接触-碰撞算法研究进展

接触-碰撞广泛存在于实际工程问题中,是影响数值计算效率与计算精度的重要因素。本文针对变形体间接触-碰撞问题的显式有限元计算,介绍接触-碰撞算法近30年来取得的主要进展。首先,简要介绍接触-碰撞问题的界面离散模型;然后,从全局接触搜索、局部搜索、接触约束施加以及接触计算的并行化四方面详细阐述目前主要算法的基本思想与特点,并分析其优势与不足;最后,对接触-碰撞算法相关研究方向给出建议。     

空间网格结构损伤探测的研究进展

空间网格结构损伤探测技术是目前结构工程领域研究的热点和难点．根据国内外的最新文献信息，讨论了损伤探测技术在空间网格结构中的应用及最新的研究进展，对现有文献中的方法进行了分类和评述．重点介绍了目前比较适用的柔度法和模型修正方法，对该领域中尚存在的问题进行了讨论，对今后的发展方向及趋势进行了展望．     

根部柔性梁的不确定性建模与确认

首先导出了包含根部挠性参数的悬臂梁动力学方程近似解,将平动和转动柔性参数作为不确定性源,根据若干预紧力矩下的模态实验结果完成了柔性参数识别,并假定其服从正态随机分布,识别了均值与标准差,进而通过回归分析建立了不确定参数随预紧力矩变化的数学模型.最后通过新的模态实验结果对所建模型进行了确认,通过确认结果,明确了该模型的使用范围.     

DYNAMIC STRESS INTENSITY FACTORS AROUND TWO CRACKS NEAR AN INTERFACE OF TWO DISSIMILAR ELASTIC HALF－PLANES UNDER IN－PLANE SHEAR IMPACT LOAD

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Applications and developments of aeroelasticity of flexible structure in flow controls

柔性结构与空气动力耦合形成的系统呈现出丰富的非定常、非线性流动和结构动力学行为,对其气动弹性效应合理地控制和利用,能够大幅度提高飞机机翼、风力机叶片等结构的气动性能,并使其具有气动自适应能力.本文总结了近年来与气弹效应应用相关的研究进展及存在的问题,具体介绍了薄膜翼型的流动控制特性、柔性壁面减阻技术以及Sinha扰流装置的发展过程、主要成果以及未来发展趋势,着重对相关试验、流固耦合数值分析、Lagrangian拟序结构动力学等理论分析方法进行总结,展示了气弹效应在流动控制方面的巨大潜力和深远的学术意义,以便更多的研究人员开展该领域的研究工作.     

部分浸入水中的柔性梁非线性自由振动

研究了浸入水中的柔性梁非线性自由振动,假设其底端具有线弹性扭转弹簧支撑,顶端附有不计体积的集中质量块.推导了梁的运动控制方程和边界条件,由于考虑了大挠度,法向运动和轴向运动是非线性耦合的,使用Morison方程给出了流体力的表达式,利用有限差分法和Runge-Kutta法数值分析了梁在真空中和在水中的自由振动,讨论了参数对振动模态、固有频率等的影响.     

Balancing aspects of numerical dissipation,dispersion, and aliasing in time-accurate simulations

The current study looks at the selection of scheme elements that are well-suited for long-time integration of unsteady flows in the absence or under-resolution of physical diffusion. A concerted assembly of numerical components are chosen relative to a target aliasing limit, which is taken as a best-case scenario for overall spectral resolvability. High-order and optimized difference stencils are employed in order to achieve accuracy; meanwhile, quasi skew-symmetric splitting techniques for nonlinear transport terms are used in order to greatly improve robustness. Finally, tunable and scale-discriminant artificial-dissipation methods are incorporated for de-aliasing purposes and as a means of further enhancing both accuracy and stability. Central finite difference methods are considered, and spectral characterizations of the scheme components are presented. Canonical test cases (the isentropic vortex [IV] and Taylor-Green vortex problems) are chosen in order to highlight the benefits associated with the proposed approach for enhancing overall algorithm robustness and accuracy.     

ANALYSIS OF PARAMETERS IN GRANULAR SOLID HYDRODYNAMICS FOR TRIAXIAL COMPRESSION TESTS

Granular materials are omnipresent in industries and in nature. For small strains, elastic-plastic and hypoplastic constitutive relations are widely used in engineering practice, but they are not a significant reflection of the underlying physics. Under a unified thermodynamics framework explaining the physics of materials, granular solid hydrodynamics （GSH） was an ex- tension towards describing granular materials, not only solid-like, but also fluid-like behaviors. In this paper, the fundamentals of GSH are briefly treated and then simplified to analyze quasi- static deformations in triaxial compressions. The calculated stress-strain relations and volumetric strain are compared with experimental results. The influences of the major parameters in GSH, especially their cross coupling influences, are analyzed and their physical meanings are further clarified. After parameters were calibrated, the calculated stress values in the characteristic stress state are found to be within 22% of tested values. Meanwhile, the energy dissipation during triaxial compression is analyzed. The above results support and partially quantify GSH.     

Computation of hydrodynamic mass and damping coefficients for a cavitating marine propeller flow using a panel method

The present paper deals with the numerical calculation of hydrodynamic mass and damping coefficients under consideration of unsteady sheet cavitation on marine propeller flows. In the first part of the paper, the mathematical and numerical background behind the numerical method is introduced. The numerical calculations carried out in this work are based on a low-order panel method. Panel methods belong to the class of collocation techniques and are applied to obtain a numerical solution of a potential flow based system of boundary integral equations. They are suitable for the present application because of their short computation time which makes them applicable in the design process of marine propellers.Additionally, two different approaches for the determination of hydrodynamic masses and damping are introduced in this work. The hydrodynamic masses and damping are important in studies of the ship motion in seaway and in the analysis of vibrations of a vessel and its appendages. The developed approaches are applied on a propeller flow in heave motion. Hereby, the calculations are performed for a non-rotating and rotating propeller under non-cavitating and cavitating conditions. The results obtained from the simulations are discussed in detail and an outlook is given.     

Modeling of two-phase flow in porous media with heat generation

The main purpose of this work is investigation of coolability of a boiling debris bed. The main governing equations are derived using volume averaging technique. From this technique some specific interfacial areas between phases are appeared and proper relations for modeling these areas are proposed. Using these specific areas, a modification for the Tung/Dhir model in the annular flow regime is proposed. The proposed modification is validated and the agreements with experimental data are good. Finally, governing equations and relations are implemented in the THERMOUS program to model two-phase flow in the debris bed in the axisymmetric cylindrical coordinate. Two typical configurations including flat and mounted beds are considered and the main physical phenomena during boiling of water in the debris bed are studied. Comparing the results with the one-dimensional analysis shows higher specific power of the bed.     

Formulation of nanofluids for natural convective heat transfer applications

The paper is concerned about formulation of aqueous based nanofluids and its application under natural convective heat transfer conditions. Titanium dioxide nanoparticles are dispersed in distilled water through electrostatic stabilization mechanisms and with the aid of a high shear mixing homogenizer. Nanofluids formulated in such a way are found very stable and are used to investigate their heat transfer behaviour under the natural convection conditions. The preliminary results are presented in this paper. Both transient and steady heat transfer coefficients are measured and the results show a systematic decrease in the natural convective heat transfer coefficient with increasing particle concentration. This is in contradiction to the initial expectation. Possible reasons for the observations are discussed.     

圆柱壳的非线性自由振动分析

本文分析了各向同性封闭圆柱壳的非线性自由振动。文中采用经典的非线性弹性力学方法推导了圆柱壳的大振幅运动方程,这些方程的静态形式与冯·卡门的板理论方程具有同样的精度。文中讨论了四种基本振动模态,并且还以数学公式的形式给出了一般的最终结果,一些例子以曲线给出结果,并进行了比较。结果还表明线性振动可以作为非线性振动的一种特例。     

Nonlinear coupled dynamics of liquid-filled spherical container in microgravity

Nonlinear coupled dynamics of a liquid-filled spherical container in microgravity are investigated. The governing equations of the low-gravity liquid sloshing in a convex axisymmetrical container subjected to lateral excitation is obtained by the variational principle and solved with a modal analysis method. The variational formulas are transformed into a frequency equation in the form of a standard eigenvalue problem by the Galerkin method, in which admissible functions for the velocity potential and the liquid flee surface displacement are determined analytically in terms of the Gaussian hypergeometric series. The coupled dynamic equations of the liquid-filled container are derived using the Lagrange＇s method and are numerically solved. The time histories of the modal solutions are obtained in numerical simulations.     

波形整形技术在Hopkinson杆实验中的应用

由于波形整形技术可减小Hopkinson杆实验在撞击过程中产生的高频振荡以及实现试样在受载过程中的恒应变率加载,因此,波形整形技术越来越受到关注。本文中详细介绍了波形整形技术在Hopkinson杆的动态压缩、拉伸、巴西圆盘、弯曲断裂等实验中的研究进展,并给出了该技术在应用中需注意的问题。     

正交各向异性厚板的边界元解法

本文利用 Hormander 算子法和平面波分解法导出了计入剪切变形的正交各向异性厚板的基本解。建立了计入剪切变形的正交各向异性厚板的边界积分方程。文中详细地讨论了基本解的数值计算,并用边界元法分析了一些算例。     

多尺度模拟与计算研究进展

简要介绍了多尺问题与研究方法.重点论述了两类常见多尺度问题的模拟计算方法与研究进展,分析了各自的优缺点和使用范围.对现有研究的局限性和存在的问题进行分析,指出了进一步研究多尺度模拟与计算的必要性.介绍了求解含有孤立缺陷问题的非局部准连续体法,MAAD等方法以及求解基于微观模型本构模拟问题的局部连续体法、HMM等方法.文章对多尺度模拟与计算的前景进行展望,提出了一些亟待解决的问题.     

Supersonic Inviscid Corner Flows with Regular and Irregular Shock Interaction

The results of a numerical and analytical investigation of steady-state supersonic inviscid flows in corners formed by intersecting compression wedges are presented. The flows considered are symmetric about the corner bisector. The distinctive features of flow pattern formation related with the reflection of wedge-generated shocks from the bisector plane are studied. The wedge angles at which transition from regular to irregular shock reflection occurs are determined both numerically and analytically using the criteria available for plane flows; the data thus obtained are found to be in agreement. Flow patterns with irregular shock reflection, namely, single, transitional, and double Mach, as well as von Neumann reflection, are identified; they are similar to the known types of reflection for plane quasi-steady-state flows. Varieties of these types not observed in the plane flows are found to exist. The effects of the angle of inclination of the plane surfaces of the corner to the freestream direction, the sweep angle of the leading edges, and the dihedral angle are investigated. Some previously unknown parameters of corner configurations for which transition may occur in accordance with the von Neumann criterion are determined.     

Flow of Oldroyd-B fluid with nanoparticles and thermal radiation

The two-dimensional boundary layer flow of an Oldroyd-B fluid in the presence of nanoparticles is investigated. Convective heat and mass conditions are considered in the presence of thermal radiation and heat generation. The Brownian motion and thermophoresis effects are retained. The nonlinear partial differential equations are reduced into the ordinary differential equation (ODE) systems. The resulting ODE systems are solved for the series solutions. The results are analyzed for various physical parameters of interest. Numerical values of the local Nusselt and Sherwood numbers are also computed and analyzed.     

Improvements on the arc-length-type method

Arc-length-type and energy-type methods are two main strategies used in structural nonlinear tracing analysis, but the former is widely used due to the explicitness and clarity in conception, as well as the convenience and reliability in calculation. It is very important to trace the complete load-deflection path in order to know comprehensively the characteristics of structures subjected to loads. Unfortunately, the nonlinear analysis techniques are only workable for tracing the limit-point-type equilibrium path. For the bifurcation-point-type path, most of them cannot secure a satisfactory result. In this paper, main arc-length-type methods are reviewed and compared, and the possible reasons of failures in tracing analysis are briefly discussed. Some improvements are proposed, a displacement perturbation method and a force perturbation method are presented for tracing the bifurcation-point-type paths. Finally, two examples are analyzed to verify the ideas, and some conclusions are drawn with respect to the arc-length-type methods. The project supported by the Special Research Fund for Doctor Program of Universities (9424702)     

Experiment and modelling of birefringent flows using commercial CFD code

It is well-known that certain fluids are birefringent and when flows are viewed in polarised light interference fringes are observed. The fringes are caused by a phase shift in the light passing through the fluid and are proportional to the integral of the maximum shear strains in the fluid. In order to understand what is happening within the three dimensional flow and overcome the difficulties due to this integration, additional computational or experimental information is needed.

In this work, a commercially available computer code (Fluent) is used for the first time to model the flows. The flow data are then exported to a spreadsheet where the shear rates are integrated across the field and then banded for graphical output. The results from this are then compared to results generated from birefringent flow experiments and the agreement is found to be good since the modelled fringes show the same patterns as those in the experiment. This novel use of computational and experimental techniques together will allow quantitative analysis of three-dimensional flows in the future.

Currently, there are still a lot of empirical variables involved in fitting the computational fringes to the experiment, but the results of this preliminary study show that this is a promising approach to this type of problem.