充气头型对超空泡轴对称体阻力特性影响的试验研究

为了了解充气空泡对不同头型的空泡特征及对轴对称体阻力特性的影响,在高速水洞中试验研究了三种典型头体在不同空泡数下,轴对称体上充气空泡的几何特征以及相对应的阻力特性,分析了充气数对轴对称体空泡特征与阻力的影响.试验结果表明,充气模型的空泡长度、厚度、空泡角随充气量增加而增大, 从模型头部充气,能够产生稳定形状的空泡覆盖模型,针对某种特定形状模型,充气产生的空泡能够形成减阻的效果.     

Optoelectronic characteristics of single ZnAlO nanotetrapod

The ZnAlO nanotetrapods were prepared by a simple thermal evaporation method. The typical characteristics of I-V curves for one of our devices illustrated that both the green light and the UV light could induce the increasing of conductance, but the conductance under the UV light is much larger than that under the illumination of green light. With the increasing of temperature, the conductance of the sample is also increased. Furthermore, the change of current versus time as the UV light is switched on and off shows the sample has good switch characteristics. These findings illustrate the sample may be used in room temperature light sensors.     

GMZRP: Geography-aided Multicast Zone Routing Protocol in Mobile Ad Hoc Networks

This paper presents the design and evaluation of a highly efficient on-demand multicast routing protocol for mobile ad hoc networks (MANETs). The protocol, called Geography-aided Multicast Zone Routing Protocol (GMZRP), eliminates as much as possible duplicate route queries by using a simple yet effective strategy for propagating the multicast route request (MRREQ) packets. GMZRP is the first hybrid multicast protocol taking the advantages of both topological routing and geographical routing. It partitions the network coverage area into small zones and guarantees that each geographic zone is queried only once. GMZRP maintains a multicast forwarding tree at two levels of granularities, i.e., the zone granularity and the node granularity. By doing this, it can easily handle route breakage since the zone level information can help recover the link failure at the node level. The results of the performance evaluation of GMZRP using simulation show that, comparing with the well-known multicast protocol ODMRP (On-Demand Multicast Routing Protocol), GMZRP has much lower protocol overhead in terms of query packets and, meanwhile, achieves competing packet delivery ratio and shorter delivery latency.

Key management issues in wireless sensor networks: current proposals and future developments

Key management has remained a challenging issue in wireless sensor networks (WSNs) due to the constraints of sensor node resources. Various key management schemes that trade off security and operational requirements have been proposed in recent years. In this article, we first examine the security and operational requirements of WSNs and then review five key management protocols: Eschenauer, Du, LEAP, SHELL, and Panja. Eschenauer's scheme is a classical random key distribution scheme for WSNs. Du's scheme improves on Eschenauer's scheme by using key matrices. LEAP provides a highly flexible key management scheme using four types of keys. SHELL focuses on achieving high robustness, and Panja is optimized for hierarchical WSNs. LEAP, SHELL, and Panja support cluster-based operations and are more aligned with current trends as shown by the new standards, IEEE 802.15.4b and the ZigBee "enhanced" standard. Future developments likely will incorporate the features of LEAP and adjustable robustness enhancements from Eschenauer or Du; extremely security-critical applications may benefit from restructuring SHELL to ease implementation and maintenance. Developments for extremely large WSNs should consider improving Panja's scheme due to its hierarchical scalability feature.     

Gossip-based cooperative caching for mobile applications in mobile wireless networks

Cooperative caching is an efficient way to improve the performance of data access in mobile wireless networks, by cache nodes selecting different data items in their limited storage in order to reduce total access delay. With more demands on sharing a video or other data, especially for mobile applications in an Internet-based Mobile Ad Hoc Network, considering the relations among data items in cooperative caching becomes more important than before. However, most of the existing works do not consider these inherent relations among data items, such as the logical, temporal, or spatial relations. In this paper, we present a novel solution, Gossip-based Cooperative Caching (GosCC) to address the cache placement problem, and consider the sequential relation among data items. Each mobile node stores the IDs of data items cached locally and the ID of the data item in use into its progress report. Each mobile node also makes use of these progress reports to determine whether a data item should be cached locally. These progress reports are propagated within the network in a gossip-based way. To improve the user experience, GosCC aims to provide users with an uninterrupted data access service. Simulation results show that GosCC achieves better performance than Benefit-based Data Caching and HybridCache, in terms of average interruption intervals and average interruption times, while sacrificing message cost to a certain degree.     

基于混沌细胞自动机的图像加密算法

提出了基于耦合混沌和触发细胞自动机的图像加密算法。首先用耦合混沌系统对图像进行加密,然后用耦合二维细胞自动机再次加密,最后用混沌序列对图像进行置乱。加密过程中同时运用了分组加密和流加密方法。仿真实验结果表明,该加密算法实现简单,扩散和混淆效果比较理想,有较强的抗攻击能力,具有良好的加密效果。     

Message and time efficient consensus protocols for synchronous distributed systems

For a synchronous distributed system of n processes with up to t potential and f   actual crash failures, where (t<n-1,f≤t)$(t<n-1,f\le t)$, the time lower bound for a protocol to achieve consensus is min(t+1,f+2)$\min (t+1,f+2)$ rounds. Currently, most researches in this field focus on the time efficiency of consensus protocols. This paper proposes consensus protocols for synchronous distributed systems that achieve both message and time   efficiency. Based on an early stopping consensus protocol for synchronous distributed system with crash failures, we propose a rotating coordinator scheme that significantly reduces message complexity. However, this protocol is not time efficient because it requires min(t+1,f+3)$\min (t+1,f+3)$ rounds to reach consensus. Thus, to achieve both time and message efficiency, we propose another protocol in which (t+1)$(t+1)$ coordinators are used to send messages in each round. Furthermore, we show that the proposed consensus protocol with crash failures can be revised to be more message-efficient with orderly crash failures. When a process is able to send more than one message to another in a round, we propose an optimal message efficient early stopping consensus protocol for synchronous distributed systems with orderly crash failures.     

Architecting and implementing distributed Web applications using the graph‐oriented approach

This paper presents a graph‐oriented framework, called WebGOP, for architecture modeling and programming of Web‐based distributed applications. WebGOP is based on the graph‐oriented programming (GOP) model, under which the components of a distributed program are configured as a logical graph and implemented using a set of operations defined over the graph. WebGOP reshapes GOP with a reflective object‐oriented design, which provides powerful architectural support in the World Wide Web environment. In WebGOP, the architecture graph is reified as an explicit object which itself is distributed over the network, providing a graph‐oriented context for the execution of distributed applications. The programmer can specialize the type of graph to represent a particular architecture style tailored for an application. WebGOP also has built‐in support for flexible and dynamic architectures, including both planned and unplanned dynamic reconfiguration of distributed applications. We describe the WebGOP framework, a prototypical implementation of the framework on top of SOAP, and a performance evaluation of the prototype. The prototype demonstrated the feasibility of our approach. Results of the performance evaluation showed that the overhead introduced by WebGOP over SOAP is reasonable and acceptable. Copyright © 2003 John Wiley & Sons, Ltd.     

WEBGOP: collaborative web services based on graph-oriented programming

WEBGOP is a programming architecture for collaborative Web services using graph-oriented programming. The motivation for the project comes from the realization that the integration of collaborative Web services lacks support. The aim of WEBGOP is to extend the Web from a client-server system to a structured multipoint system. A graph abstraction of the network provides the structure for the integration of Web services and facilitates their configuration and programming. Using WEBGOP, a logical graph representing a virtual-overlay network over the Internet is created to link up collaborative Web services. Web services are individually or jointly invoked through either unicast or multicast messages within the overlay network. All messages are based on the simple-object access protocol (SOAP). This forms an extension of the hypertext transfer protocol (HTTP) to support the distributed invocation of Web services. The Web services on different servers work collaboratively for a multipoint network application. This project provides a structured integration of Web services by extending the support of intermediary processing in a multipoint service. It also provides a rich network-programming interface for a new class of integrated Web applications while retaining the use of the Internet protocol and HTTP.