Parameters Inversion of Fluid-Saturated Porous Media Based on Neural Networks

The multi-layers feedforward neural network is used for inversion of material constants of fluid-saturated porous media. The direct analysis of fluid-saturated porous media is carried out with the boundary element method. The dynamic displacement responses obtained from direct analysis for prescribed material parameters constitute the sample sets training neural network. By virtue of the effective L-M training algorithm and the Tikhonov regularization method as well as the GCV method for an appropriate selection of regularization parameter, the inverse mapping from dynamic displacement responses to material constants is performed. Numerical examples demonstrate the validity of the neural network method.

The Effect of Conical Flowfields on the Performance of Waveriders at Mach 6

The performance of 23 kinds of waveriders, derived from different conical flowfields, is analyzed by the numerical computation under the conditions of fight speed of Mach 6, attack angle of 0° and flight altitude of 30 km. These results indicate that the performance is influenced by the shapes and the width to height ratios (W/H ) of generating cones. The geometrical parameter and the lift coefficient are proportional to W/H, while the drag coefficient and the lift to drag ratio (L/D ) have extreme values. Considering the base drag and the computation errors, the waverider with the highest L/D is cut from the elliptical cone’s flowfield (W/H = 1.5―1.618), and the configuration with the lowest drag can also be obtained at W/H = 1:1.5. Accordingly, good suggestions are proposed for practical design based on these computational results.

Nanoindentation of thin-film-substrate system: Determination of film hardness and Young's modulus

In the present paper, the hardness and Young's modulus of film-substrate systems are determined by means of nanoindentation experiments and modified models. Aluminum film and two kinds of substrates; i.e. glass and silicon, are studied. Nanoindentation XP II and continuous stiffness mode are used during the experiments. In order to avoid the influence of the Oliver and Pharr method used in the experiments, the experiment data are analyzed with the constant Young's modulus assumption and the equal hardness assumption. The volume fraction model (CZ model) proposed by Fabes et al. (1992) is used and modified to analyze the measured hardness. The method proposed by Doerner and Nix (DN formula) (1986) is modified to analyze the measured Young's modulus. Two kinds of modified empirical formula are used to predict the present experiment results and those in the literature, which include the results of two kinds of systems, i.e., a soft film on a hard substrate and a hard film on a soft substrate. In the modified CZ model, the indentation influence angle, phi, is considered as a relevant physical parameter, which embodies the effects of the indenter tip radius, pile-up or sink-in phenomena and deformation of film and substrate.

平行于裂纹方向的载荷对于疲劳裂纹扩展速率的影响

<正> 1. 引言断裂力学在疲劳裂纹扩展的研究中得到了成功的应用。但是,现有的疲劳裂纹扩展数据大都是从单向加载的试件上得到的。而实际构件通常处于双向载荷或多向载荷的状态下。在对构件作寿命估计时,双向载荷对于疲劳裂纹扩展速率的影响是一个必须要考虑的因素。在1966年Christensen等曾指出,实验表明在双向载荷下疲劳裂纹扩展速率为单向载荷时的三倍,但它只给出了实验结果的示意图。后来有限的一些实验数据得出各不相同的结论。大部分试验结果表明,平行于裂纹方向的载荷对于裂纹扩展的影响很小。也有数据表明它会加快或减慢裂纹扩展。

地形对洋流影响的初步实验模拟

在旋转流动系统中,由于柯氏力的作用,地形对流动的影响常呈现出与惯性系中很不相同的特点,近年来在地球物理流体力学研究中受到很大重视。 本文介绍了作者们在力学所旋转实验台上,针对近海洋流问题所作的一些旋转流体中地形影响的初步实验研究,在一扇形容器中用源汇法模拟了洋流的陆架地形绕流,过海脊流,绕岛屿以及沿径向有变深度地形的环流,这些初步的定性的结果表明,均质旋转流动系统确能反映大尺度旋转流动的某些动力学特征。

激光器谐振腔中辐射能量密度须按场强叠加计算

原则上,激光腔中相干光的强度应由往返光束按场强叠加计算。本文比较按场强叠加和通常按光强叠加的计算,指出:腔中各处的饱和增益和光强确有可观的偏差,但场分布和输出功率很好地相符。

Diffusion dominated process for the crystal growth of a binary alloy

The pure diffusion process has been often used to study the crystal growth of a binary alloy in the microgravity environment. In the present paper, a geometric parameter, the ratio of the maximum deviation distance of curved solidification and melting interfaces from the plane to the radius of the crystal rod, was adopted as a small parameter, and the analytical solution was obtained based on the perturbation theory. The radial segregation of a diffusion dominated process was obtained for cases of arbitrary Peclet number in a region of finite extension with both a curved solidification interface and a curved melting interface. Two types of boundary conditions at the melting interface were analyzed. Some special cases such as infinite extension in the longitudinal direction and special range of Peclet number were reduced from the general solution and discussed in detail.

6马赫锥体流场对乘波体性能的影响及规律

以CFD计算为分析工具，在6马赫飞行速度、0°飞行攻角和30km飞行高度的设计条件下，综合分析了23种源自不同锥体流场所获乘波体的性能．分析结果表明，基本锥体的截面形状及截面宽高比均对乘波体性能有较大影响；当基本锥体的宽高比变化时，相应乘波体几何参数和升力系数的变化基本与之呈正比关系，而阻力系数及升阻比则出现极值．此外，我们发现在此飞行状态下，综合考虑乘波体底部阻力及计算误差等因素，当基本锥体为椭圆锥，且截面椭圆宽高比在1．5～1．618时，所获得的乘波体具有最大的升阻比；而当截面椭圆宽高比约为1：1．5时，所获得乘波体阻力最小．依据所得的计算结果，对于乘波飞行器的实际设计给出了相应的建议．

流体力学新进展——第五届国际流体力学会议概况

介绍了第五届国际流体力学会议(Proceedings of the 5th International Conference on Fluid Mechanics,2007年8月15～19日,上海)的基奉情况.大会报告涉及生物流体力学、湍流与流动控制、超声速流动、超空泡减阻、破碎波与海岸泥沙输运等当前流体力学前沿领域的研究进展;按流体力学学科分支总结了本次会议分会场交流论文的基本情况,阐述了若干学科分支方向的研究进展.……

板料激光弯曲过程的数值模拟

采用MSC.Marc非线性有限元软件,对板料激光弯曲过程进行了数值模拟.试验与数值模拟的相互印证表明:数值模拟的模型正确,计算过程可靠,计算结果可为板料激光弯曲工艺参数的选择提供依据.通过模拟多道次直线照射的过程可知,变形效果受照射路径次序的影响很大,应选择从固定端向自由端方向交替的双向照射路径,以保证板料在宽度方向上变形的均匀.模拟结果可为带筋结构壁板激光弯曲成形提供实用方案.

利用块状介质进行飞秒强激光脉冲的腔外压缩

高强度飞秒激光脉冲的腔外压缩是获得高次谐波阿秒脉冲驱动源的必要手段。实验研究了超强超短飞秒激光脉冲在经过块状介质后的光谱展宽和色散补偿压缩现象。单脉冲能量0．26mJ，脉宽50fs的激光脉冲经透镜在空气中聚焦后再入射到块状材料上，出射脉冲光谱被展宽到接近40nm。由于在块状材料中的自聚焦效应，出射光束质量变好并保持较小的空间啁啾。利用熔融石英棱镜对补偿带有正色散的出射脉冲，最后得到〉0．1mJ，19fs的压缩脉冲。利用SPIDER装置测量了出射脉冲的脉宽和光谱相位。整个系统的能量效率大约为35％，压缩后的