论高速切削加工技术及应用

高速切削加工技术因其具有高效、高精度、工序简化等优点而广泛应用于汽车、航空航天、模具等行业,为提高企业的生产效率和经济效益作出了积极贡献.文章通过分析高速切削加工的关键技术和应用,展望高速切削加工技术的发展前景.     

SRC柱和CFST柱在车辆撞击下的动力响应分析

为了研究框架柱在冲击荷载作用下的抗撞击性能,基于非线性有限元软件LS-DYNA建立了精细化车辆-框架柱碰撞模型。首先,根据轴压承载力相同的原则设计2类框架柱：钢骨混凝土(SRC)柱和钢管混凝土(CFST)柱;其次,对轴压比、撞击角度、撞击速度和车型等影响方形框架柱抗撞击性能有关因素进行参数化分析,探讨框架柱在汽车撞击下的塑性损伤以及破坏模式;最后,比较SRC柱和CFST柱的抗撞击性。结果表明,SRC柱在柱顶受到斜向剪切破坏,在柱底受到弯剪破坏,而CFST柱的破坏模式是弯曲破坏。在车辆侧向撞击下,角度为15°撞击SRC柱时更危险,而CFST柱在正面撞击时更危险。车辆速度和车辆质量的增大,使得框架柱受到的碰撞损伤增大。CFST柱的抗撞击性能优于SRC柱。研究结果证明SRC柱和CFST柱具有优越的抗撞击性能,可为国家相关标准的修订提供参考。     

近20年来我国城市化发展速度的省区间比较--基于"五普"口径的修正

首先运用联合国法．以“五普”口径为标准．以各省区历年的非农人口为基础．系统构建了三普以来全国各省区历年的城市化水平：进而采用修正后的城市化水平数据．通过回归分析和相关性分析,从空间分布和时间序列变化两个方面分析了近20年来我国城市化发展速度的省区间差异。通过分析发现,我国1980年代城市化发展速度整体上呈现“东高西低”的格局,1990年代呈现“南高北低”的格局;20年来各省区间的城市化发展具有同步性,但发展速度的波动性不同．总体表现为城市化水平越高,波动性越小：最后对导致城市化发展速度差异的因素进行了初彤分析．     

The M/M\1 queue in a heavy-tailed random environment

We consider an M / M /1 queue with the special feature that the speed of the server alternates between two constant values s_L and s_H > s_L . The high-speed periods are exponentially distributed, and the low-speed periods have a regularly varying distribution. We obtain explicit asymptotics for the tail of the workload distribution. The two cases in which the offered traffic load is smaller respectively larger than the low service speed are shown to result in completely different asymptotics.     

Product quality and competition: evidence from the broadband industry

Access to high-quality broadband internet is important in many different respects such as in communication, education, commerce and information acquisition. We investigate the relationship between the number of broadband providers in an area and the quality of broadband service provided where download speed is used as a proxy for quality. We find that increased competition in an area positively affects the access to fast download speeds. This finding is robust to three different top download speeds tested. Additionally, a theoretical model is provided which shows that competition can decrease the profits associated with offering slow speeds; this gives an incentive for providers to offer higher speeds as a way to increase profits.     

Determining the relationship between ownership and technical efficiency by using a dynamic stochastic production frontier approach

This study adopts dynamic stochastic production frontier approach. Because of the impact of quasi-fixed inputs, airports cannot immediately adjust their production processes. Based on the heterogeneity of public and private airports’ operation models, this study estimates their individual production adjustment speed and their short-run and long-run technical efficiencies to compare transnational airport business performance. The results reveal the differences in the production adjustment of airport groups under different ownerships and performance. Private airports have faster production adjustment, whereas public airports have higher short-run and long-run technical efficiencies. As their primary goal, public airports should strive to increase the elasticity of production adjustment to increase the production adjustment speed, thus improving their long-run technical efficiency. On the other hand, private airports should prioritize eliminating short-run inefficiency to increase their actual output level, thereby improving their long-run technical efficiency.     

基于CFD的超声波流体测量影响因素的模拟

针对超声波流量计，通过实验实测和模拟结果的对比，提出采用CFD模拟仿真的方法，研究影响因素对超声波流量计内部流场及速度分布的影响。借助模拟方法分析了热电偶的安装、不同流量和流体温度对流量计内部流场及修正系数K的影响，结果表明，热电偶不仅仅影响了下游的工质流动，扰动的范围已经蔓延到上游接近超声波探头附近，会对流量测量结果产生影响；同一温度下，修正系数K随着流量的增大，先快速减小，再缓慢减小；当流量较小时，流动本身因温度的变化所带来的测量误差已经超过了可接受的范围，表明在流量较小时测量会产生较大的误差，应考虑温度补偿。模拟仿真为超声波流量计的进一步研究和优化提供了简单便捷的方法。     

Contextualizing speed and cross-border acquisition performance: Labor market flexibility and efficiency effects

There is general agreement that acquisition integration is decisive for acquisition performance. Despite this consensus, there are heterogeneous results on integration measures, such as integration speed with empirical research supporting the benefits of either fast or slow integration. We argue that the business environment surrounding acquisitions has the potential to reconcile conflicting findings. We develop how institutional factors (i.e., labor market flexibility and efficiency) influence the relationships between speed of human and functional integration on acquisition performance. With a sample of 203 transactions from acquirers from central Europe and Scandinavia, we find human and functional integration speed have divergent effects on acquisition performance. Further, both relationships are moderated by labor market flexibility and efficiency, but in different ways. Implications for research and practice are discussed.     

高速铣削铝合金时铣刀材料和几何角度的合理选择

高速切削是随着数控机床主轴转速和进给速度大幅度提高而逐渐发展的新型制造技术。文章阐述了高速切削刀具基本发展状况和高速铣削对刀具的要求,并论述了高速铣削铝合金时铣刀材料和几何角度的合理选择。     

关于高速信号处理算法的研究

高速信号处理算法的研究主要是针对目前日益激增的网络数据流量给各大运营商带来的巨大难题而提出。本文针对几种目前研究比较集中的信号处理算法进行分析,并进行对比,能够为高速信号处理算法的选择提供一定的理论支持。