基于位移型Gurtin变分原理计算动力响应的逐步积分法

本文利用位移型Ｇｕｒｔｉｎ变分原理,在时间域上采用三次Ｈｅｒｍｉｔｅ插值函数进行离散,给出了一种计算结构动力响应的逐步积分方法。通过稳定性分析研究了该方法的稳定区情况表明,当１．６４≤θ≤２．０８时,该方法的数值计算精度很高,但是条件稳定积分格式。当θ≥４．１时,该方法是无条件稳定的积分格式,精度较高。     

A method for transforming a full computation of the effects of a complex-explosion scenario to a simple computation by ConWep

When a large amount of explosive that is stored inside a structure explodes, a blast wave is generated. The strength of the blast depends on both the explosive (type and weight) and the properties of the storing structure, since part of the blast energy is needed to destroy the structure. If one is to determine the dynamic load (e.g., pressure and impulse) that is exerted by the blast on a neighboring structure, he needs to conduct a full numerical simulation with a validated numerical code. If the amount of the stored explosive changes while the storing structure remains the same a new full hydrodynamic computation is required. In the present study, we present a method by which, for a given structure inside which explosive is stored, only a few full hydrodynamic computations with different amounts of explosive are required in order to obtain the resulted blast wave load on a neighboring structure that is located at different distances from the exploding structure for any amount of explosive between the minimal and the maximal amounts that were used in the full hydrodynamic computations. By means of the proposed method, the problem, which as mentioned above needs a full hydrodynamic computation, is replaced by an equivalent problem of the explosion of a bare hemispherical charge. The equivalent problem can be solved by means of an empirical model such as ConWep, which is very simple to apply. The solution of the equivalent problem results in identical peak pressure and peak impulse at close and far ranges from the explosion source and very similar pressure and impulse profiles at far ranges. The proposed method is demonstrated by an explanatory example in which the effect of an explosion inside an ammunition magazine on the surroundings is studied.     

Finite element computation of a turbulent flow over a two-dimensional backward-facing step

The present paper is devoted to the computation of turbulent flows by a Galerkin finite element method. Effects of turbulence on the mean field are taken into account by means of a (k-ε) turbulence model. The wall region is treated through wall laws and, more specifically, Reichardt's law. An inlet profile for ε is proposed as a numerical treatment for physically meaningless values of k and ε. Results obtained for a recirculating flow in a two-dimensional channel with a sudden expansion in width are presented and compared with experimental values.     

Finite element computation of a turbulent flow over a two-dimensional backward-facing step

This paper is devoted to the computation of turbulent flows by a Galerkin finite element method. Effects of turbulence on the mean field are taken into account by means of a k-? turbulence model. The wall region is treated through wall laws and, more specifically, Reichardt's law. An inlet profile for ? is proposed as a numerical treatment for physically meaningless values of k and ?. Results obtained for a recirculating flow in a two-dimensional channel with a sudden expansion in width are presented and compared with experimental values.     

The convergent condition and united formula of step reduction method

The step reduction method was first suggested by Prof. Yeh Kai-yuan^[1]. This method has more advantages than other numerical methods. By this method, the analytic expression of solution can he obtained for solving nonuniform elastic mechanics. At the same time, its calculating time is very short and convergent speed very fast. In this paper. the convergent condition and united formula of step reduction method are given by mathematical method. It is proved that the solution of displacement and stress resultants obtained by this method can converge to exact solution uniformly. when the convergent condition is satisfied. By united formula, the analytic solution can be expressed as matrix form, and therefore the former complicated expression can be avoided. Two numerical examples are given at the end of this paper which indicate that by the theory in this paper, a right model can be obtained for step reduction method.Project Supported by Science and Technic Fund of the National Education Committee.The author would like to thank Prof. Yeh Kai-yuan for his directing.     

The computation of the eddy along the upper wall in the three‐dimensional flow over a backward‐facing step

A three‐dimensional laminar flow over a backward‐facing step is studied as a numerical experiment by solving the steady‐state, isothermal and incompressible Navier–Stokes equations using two different finite element codes. The Reynolds number ranges from 100 to 1050. The expansion ratio is 1:1.94, and the aspect ratio is 1:36.7. The numerical experiment reveals both eddies along the lower and upper walls downstream of the step. Results of computations regarding positions of detachment of the eddy along the upper wall and positions of reattachments of the eddies along both the lower and upper walls are tabulated along with positions and magnitudes of global extrema of shear rate within the eddies. The wall effects are shown by calculating streamlines along planes parallel/normal to the lateral walls of the domain and depicting how the streamlines are distorted close to the walls and how they assume a two‐dimensional configuration in the plane of symmetry. Comparisons are made with available numerical results and laboratory measurements. Copyright © 2011 John Wiley & Sons, Ltd.     

一种有限元并行计算前处理方法

本文研究一种有限元并行计算前处理方法,只需将各子结构的超单元信息发送给各处理机,就可并行自动剖分各子结构和并行优化各子结构单元编号,以达到极小最大波宽和波宽的均主根,算例表明,该方法可大大减少有限元并行计算前处理时间。     

最大熵法在可靠度计算中的应用

在可靠度计算中,任意对立随机变量往往为不同概率分布,不利于可靠度计算。通常采用正态转换法将其转换成具有统一形式的正态分布,以便进行可靠度计算。但是,正态转换并不能保证原有随机变量概率分布特性,从而一定程度上造成可靠度计算误差。针对上述问题,本文提出在可靠度计算中利用最大熵法将任意随机变量概率密度函数转换成近似精度更高且具有统一标准形式的最大熵概率密度函数(MEPDF),然后提出对应的一阶可靠度计算方法进行可靠度求解,最后通过实例分析证明该方法的有效性。     

结构特征向量灵敏度分析的共轭梯度迭代法

在特征向量灵敏度模态法的基础上,将未知模态对特征向量灵敏度的贡献用线性方程组的解来表示,从而提出了不用求Ｋ＾－１的特征向量灵敏度的共轭梯度迭代法。该方法仅需少数几阶模态和少量的迭代次数,便能获得结构高精度特征向量灵敏度,文中给出几种模态法与本文方法的算例比较,显示了该方法的有效性和精确性。     

RKDG有限元方法计算流体与刚体耦合

用Level set方法配合Runge-Kutta discontinuous Galerkin (RKDG)有限元方法求解流体与刚体耦合问题。用RKDG有限元方法求解欧拉方程,通过求解Level set方程对界面进行追踪,并用推广的Ghost fluid方法对流刚界面进行处理。数值实验表明,该方法具有较高的分辨率。由于该方法不需要对移动网格进行处理,因此可以处理任意形状的拓扑问题,并且很容易推广到三维。     

The ray method for the deflection of a floating flexible platform in short waves

In this paper, we explain how the ‘ray method’ can be used to describe the deflection, due to short waves, of a very large floating platform in finite or infinite water depth. The elastic properties of the platform are isotropic, but may be distributed inhomogeneously. In the first section, we give a derivation of the equation for the phase and amplitude functions. Then an integro-differential equation for the determination of the deflection is used to find the initial condition for amplitude along the characteristics. For the homogeneous two-dimensional platform in water of finite depth, an exact solution in the form of a superposition of modes can be obtained. This simplified problem serves as a ‘canonical’ problem for problems with the same structure locally. In the last section, we give some result for a semi-infinite platform with varying elasticity coefficient, the mass distribution being taken constant.     

A Godunov method for the computation of erosional shallow water transients

A Godunov method is proposed for the computation of open‐channel flows in conditions of rapid bed erosion and intense sediment transport. Generalized shallow water equations govern the evolution of three distinct interfaces: the water free‐surface, the boundary between pure water and a sediment transport layer, and the morphodynamic bottom profile. Based on the HLL scheme of Harten, Lax and Van Leer (1983), a finite volume numerical solver is constructed, then extended to second‐order accuracy using Strang splitting and MUSCL extrapolation. Lateralisation of the momentum flux is adopted to handle the non‐conservative product associated with bottom slope. Computational results for erosional dam‐break waves are compared with experimental measurements and semi‐analytical Riemann solutions. Copyright © 2003 John Wiley & Sons, Ltd.     

基于表面肌电信号改进步长估计模型的个人定位方法

针对个人室内定位中步长估计方法存在估计精度不高、适用范围有限的问题,提出一种采用表面肌电信号和加速度信息的改进步长估计模型.通过采集行人在行走过程中的表面肌电信号和加速度信息,利用平滑滤波算法对该信号进行预处理,并通过峰值检测算法进行步数检测,在此基础上提出一种改进的步长估计模型,利用该模型进行了个人室内定位测试.实验...     

Experimental investigation of flow over a backward-facing step in proximity to a flexible wall

Turbulent flow between a flexible wall and a solid surface containing a backward-facing step (BFS) was investigated using digital particle image velocimetry and high-speed photography. Stationary sheet of paper under tension was positioned above the solid surface in proximity to the BFS. The incoming air flow emerged from a planar nozzle that was located in the solid wall upstream of the BFS. Flows corresponding to two values of the Reynolds number (3,000 and 3,600) based on the step height and the maximum flow velocity at the step location were characterized in terms of patterns of time-averaged velocity, out-of-plane vorticity, streamline topology, and turbulence statistics. In addition, paper sheet oscillation was characterized using high-speed photography. For the control case of a solid upper wall with the geometry that represented the time-averaged paper profile, hydrodynamic frequencies were characterized using unsteady pressure measurements. Frequencies of the natural vibration modes of the paper sheet were well separated from the hydrodynamic frequencies corresponding to the oscillations of the shear layer downstream of the BFS. As the inflow velocity increased, the paper sheet was pulled closer to the solid surface, which resulted in increased confinement of the incoming jet. The flow reattachment length calculated on the basis of time-averaged flow patterns increased with the increasing Reynolds number.     

A multigrid pseudospectral method for steady flow computation

In this work two‐dimensional steady flow problems are cast into a fixed‐point formulation, Q = F(Q). The non‐linear operator, F, is an approximate pseudospectral solver to the Navier–Stokes equations. To search the solution we employ Picard iteration together with a one‐dimensional error minimization and a random perturbation in case of getting stuck. A monotone convergence is brought out, and is greatly improved by using a multigrid strategy. The efficacy of this approach is demonstrated by computing flow between eccentric rotating cylinders, and the regularized lid‐driven cavity flow with Reynolds number up to 1000. Copyright © 2003 John Wiley & Sons, Ltd.     

体系可靠度计算中改进的等价平面法

具有多个失效模式的结构体系可靠度计算是一个复杂的多维积分问题,很难直接得到结果,往往需要采用近似计算。在失效模式间的相关性较弱时一些近似计算方法可以得到较好的估计值,但是当各失效模式相关性较强时这些近似方法的误差较大。最初的等价平面法是用来计算并联体系失效概率的,但是需要进行迭代求解,而且其误差较大。本文推导了串联体系的等价平面法(EPM法)的解析公式,在此基础上提出了改进的等价平面法(IEPM法),该方法既可以计算串联体系也可以计算并联体系的失效概率,解决了体系可靠度在各失效模式相关性较强时近似计算误差较大的问题。经大量的算例验证,说明该方法在失效模式相关性较强时具有较高的计算精度。