模型为后续更高精度的二相流计算模型开发提供了一种研究思路.      

纤维─基体界面剪切强度与断裂韧性的表征和测定

根据单向复合材料纤维分布、剪切变形与裂纹扩展的细观分析,提出纤维一基体界面剪切强度和断裂韧性的简易细观表征与测定方法．     

风浪充分发展下海面阻力系数的数值研究

考虑海气界面附近大气边界层内海浪充分发展情况下的海气动量交换．根据Ｄｏｎｅｌａｎ（１９８２）的观点建立海气界面模型，采用ｋ ε模式以计算湍流流场．所得流场结构包括风速、湍流动能、湍能耗散及湍流粘性系数等都较合理；计算的海面阻力系数ＣＤ在中等风速时与实测结果符合良好．     

岩体—界面系统剪切不稳定性分析

基于界面的刚塑性应变软化假设，分析了岩体－界面系统在端部剪力和岩体中分布剪切载荷共同作用下的变形、应力和损伤演化。利用位能原理和稳定性的能量准则，得到了岩体界面系统的不稳定性条件。分析结果表明，损伤区达到边界之前，系统可能是稳定的或者是不稳定的。依赖于分布剪切载荷和界面摩擦力的比较，若均布剪切载荷大于界面摩擦力，则系统不稳定，否则系统稳定；当损伤区达到边界之后，系统的不稳定性决定于载荷及界面材料性质，界面软化刚度系数和界面强度对于不稳定性有明显影响。     

微粘结实验影响因素的边界元法分析

微粘结实验是一种新的简便的测量复合材料界面剪切强度的方法，本文运用数值方法－边界单元法，对一个实际试件进行了界面应力计算，讨论了材料常数，刀口宽度和新月尖角三种主要因素对界面应力分布可能产生的影响，从而为实验结果分析，实验方法改进和评估提供依据。     

孤立波与淹没平板相互作用的三维波面和水动力实验研究

在波浪水池中进行了孤立波作用下有限长度和有限宽度淹没平板的三维模型水池实验. 首次应用多目视觉立体重构技术测量局部三维自由表面变形, 该系统的有效测量水平范围为1.7 m$\times $1.6 m. 用4个三分力测力传感器组成水下测力系统, 在不影响波面的情况下测量孤立波对平板的作用力和力矩. 针对波浪不破碎的情况, 选择0.4 m水深和0.16 m波高的来波条件, 平板淹没深度为0.1 m. 实验结果表明, 孤立波经过淹没平板时自由面有明显的三维变形, 导致孤立波波幅的时空变化. 波幅在平板尾缘中心线处达到最大值, 并沿展向逐渐减小. 利用多目视觉立体重构系统得到的波面变化过程与浪高仪给出定点波面时间序列相互印证, 表明建立标识码波面测量方法是有效的. 孤立波对淹没平板作用的水动力载荷变化分为6个典型阶段, 并与利用波面三维重构得到的波面测量标识码并讨论. 基于多目视觉立体重构技术得到了垂向力和俯仰力矩极值点出现时的三维波面形态. 建立的多目视觉立体重构系统将为海洋工程结构物的水池物理模型实验提供新的波面测量手段.      

Numerical simulation of orbitally shaken viscous fluids with free surface

Orbitally shaken bioreactors are an emerging alternative to stirred‐tank bioreactors for large‐scale mammalian cell culture, but their fluid dynamics is still not well defined. Among the theoretical and practical issues that remain to be resolved, the characterization of the liquid free surface during orbital shaking remains a major challenge because it is an essential aspect of gas transfer and mixing in these reactors. To simulate the fluid behavior and the free surface shape, we developed a numerical method based on the finite element framework. We found that the large density ratio between the liquid and the gas phases induced unphysical results for the free surface shape. We therefore devised a new pressure correction scheme to deal with large density ratios. The simulations operated with this new scheme gave values of wave amplitude similar to the ones measured experimentally. These simulations were used to calculate the shear stress and to study the mixing principle in orbitally shaken bioreactors. Copyright © 2012 John Wiley & Sons, Ltd.     

Invariant submodels of the model of thermal motion of gas in a rarefied space

A model describing the thermal motion of a gas in a rarefied space is investigated. This model can be used in the study of the motion of gas in outer space, and the processes occurring inside the tornado, and the state of the medium behind the shock front of the wave after a very intense explosion. For a given initial pressure distribution, a special choice of mass Lagrange variables leads to a reduced system of differential equations describing this motion, in which the number of independent variables is one less than the original system. This means that there is a stratification of a highly rarefied gas with respect to pressure. Namely, in a strongly rarefied space for each given initial pressure distribution, at each instant of time all gas particles are localized on a two-dimensional surface moving in this space. At each point of this surface, the acceleration vector is collinear with its normal vector. The resulting system admits an infinite Lie transformation group. All significantly various submodels that are invariant with respect to the subgroups of its eight-parameter subgroup generated by the transfer, extension, rotation, and hyperbolic rotation operators (the Lorentz operator) are found. For invariant submodels of rank 1, the basic mechanical characteristics of the gas flow described by them are obtained. Conditions for the existence of these submodels are given. For invariant submodels of rank 2, integral equations describing these submodels are obtained. For some submodels, the problem of describing the gas flow from the initial location of its particles and the distribution of their velocities has been investigated.     

界面剪切力作用下波状液膜流的水动力稳定性

液膜流的水动力稳定性作为保障其高效传热传质性能的重要因素之一,受多种因素的制约和影响. 当气液界面处存在因气流流动而产生剪切力作用时,剪切力将通过改变界面处的边界条件,从而影响液膜流动的稳定性. 基于边界层理论,采用积分法建立了剪切力作用下降液膜表面波演化方程,分析了界面剪切力对水动力稳定性的影响. 研究表明,正向剪切力为不稳定性因素,反向剪切力在较小雷诺数时为不稳定因素,在大雷诺数时为稳定性因素;正向剪切力使临界波数和临界波速增大,反向剪切力使其减小;剪切力对临界波速的影响在不同雷诺数下也有所不同.      

Measurements of wave-variance and velocity spectra in breaking waves

Displacements of mechanical waves superposed onto wind waves were measured with a laser displacement gauge in a wind-wave tank. The effects of wave breaking, especially the spilling breaking type, on the wave-variance spectra are investigated. In the absence of wave breaking, the quasi-equilibrium spectrum consists of an f ^–7/3 subrange in the capillary regime, and its spectral density increases with increasing wind speed. When intense spilling breaking occurs, the water surface is saturated with small-scale features that cause not only an increase in the spectral density but also a reduction in the slope of the spectrum at high frequencies. Velocity components under the water surface were measured with a laser Doppler velocimeter. The energy spectra of the vertical and longitudinal velocity components in breaking waves are practically identical in the frequency range near the dominant wave frequency. At higher frequencies, the spectra generally follow Kolmogorov's –5/3 law. In the intermediate frequency range, we observed a higher spectral density for the vertical velocity component than for its longitudinal counterpart. These results suggest that turbulence energy is transferred from the vertical component to the longitudinal component in breaking waves. The acceleration of the water motion becomes as large as gravitational acceleration when intense wave breaking takes place. The flow field in breaking waves is highly dissipative.     

Large eddy simulation and experiment of shear breakup in liquid-liquid jet: Formation of ligaments and droplets

Understanding the shear breakup in jet flows and the formation of droplets from ligaments is important to determine the final droplet size distribution (DSD). The initial droplet size, which affects the final DSD, is considered to be generated by the shear breakup. Large eddy simulation (LES) was performed to investigate the shear breakup in liquid-liquid jet flows. The explicit Volume of Fluid (VOF) model with the geometric reconstruction scheme was used to capture the oil-water interface. The estimated oil distribution including wave peaks, ligaments, droplets and water streaks were compared to the experiments with a good agreement. The estimated DSD matched with the measurements favorably well. In the simulation, the formation of droplets with a smooth and curved surface from ligaments or sheet-like structures was obtained. Different mechanisms were observed along with the shear layer including the formation of droplets from ligament through the capillary forces, breakage of a droplet into smaller ones and attachment of a droplet to a ligament. The destructive shear forces and resisting surface tension forces were quantified on stretching and retracting ligaments. The influence of internal viscous force was found to be negligible due to low oil viscosity. The critical capillary number was found to be larger than 5.0 for ligaments breaking with the shear breakup. The capillary number was below unity for retracting ligaments. The coalescence of two equal-sized droplets was obtained in the shear breakup region. The shear stress magnitude at the contact region increased more than two folds. The total surface area decreased nearly 20% after the coalescence.     

Wärmeübergang bei der Fallfilmverdampfung

At film evaporation in a vertical tube the heat transfer depends on the flow pattern of the falling film and the shear stress at film surface due to a significant flow in the gas phase. For heat transfer without superimposed gas flow equations exist which are checked experimentally. These equations have general validity for film evaporation and film condensation. By describing the influence of gas flow great differences occur between condensation and evaporation. The present work deals with a calculation method which was verified by condensation data. This method is compared and discussed with experimental results of film evaporation. It is shown that the presented method describing the shear stress influence is also applicable.     

Certain optimum forms of radiating bodies in supersonic gas flow

We solve two variational problems on determining the optimum form of a radiating body of given dimensions in a gas flow at high supersonic velocity with a laminar flow regime in the boundary layer. We consider the case in which two heat transfer processes are significant: convective heat transfer from the gas to the body and radiation from the body surface. The first problem involves finding the contour of the body which receives the minimal thermal flux. In the solution of the second problem we seek that form of the thermally isolated body for which its surface temperature will be minimal for given parameters of the approaching stream or for which the motion velocity will be maximal for a given wall temperature and flight altitude.     

Laboratory study on the evolution of waves parameters due to wave breaking in deep water

Understanding of the occurrence of the wave breaking, the process of the wave breaking and evolution of waves after they break in deep water is crucial to simulate the growth of wind wave in ocean. In this study, deep-water breaking waves with various spectral types, center frequencies and frequency bandwidths are generated in a wave flume based on energy focusing theory. The time series of the wave surface elevation along the flume are obtained by 22 wave probes mounted along the central line of the flume. The characteristics of deep-water wave breaking are analyzed using the spectrum analysis based on the Fast Fourier Transform (FFT). For small center frequency the maximum height of wave surface generated using the Pierson–Moskowitz (P–M) spectrum is produced and the impact of the frequency width is small in wave breaking zone. While the spectral type has a significant impact on the local wave steepness during breaking, the influence of center frequency and frequency width on the local wave steepness is very weak. The significant wave steepness changes significantly after wave breaking, but it remains stable in the upstream or the downstream of wave breaking zone. After wave breaking, the peak frequency remains stable, but the spectrally weighted wave frequency changes significantly. The relationship between the level of downshift and the incident wave steepness is approximately linear. By analyzing the energy spectra, it is found that the energy loses near high frequency of controlling frequencies range and increases near peak frequency during the wave breaking. After wave breaking, the total energy dissipates remarkably with increasing breaking intensity.