The Structure Functions of the Velocity and Temperature Fields from the Perspective of Dimensional Scaling

In this paper, dimensional scaling is used to describe the turbulence structure of the velocity and temperature fields in the inertial range and the far dissipation range as well as the intermediate transition range under locally isotropic conditions at sufficiently large Reynolds numbers. This kind of scaling is expressed in a strictly mathematical manner employing dimensional π -invariants analysis. It is shown that in the case of the asymptotic solutions for either the inertial range or the far dissipation range only one π number occurs that has to be considered as a non-dimensional universal constant. This π number may be determined theoretically or/and empirically. In the case of the transition range two π numbers occur. Consequently, a universal function is established that has to be derived theoretically or/and empirically, too. Here, Batchelor's [7] classical interpolation formula for the turbulence structure of the velocity field and the empirical one of Stolovitzky et al. [59], both may serve as universal functions, are compared with the results provided by numerical solutions of Kolmogorov′s [32] structure equation for the velocity field. It is shown that these interpolation formulae match not only the asymptotic solutions of the inertial range and the far dissipation range, respectively, but also these numerical results in an excellent manner. The former may be considered as necessary condition and the latter as sufficient condition. In the case of the temperature field results of the corresponding universal function are predicted using Yaglom's [63] structure equation. These results also match the corresponding asymptotic solutions of both the inertial range and the far dissipation range. However, in contrast to the case of the velocity field, the predicted universal function for the temperature field may notably overshoot its asymptotic solution for the inertial range. This overshooting occurs in the transition range and may be considered as an analogue to the so-called Hill ‘bump’ that usually occurs in the high-wave number portion of the temperature spectrum.     

双机头电脑刺绣机横梁的模态分析

以GY902A型电脑刺绣机的主横梁作为研究对象，分别运用实验测试和有限元模拟方法分析了电脑刺绣机横梁弯曲振动的固有频率，并利用经过验证的有限元模型，在计算机上作了结构改进仿真设计，实现了在不改变电脑刺绣机重量的前提下，电脑刺绣机横梁的固有频率提高22％，使电脑刺绣机更不易发生共振．     

厚热塑性复合材料AS4/PEEK角铺设层合板的层间应力分析

将表征热塑性复合材料AS4 /PEEK非线性行为和应变速率相关行为的三维弹塑性模型通过程序加以实现。将程序计算结果和文献实验结果相比较可以发现,二者吻合较好,验证了所生成程序的有效性。计算了厚的AS4 /PEEK角铺设层合板[±25]s4在不同界面上的层间应力。由层间应力的三维分布图,分析了不同界面上层间应力的分布特征,并说明了可能引起层间分层的主要因素。     

Uniform strain fields and exact results in an elastoplastic fibre-reinforced composite

This work is concerned with a two-phase material consisting of an elastoplastic matrix reinforced by linearly elastic fibres. It is first shown that uniform strain fields can be generated in this heterogeneous material. A return-mapping based algorithm is then proposed and used to find uniform strain loading paths. With the help of uniform strain fields, exact results, independent of the transverse geometry and arrangement of the fibres, are derived for the effective elastic properties and for the effective initial and current yield surfaces. To cite this article: Q.-C. He, H. Le Quang, C. R. Mecanique 332 (2004).     

基于反应谱值分析的爆破震动破坏评估研究

采用反应谱方法研究了爆破震动信号的振动速度和频率特征,提出了采用反应谱曲线积分值来评估爆破震动破坏效应。通过对289组爆破地震波数据的统计表明,在反应谱曲线的积分值小于5时,爆破地震动不会对所研究的普通民房结构物产生震动破坏效应。     

Turbulent Channel Flow with an Artificial Two-Dimensional Wall Layer

Turbulent flow of an incompressible fluid in a plane channel with parallel walls is considered. The three-dimensional time-dependent Navier-Stokes equations are solved numerically using the spectral finite-difference method. An artificial force which completely suppresses lateral oscillations of the velocity is introduced in the near-wall zone (10 % of the channel half-width in the neighborhood of each wall). Thus, the three-dimensional flow zone, in which turbulent oscillations can develop, is separated from the wall by a fluid layer. It is found that the elimination of three-dimensionality in the neighborhood of the walls leads to a significant reduction in the drag. However, complete laminarization does not occur. The flow in the stream core remains turbulent and can be interpreted as a turbulent flow in a channel with walls located on the boundary of the two-dimensional layer and traveling at the local mean-flow velocity. The oscillations developing inside the two-dimensional layer, which have significant amplitude, distort the flow only in the adjacent zone. Beyond this zone the distributions of the mean characteristics and the structure of instantaneous fields completely correspond to ordinary turbulent flow in a channel with rigid walls. The results obtained confirm the hypothesis of the unimportance of the no-slip boundary conditions for the fluctuating velocity component in the mechanism of onset and self-maintenance of turbulence in wall flows.     

Transient gas flow simulation using an Adaptive Method of LinesSimulation d'écoulements dans un gazoduc par la méthode adaptative de lignes

Designing natural gas pipelines to safely and efficiently handle unsteady flows, requires knowledge of pressure drop, flowrate and temperature distribution throughout the system. The accurate prediction of these parameters is essential in order to achieve optimum cumulative deliverability, and safe and reliable operation. An Adaptive Method of Lines algorithm is formulated for the solution of Euler system of equations, which fully simulates slow and fast transients. Two test cases present the improvement of the numerical solution from grid adaptation. Good results are obtained both for slow and fast transients simulations proving that the suggested numerical procedure is appropriate for such predictions. To cite this article: E. Tentis et al., C. R. Mecanique 331 (2003).     

裂纹转子分岔、混沌行为研究

分析非线性涡动中裂纹转子在裂纹存在和裂纹扩展两种情形下的典型非线性动力学行为－－混沌和分岔现象。在2／3倍临界转速区，裂纹深度浅于R／2的裂纹转子有分岔和拟周期响应出现；深度超过R／2的裂纹转子会出现分岔及混沌现象。裂纹角β对这些非线性动力学行为有很大影响。     

Effects of static electric field on the fracture behavior of piezoelectric ceramics

The paper gives an overview on experimental observations of the failure behavior of electrically insulating and conducting cracks in piezoelectric ceramics. The experiments include the indentation fracture test, the bending test on smooth samples, and the fracture test on pre-notched (or pre-cracked) compact tension samples. For electrically insulating cracks, the experimental results show a complicated fracture behavior under electrical and mechanical loading. Fracture data are much scattered when a static electric field is applied. A statistically based fracture criterion is required. For electrically conducting cracks, the experimental results demonstrate that static electric fields can fracture poled and depoled lead zirconate titanate ceramics and that the concepts of fracture mechanics can be used to measure the electrical fracture toughness. Furthermore, the electrical fracture toughness is much higher than the mechanical fracture toughness. The highly electrical fracture toughness arises from the greater energy dissipation around the conductive crack tip under purely electric loading, which is impossible under mechanical loading in the brittle ceramics. The project supported by an RGC grant from the Research Grant Council of the Hong Kong Special Administrative Region, China