Flow Characteristics and Aerodynamic Heating of Tube Trains in Choked/Unchoked Flow:A Numerical Study

Hyperloop has become one of the key reserve technologies for future high-speed rail transit.The gas in the tube is compressed and rubbed,leading to a strong aerodynamic heating effect.The research on the flow field characteristics and aerodynamic heating effect of hyperloop is in its infancy,and that on the flow field structure is lacking.In this study,the nozzle theory was used to make a preliminary judgment on the choked flow phenomenon in the hyperloop.Based on the flow results obtained under different working conditions,the identification basis of the choked flow phenomenon in the hyperloop was obtained.Furthermore,the effect of the choked/unchoked flow on the flow structure,temperature,and pressure distribution of the annular space in the tube was analyzed.Based on traditional high-speed railway aerodynamics,according to relevant theories and calculation in aerospace field,and combined with the model test data,the reliability verification analysis on the characteristics of the flow field are carried out.The structure of the flow filed in the tube can be divided into choked and unchoked.The judgment is dependent on whether the throat reaches the speed of sound.Under the choked flow,a normal shock wave is formed in front of the tube train.The temperature rise of the local flow field exceeds 50 K;the temperature rise of the stagnation region exceeds 88 K,and the pressure is approximately 1.7 times that of the initial pressure in the tube.When the flow is unchoked,differences arise in the distribution of the flow field corresponding to different incoming Mach numbers.When the incoming flow is supersonic,the flow field maintains a supersonic speed,and a bow-shaped shock wave is formed at the front of the tube train.Owing to the shock wave or expansion wave,the local flow field exhibits significant fluctuations in temperature and pressure.Conversely,when the incoming flow is subsonic,the flow field in the tube maintains a subsonic speed,and no shock wave structure is observed.

一种嵌入式BitTorrent优化策略

BitTorren(tBT)的Optimistic Unchoke算法为BT下载带来了一定程度的不公平性。利用该缺陷,消极上节点可以通过维护大量的伙伴节点来获得较高的下载速率。普通BT客户端维护伙伴节点的机制不适合于内存和网络连接数受限的嵌入式平台。论述了一种针对嵌入式BT的伙伴节点维护策略。该策略通过有效维护大量的可用节点,在满足嵌入式平台内存和网络连接数受限、上传速率低的条件下,利用Optimistic Unchoke机制来增加获得节点服务的概率,从而提高客户端的下载性能。

BitTorrent改进方法

改进了传统的BitTorrent实现,通过一定的策略来选择邻居节点从而提高整体的下载性能,通过自适应地调整unchoke节点数目,来更加有效地利用每个节点的上行带宽。经过NS2仿真测试,发现改进的BitTorrent在网络带宽总体偏差的情况下,相对于传统的算法能够缩短将近25%的节点平均下载时间。

一种抵抗搭便车行为的概率式连接交换unchoking策略

Bit Torrent文件共享系统中的搭便车(free-riding)节点会使系统性能大幅下降,目前,Bit Torrent主要采用choking策略来抑制搭便车行为,但与choking合作而存在的随机选择节点的unchoking策略仍然给搭便车行为提供了机会.提出了一种基于概率连接交换(probabilistic link exchange,简称PLX)的unchoking策略,在实现unchoking功能的同时,有效地抑制了搭便车行为.由于搭便车节点不提供上传,所以choking以后没有指向搭便车节点的连接,此时,PLX的连接交换机制就能抑制搭便车节点进入文件共享系统.另外,通过对连接交换概率的数学控制,PLX可以区分节点对共享系统的贡献,并根据贡献大小调整其在共享网络中的位置,进一步保证了公平性.最后,对PLX的影响进行了深入的理论分析和模拟实验验证,结果表明:PLX unchoking策略较现有的抵抗搭便车的方法更简单、直接,在效果上有明显提升.

BitTorrent协议Choking／Unchoking机制的度量和分析

BitTorrent是一个用于内容分发的P2P协议,现在已经发展成为互联网的一项重要的应用。本文从性能的角度,度量BitTorrent的行为,解释BitTorrent协议的关键元素,分析BitTorrent是否是高效的。本文有以下贡献:①提出一种有效度量BitTorrent式的内容分发协议的方法。②确认BitTorrent的Choking/Unchoking机制存在缺陷,不是高效的。③设计ShareStorm协议,证明BitTorrent的缺陷可以避免。经初步验证,在下载完成时间这个最主要的性能指标上,ShareStorm比BitTorrent至少减少50+%。

ShareStorm协议的Choking/Unchoking机制

提出一种有效度量P2P内容分发协议的实验方法,并设计了内容分发协议ShareStorm用于判定是否提供内容上载服务的Choking/Unchoking机制,以避免BitTorrent中竞争特性对peers之间协作的制约.实验表明,ShareStorm在下载完成时间这个最主要的性能指标上相比BitTorrent效率更高.

基于BT协议的数据分发机制

BT协议是一种用于文件分发的P2P协议,目前是internet网上应用最为广泛的协议之一。它所采用基于tit-for-tat算法的Choking/Unchoking机制来选择意愿进行数据传输的对等节点,但该机制对新加入网络,并没有相关数据的节点而言,意味着更少的机会获得数据,难以快速融入整个数据交换网络中以快速完成文件的下载。本文在分析BitTorrent协议的基础上,介绍一种改进方法以解决上述问题。