科学史上关于寻找地外文明的争论——人类应该在宇宙的黑暗森林中呼喊吗?

尽管地外文明是否存在的问题,目前尚无定论,但与其相关的探讨,已成为科学史领域的重要研究课题。1970年代,随着搜寻地外文明计划（简称SETI）的一无所获,与其相对的另一种试图接触地外文明的实践手段——主动向地外文明发送讯息（简称METI）,也被提上日程,并在科学界引发颇多争议。本文对争论双方的观点以及争论所产生的影响进行了考察,并研究了与MET1有若的几种费米佯谬的解决方案。本文认为,在尚未做好接触地外文明的准备之前,实施MET1是非常危险的。     

公铁联运规模效应和枢纽拥挤效应的联动分析

公铁联运最主要的优势就是采用轴辐式网络结构实现规模经济,降低货物的运输成本,货运量越大,规模效应越明显;另一方面,过多的货物在枢纽集结会造成枢纽拥挤,增加货物在枢纽的停留时间,从而降低了公铁联运的竞争力.本文分别采用与流量相关的分段成本函数和M/M/c的排队模型来描述公铁联运的规模效应和拥挤效应,建立了非线性的整数规划模型,用于综合分析规模效应和枢纽拥挤效应对公铁联运的影响.采用混合禁忌搜索算法求解本文的模型,该算法具有较好的收敛效果.最后,分析了铁路服务固定成本、公路运输成本、货物运到期限和枢纽能力对公铁联运竞争力的影响.     

Stern waves consisting of forward-oriented breaking waves and the remaining following waves

When a ship with a wide, immersed transom stern runs on a deep draft, forward-oriented wave breaking often occurs just behind the transom stern. In such conditions, stern waves can be considered to consist of two main components: the forward-oriented breaking wave and the remaining following waves. In our previous study, which was the first part of the present study, we developed a method to treat the forward-oriented breaking wave, and have clarified that it has a scale effect and its resistance coefficient decreases with an increase in the size of the model ship. On the other hand, the study also indicated that the height of the remaining following waves increases with an increase in the model ship size. The purpose of this paper is a more complete understanding of the two different component characteristics of the stern waves. We have developed a method to estimate the resistance due to the remaining following waves. Using this method, we have reached three main conclusions. The resistance increases considerably if the remaining following waves change to forward-oriented breaking waves. The resistance coefficient of the remaining following waves increases only slightly with an increase in model ship size. This stern wave resistance coefficient is strongly affected by the forward-oriented breaking waves, and therefore decreases with an increase in model ship size. Received: January 5, 2001 / Accepted: May 28, 2001     

基于CFD的导管调距桨尺度效应研究

常规螺旋桨在进行水动力实验时,需要满足临界雷诺数的要求,这样,实桨的尺度效应就可以忽略。但是,对于非常规螺旋桨,如导管螺旋桨,尺度效应能否忽略值得研究,因为机翼型导管和螺旋桨水动力性能各自随几何尺寸变化的规律不一致。基于CFD计算方法,对某导管调距桨的尺度效应进行数值研究。在获得可靠的数值模拟方法后,数值模拟了两个螺距比和两个进速下不同尺寸导管螺旋浆的流场。比尺包括1、1.7、3.4、4.25、8.5和17共6个几何尺寸。计算结果表明,导管螺旋桨敞水性能的尺度效应比预期的要明显。随着尺寸增大即雷诺数增大,导管的推力先略有增加后基本不变,螺旋桨的扭矩和推力略有下降,但效率有所增加;小螺距比工况下的尺度效应要比大螺距比的大。     

U.S. public transit earnings, employment and privatization

This paper uses individual worker and municipal information to examine privatization's influence on public transit workers' earnings and employment. OLS findings on labor earnings reveal that privatization is associated with an erosion of the public transit union premium. These labor earning findings do not change when correcting for privatization heterogeneity, as unobserved worker characteristics do not differ with differing levels of municipal privatization of public transit services. The employment findings reveal that union public transit workers are more likely to be employed in the public sector and this union-nonunion employment probability differential, declines with increasing levels of privatization. This employment result is interpreted as suggesting that labor cost savings from privatization can be derived from the enhanced employment of relatively low wage nonunion workers in the public sector of public transit services.     

物流全球化促进船公司运输链整合

在经济和物流全球化的环境下,船公司通过运输链上的纵向整合开展各种增值物流服务、实现范围经济,并且通过技术革新和兼并联盟开展横向整合,从而实现规模经济。     

Factors affecting the operational success of strategic airline alliances

With increased liberalisation in major air transport markets, the intensity of competition has increased amongst air carriers. Airlines have responded to the competitive pressures in many ways, one of which has been the formation of alliances. These alliances are linkages between the firms at various operational levels. They go beyond the common interlining agreement to encompass certain marketing and cost-reducing features. However, the question is how the success of these alliances can be ensured? While companies' culture compatibility is important and much has been written in that area, this paper focuses on factors that affect operational success of airline alliances. The operational success is measured by the change in the level of partners' inter-hub traffic due to formation of the alliance. This research has developed a methodology which could be used as a management tool to measure alliances' operational success before embarking on such agreements. The analysis of recent major alliances covering 52 inter-hub routes suggests that the main factors ensuring the alliances' operational success are: the partners' network size and their compatibility, the frequency of service between the hubs of the partners, the flight connection time at the hub and the level of competition on their network.

     

基于统计推断的动态轨道衡应用可靠性评价方法

针对轨道衡称量结果的可靠性问题,按照统计学原理,根据称重过程中所记录的传感器数据,阐述一种评价轨道衡称重结果可靠性的方法。     

放松规制下的出租车服务定价博弈模型

依据出租车行业规制特征,探讨了出租车行业在放松规制条件下的定价博弈模型.通过对政府与出租车公司之间的博弈关系分析,建立了斯坦克尔伯格双层规划模型.在出租车价格规制双层规划模型中,上层规划目标是社会福利最大化,下层规划目标是出租车公司利益最大化.模型考虑了出租车运营的实载里程.考虑双层规划模型求解的复杂性,使用遗传算法求解了放松规制后的规制价格上限值,然后结合案例,对放松规制定价博弈模型进行了研究.计算结果表明:该模型能够有效反映定价过程,模型与算法是有效的.最后考虑时间因素,建立了动态规制定价博弈模型.     

公路快速客运应对高铁时代的对策研究

针对公路快速客运的现实状况,阐述了公路快速客运与高铁相比具备的优势,提出公路快速客运企业的发展对策.