正交各向异性圆柱体在轴压作用下的应力场

基于材料体积不可压假设,对轴向压缩作用下圆柱试件在加载面内的环向和径向应力分布进行理论分析,计算结果表明:当试件材料本构为正交各向异性时,环向和径向应力分布为半径的幂函数形式;试件材料为横观各向同性时,环向和径向应力为半径的二次函数.在圆柱试件轴线上环向和径向应力相等,且均具有最大值;试件圆周边界上径向应力为0,环向应力具有极小值.通过最大拉伸应变破坏理论对试件环向应变进行分析,获得了产生环向拉伸破坏时的临界轴向载荷;并采用Hill-蔡强度理论对试件圆周边界上计算得到的应力参量进行描述,得到了轴压作用下圆柱试件的Hill-蔡强度理论表达式,其不仅取决于轴向应力和试件材料的基本力学性能,还与试件轴向变形的应变率及应变率随时间的变化率相关.     

Stress State of Multilayer Structural Elements Subjected to Static Loading and Optimization of Laminated Beams and Bars

The results of FEM investigation of the triaxial stress state in multilayer structural elements subjected to axial and bending loads are presented. The distribution regularities of the stiffness and stresses or strains depending on the geometric and mechanical characteristics of layers and their position in the cross section of beams and bars are examined. The optimization of these elements is carried out using the dependences of the Bareisis—Paulauskas method and the Optim-98 computer program created by the present authors. As the optimization criteria, the strength, stiffness, mass, and cost of the structural elements are considered.     

Weibull分布损伤失效率模型常应力下的参数估计

Bhattacharyya和Soejoeti(1989)对步进应力加速寿命试验提出损伤失效率模型(TFR模型)．本文在全加速步加试验场合下指出文献[1]，[2]，[5]中用通常的回归分析方法求取常应力下参数的估计是不合理的，同时给出了如何求取常应力下参数估计的一种方法．     

随机模拟算法在体育教学评价中应用

针对传统体育教学评价中优劣判别的绝对性,以及多评价结论非一致性问题,构建一种凸显自身优势的自主优势评价方法,评价中运用一种基于概率型随机模拟算法,通过计算各评价对象之间的优胜度,来评判评价对象的优势.经过算例验证方法的有效性,得到带有概率信息的评价结论.     

主动冷却点阵夹层防热结构温度响应计算模型北大核心CSCD

针对点阵夹层结构主动热防护问题,建立了夹层结构面板和芯体导热与冷却剂对流耦合的非稳态传热理论模型,利用有限体积法离散控制方程并在MATLAB中进行了迭代求解.模型首次考虑了面板与夹芯杆之间的收缩热阻,并利用分离变量法得到了收缩热阻的近似解析解.基于单胞模型和周期性边界条件,模拟得到了模型所需的表面对流传热系数h_(b)和h_(fin).最后,选取多单胞计算工况进行数值模拟和理论模型对比,并讨论了收缩热阻对模型预测精度的影响.结果表明:理论模型能够准确预测夹层结构及内部流体的温度变化,理论与仿真之间的最大误差不超过1%;随着外加热流密度不断增大,忽略收缩热阻使得计算结果造成的误差不断增大;与数值模拟相比,理论模型可显著地减少计算时间并节省计算资源,尤其适用于非均匀、非稳态复杂热载荷下点阵夹层结构的温度响应计算.     

基于加速失效时间模型模拟生存时间应用于检测生存性状位点

对于考察预指定情形下的统计模型的性能、性质及适应性,模拟研究是非常重要的统计工具.作为生存分析中两个最受欢迎的模型之一,由于加速失效时间模型中的因变量是生存时间的对数,且此模型能够以线性形式回归带有易解释的参数的协变量,从而加速失效模型比COX比例风险模型更便于拟合生存数据.首先提出了关于带有广义F-分布的加速失效模型的模拟研究中生成生存时间的方法,然后给出了描述加速失效时间模型的误差分布和相应的生存时间之间的一般的关系式,并给出了广义F-分布是如何生成生存时间的.最后,为证实所建议模拟技术的性能和有效性,将此方法应用于检测生存性状位点的模型中.     

Challenge Versus Exchange in Collective Decision Making: A Comparison of Two Simulation Models Based on Simulated Data

In collective decision making, actors can use different influence strategies to get their way. Differences in influence strategies may, or may not, be connected to differences in collective outcomes. This research studies two influence strategies: the exchange strategy and the challenge strategy. In the existing literature, these strategies are analyzed and compared using simulation models in which actor behavior regarding influence attempts based on one of the strategies is modeled explicitly. Until now, these models have been tested only empirically on limited data sets. However, a theoretical test is necessary to gain more precise insights in the effect of characteristics of collective decision making situations on the collective outcomes. In the present research, computer simulations are used in a structured comparison of two competing models (the iterative exchange model and challenge model). The analyses show that the outcomes of both models are captured for a large part in the actor characteristics on the issues. Besides this, the expected directions of challenges and exchanges play a major part in explaining the outcomes of the models. This research shows that the use of simulated data allows a structured search of the input space, which led to new insights into the iterative exchange model and challenge model, and therefore in the exchange strategy and the challenge strategy.     

含虫龄和虫态结构的瓢虫与棉蚜种群差分方程模型

根据棉蚜、十一星瓢虫的生物学特性建立了具有不同虫龄结构和虫态的种群差分方程模型,利用新疆棉区十一星瓢虫对棉蚜捕食作用的调查和实验数据,确定了模型的各种参数,模拟了一个夏季92天内棉田棉蚜、十一星瓢虫数量的变化趋势.利用该模型,对不同温度、不同植株营养状况下十一星瓢虫的发育及对棉蚜的捕食状况进行了对比模拟计算.通过与实际调查数据对比分析,验证了模型的准确性和实用性.     

A dynamical approach to large eddy simulation of turbulent flows: existence of weak solutions

We consider a system of equations coming from turbulence models using a large eddy simulation (LES) technique. The idea of this approach bases on decomposing the velocity into a part containing large flow structures and a part consisting of small scales. The equations for large‐scale quantities are derived from the Navier–Stokes equations with an additional constitutive relation for the contribution of small eddies. The mathematical difficulties in this paper focus on the non‐linear and non‐local turbulent term. Copyright © 2005 John Wiley & Sons, Ltd.     

一种提高风能利用率的风力发电机控制策略

在变速恒频风力发电机组的各类并网逆变器控制策略中,零d轴电流控制和空间矢量脉宽调制(SVPWM)以能够提高风能利用系数和稳定逆变器输出电压的优势得到了广泛的应用.在理论研究阶段需要对其进行仿真,然而在Matlab/Simulink中并没有可以实现上述功能的并网仿真模型.因此推导了变速恒频风力发电机组及控制策略的数学模型,并在Matlab/Simulink中完成搭建、当风速恒定不变时,对发电系统的运行情况进行了仿真.结果表明,零d轴电流控制与SVPWM的结合,既提高了风能利用系数,又有效地提高了逆变器的输出电压,保证当电网电压升高时不影响发电系统向电网输送功率的效率.