排序方式: 共有11条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
A. I. Toumpis A. M. Galloway L. Arbaoui N. Poletz 《Science & Technology of Welding & Joining》2014,19(8):653-663
Friction stir welding is a solid state thermomechanical deformation process from which the plasticisation behaviour of the stirred material can be evaluated through the study of flow stress evolution. Flow stress data also supporting the development of a local microstructural numerical model have been generated. Hot compression testing of DH36 steel has been performed at a temperature range of 700–1100°C and strain rates from 10?3 to 102 s?1 to study the alloy’s thermomechanical deformation behaviour in conditions that simulate the actual friction stir welding process. It has been found that the evolution of flow stress is significantly affected by the test temperature and deformation rate. The material’s constitutive equation and constants have been calculated after analysis of these data. Preliminary numerical analysis results are in good agreement with experimental observations. 相似文献
5.
We study the transport capacity of the Gaussian multiple access channel (MAC), which consists of multiple transmitters and
a single receiver, and the Gaussian broadcast channel (BC), which consists of a single transmitter and multiple receivers.
The transport capacity is defined as the sum, over all transmitters (for the MAC) or receivers (for the BC), of the product
of the data rate with a reward r(x) which is a function of the distance x that the data travels.
In the case of the MAC, assuming that the sum of the transmit powers is upper bounded, we calculate in closed form the optimal
power allocation among the transmitters, that maximizes the transport capacity, using Karush-Kuhn-Tucker (KKT) conditions.
We also derive asymptotic expressions for the optimal power allocation, that hold as the number of transmitters approaches
infinity, using the most-rapid-approach method of the calculus of variations. In the case of the BC, we calculate in closed
form the optimal allocation of the transmit power among the signals to the different receivers, both for a finite number of
receivers and for the case of asymptotically many receivers, using our results for the MAC together with duality arguments.
Our results can be used to gain intuition and develop good design principles in a variety of settings. For example, they apply
to the uplink and downlink channel of cellular networks, and also to sensor networks which consist of multiple sensors that
communicate with a single central station.
Work was carried out while all authors were with the Telecommunications Research Center Vienna (ftw.), and supported by K
plus funding for the ftw. project I0 “Signal and Information Processing.”
Parts of this work have appeared, in preliminary form, in [1,2,3],
Gautam A. Gupta holds a joint B.S./M.S. degree in mathematics and computing at the Department of Mathematics of the Indian Institute of Technology
at New Delhi. During the summer of 2003, he attended a summer course on Probability and Statistical Mechanics organized by
the Scoula Normale Superiore, in Pisa, Italy. During the summers of 2004 and 2005 he worked at the Telecommunications Research
Center Vienna (ftw.) as a summer intern. During the spring of 2006, he was a visitor at the Norwegian University of Science
and Technology, working toward his M. S. Thesis.
Stavros Toumpis received the Diploma in electrical and computer engineering from the National Technical University of Athens, Greece, in
1997, the M.S. degrees in electrical engineering and mathematics from Stanford University, CA, in 1999 and 2002, respectively,
and the Ph.D. degree in electrical engineering, also from Stanford, in 2003. From 1998 to 1999, he worked as a Research Assistant
for the Mars Global Surveyor Radio Science Team, providing operational support. From 2000 to 2003, he was a Member of the
Wireless Systems Laboratory, at Stanford University. From 2003 to 2005, he was a Senior Researcher with the Telecommunications
Research Center Vienna (ftw.), in Vienna, Austria. Since 2005, he is a Lecturer at the Department of Electrical and Computer
Engineering of the University of Cyprus. His research is on wireless ad hoc networks, with emphasis on their capacity, the
effects of mobility on their performance, medium access control, and information theoretic issues.
Jossy Sayir received his Dipl. El.-Ing. degree from the ETH Zurich in 1991. From 1991 to 1993, he worked as a development engineer for
Motorola Communications in Tel Aviv, Israel, contributing to the design of the first digital mobile radio system ever produced
by Motorola. He returned to ETH from 1993 to 1999, getting his PhD in 1999 under the supervision of Prof. J.L. Massey. The
title of his thesis is “On Coding by Probability Transformation.” Since 2000, he has been employed at the Telecommunications
Research Center (ftw) in Vienna, Austria, as a senior researcher. His research interests include iterative decoding methods,
joint source and channel coding, numerical capacity computation algorithms, Markov sources, and wireless ad hoc and sensor
networks. Since July 2002, he manages part of the strategic research activities at Ftw and supervises a group of researchers.
He has taught courses on Turbo and related codes at Vienna University of Technology and at the University of Aalborg, Denmark.
He has served on the organization committees of several international conferences and workshops.
Ralf R. Müller was born in Schwabach, Germany, 1970. He received the Dipl.-Ing. and Dr.Ing. degree with distinction from University of Erlangen-Nuremberg
in 1996 and 1999, respectively. From 2000 to 2004, he was with Forschungszentrum Telekommunikation Wien (Vienna Telecommunications
Research Center) in Vienna, Austria. Since 2005 he has been a full professor at the Department of Electronics and Telecommunications
at the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway. He held visiting appointments at Princeton
University, U.S.A., Institute Eurecom, France, The University of Melbourne, Australia, and The National University of Singapore
and was an adjunct professor at Vienna University of Technology. Dr. Müller received the Leonard G. Abraham Prize (jointly
with Sergio S. Verdú) from the IEEE Communications Society and the Johann-Philipp-Reis Prize (jointly with Robert Fischer).
He was also presented an award by the Vodafone Foundation for Mobile Communications and two more awards from the German Information
Technology Society (ITG). Dr. Müller is currently serving as an associate editor for the IEEE Transactions on Information
Theory. 相似文献
6.
Optimal deployment of large wireless sensor networks 总被引:2,自引:0,他引:2
Toumpis S. Tassiulas L. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2006,52(7):2935-2953
A spatially distributed set of sources is creating data that must be delivered to a spatially distributed set of sinks. A network of wireless nodes is responsible for sensing the data at the sources, transporting them over a wireless channel, and delivering them to the sinks. The problem is to find the optimal placement of nodes, so that a minimum number of them is needed. The critical assumption is made that the network is massively dense, i.e., there are so many sources, sinks, and wireless nodes, that it does not make sense to discuss in terms of microscopic parameters, such as their individual placements, but rather in terms of macroscopic parameters, such as their spatial densities. Assuming a particular interference-limited, capacity-achieving physical layer, and specifying that nodes only need to transport the data (and not to sense them at the sources, or deliver them at the sinks once their location is reached), the optimal node placement induces a traffic flow that is identical to the electrostatic field created if the sources and sinks are replaced by a corresponding distribution of positive and negative charges. Assuming a general model for the physical layer, and specifying that nodes must not only transport the data, but also sense them at the sources and deliver them at the sinks, the optimal placement of nodes is given by a scalar nonlinear partial differential equation found by calculus of variations techniques. The proposed formulation and derived equations can help in the design of large wireless sensor networks that are deployed in the most efficient manner, not only avoiding the formation of bottlenecks, but also striking the optimal balance between reducing congestion and having the data packets follow short routes. 相似文献
7.
This work is based on the observation that, as the number of nodes in a wireless network approaches infinity, minimum cost routes become smooth curves that observe the same laws followed by rays of light in properly defined optical media. Accordingly, in this paper an analogy between optimal routing in large wireless networks and Geometrical Optics is first formally defined. The analogy is based on the concept of the cost function, which plays the role of the refractive index in the networking context. Then, the relevance of the principle of Fermat and the eikonal equation in routing problems is shown, and a methodology for calculating the cost function is proposed and applied in two cases of special interest, i.e., bandwidth limited and energy limited networks. The applicability of the Optics-Networking analogy is also discussed in the case of networks with large but finite numbers of nodes. Finally, novel, distributed route discovery protocols that make use of the analogy are outlined. 相似文献
8.
9.
Bilal Ahmad Alexander Galloway Athanasios Toumpis 《Science & Technology of Welding & Joining》2013,18(6):548-558
ABSTRACTSignificant progress has been made on the implementation of friction stir welding (FSW) in the industry for aluminium alloys. However, steel FSW and other high-temperature alloys is still the subject of considerable research, mainly because of the short life and high cost of the FSW tool. Different auxiliary energies have been considered as a means of optimising the FSW process and reducing the forces on the tool during the plunge and traverse stages, but numerical studies on steel are particularly limited. Building on the state-of-art, laser-assisted steel FSW has been numerically developed and analysed as a viable process amendment. Laser-assisted FSW increased the traverse speed up to 1500?mm?min?1, significantly higher than conventional steel FSW. The application of laser assistance with a distance of 20?mm from the rotating tool reduced the reaction force on the tool probe tip up to 55% when compared to standard FSW. 相似文献
10.
Hongjun Li Jian Gao Qinchuan Li Alexander Galloway Athanasios Toumpis 《Science & Technology of Welding & Joining》2013,18(2):156-162
ABSTRACTEnhancing the heat transfer to the material being welded, instead of the tool, will improve the welding thermal efficiency. Friction stir welding of 5?mm thick 6061-T6 aluminium alloy plates was carried out with the newly produced tools. It was found that the thermal efficiency increased by 4.2% using a tool with all the new design features (i.e. hollow, fluted and thermally insulated) compared to the conventional tool for aluminium welding. To assess the benefits of the new tool design on steel FSW, a finite element numerical simulation study was undertaken. In this case, the simulation results yielded a welding thermal efficiency increase of 10–15% using a thermally coated tool, thereby offering potential productivity gains. 相似文献