The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Adrian Bejan - One of the best experts on this subject based on the ideXlab platform.
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constructal design of distributed cooling on the landscape
International Journal of Energy Research, 2011Co-Authors: Liang Xia, S Lorente, Adrian BejanAbstract:Here we constructal design to determine the area size (or number of users N) to be allocated to a central refrigeration plant, and to configure the distribution Network for the distributed cooling system. The main objective is to maximize the net cooling capacity delivered to every user, and to minimize the pumping power required for transporting the chilled water. Two types of distribution Networks are investigated: radial and Tree-shaped Networks. First, when ducts with a single diameter are used, the net cooling capacity delivered to every user is almost the same, while the pumping power requirement for the Tree Network is greater than that for the radial Network. Multi-diameter Tree Networks are investigated next. The pumping power requirement for multi-diameter Tree-shaped Network is less than for radial Network when N is greater than 19 if the flow is laminar, and when N is greater than 80 if the flow is turbulent. The global performance of the area-based cooling system can be improved by increasing the freedom to morph the configuration. Copyright © 2010 John Wiley & Sons, Ltd.
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Tree shaped flow structures designed by minimizing path lengths
International Journal of Heat and Mass Transfer, 2002Co-Authors: S Lorente, W Wechsatol, Adrian BejanAbstract:Abstract This paper outlines a direct route to the construction of effective Tree-shaped flow structures. Dendritic flow structures dominate the design of natural and engineered flow systems, especially in thermal and fluid systems. The starting point is the optimization of the shape of each elemental area or volume, such that the length of the flow path housed by the element is minimized. Proceeding toward larger and more complex structures – from elements, to first constructs, second constructs, etc. – the paper develops Tree-shaped flow structures between one point and a straight line, one point and a plane, a circle and its center, and a point and many points distributed uniformly over an area. In the latter, the construction method is applied to a fluid flow configuration with laminar fully developed flow. The constructions reveal several features that are supported by empirical observations of natural Tree-shaped flows: asymmetry, flow rate imbalance, pairing or bifurcation, angles between branches, and Y-shaped constructs that lie in a plane. It is shown that these basic features are necessary because of “packing”, i.e., assembling optimized elements into a fixed space, and filling the space completely. For the flow between an area and one point, the best elemental shape is the regular hexagon. It is shown that the emergence of string-shaped links that connect two or more elements are necessary features, which are also required by packing. Strings cover some of the inner zones of the Tree Network, particularly the inner zones of large and complex Trees. Dichotomous Y-shaped constructs dominate the Tree structure, especially the peripheral zones of the Tree canopy. The practical importance of the simplified design method is discussed.
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how nature takes shape extensions of constructal theory to ducts rivers turbulence cracks dendritic crystals and spatial economics
International Journal of Thermal Sciences, 1999Co-Authors: Adrian BejanAbstract:The constructal theory of the origin of geometrical form in natural flow (open) systems began with the discovery that, contrary to the established view, the Tree Network can be deduced from a single principle: the geometric minimization of resistance in volume-to-point flow. This article reviews a series of developments that extend the constructal law over naturally shaped flow phenomena other than the Tree. Examples include the proportionality between width and depth in rivers of all sizes, the nearly round cross-sections of all blood vessels and bronchial passages, the dendritic shape of the snowflake, the pattern formed by cracks in a solid that shrinks upon cooling or drying (e.g., mud cracks), the onset and multiplication of rolls in B~nard convection, the transition (first eddy) and stepwise growth of all turbulent mixing regions, and the very existence of economics spatial structure (minimal cost routes between an area and one point). ~) Elsevier, Paris. constructal therory / Tree Network / minimization of resistance / economic optimization / transition to turbulence / B~nard rolls / shape of the cross-sections
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sTreets Tree Networks and urban growth optimal geometry for quickest access between a finite size volume and one point
Physica A-statistical Mechanics and Its Applications, 1998Co-Authors: Adrian Bejan, Gustavo A LedezmaAbstract:The geometric form of the Tree Network is deduced from a single mechanism. The discovery that the shape of a heat-generating volume can be optimized to minimize the thermal resistance between the volume and a point heat sink, is used to solve the kinematics problem of minimizing the time of travel between a volume (or area) and one point. The optimal path is constructed by covering the volume with a sequence of volume sizes (building blocks), which starts with the smallest size and continues with stepwise larger sizes (assemblies). Optimized in each building block is the overall shape and the angle between constituents. The speed of travel may vary from one assembly size to the next, however, the lowest speed is used to reach the infinity of points located in the smallest volume elements. The volume-to-point path that results is a Tree Network. A single design principle – the geometric optimization of volume-to-point access – determines all the features of the Tree Network.
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constructal theory Network of conducting paths for cooling a heat generating volume
International Journal of Heat and Mass Transfer, 1997Co-Authors: Adrian BejanAbstract:Abstract This paper develops a solution to the fundamental problem of how to collect and ‘channel’ to one point the heat generated volumetrically in a low conductivity volume of given size. The amount of high conductivity material that is available for building channels (high conductivity paths) through the volume is fixed. The total heat generation rate is also fixed. The solution is obtained as a sequence of optimization and organization steps. The sequence has a definite time direction, it begins with the smallest building block (elemental system) and proceeds toward larger building blocks (assemblies). Optimized in each assembly are the shape of the assembly and the width of the newest high conductivity path. It is shown that the paths form a Tree-like Network, in which every single geometric detail is determined theoretically. Furthermore, the Tree Network cannot be determined theoretically when the time direction is reversed, from large elements toward smaller elements. It is also shown that the present theory has far reaching implications in physics, biology and mathematics.
Pangfeng Liu - One of the best experts on this subject based on the ideXlab platform.
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Distributed throughput optimization for ZigBee cluster-Tree Networks
IEEE Transactions on Parallel and Distributed Systems, 2012Co-Authors: Yu-kai Huang, Pi-cheng Hsiu, Ai Chun Pang, Pangfeng LiuAbstract:ZigBee, a unique communication standard designed for low-rate wireless personal area Networks, has extremely low complexity, cost, and power consumption for wireless connectivity in inexpensive, portable, and mobile devices. Among the well-known ZigBee topologies, ZigBee cluster-Tree is especially suitable for low-power and low-cost wireless sensor Networks because it supports power saving operations and light-weight routing. In a constructed wireless sensor Network, the information about some area of interest may require further investigation such that more traffic will be generated. However, the restricted routing of a ZigBee cluster-Tree Network may not be able to provide sufficient bandwidth for the increased traffic load, so the additional information may not be delivered successfully. In this paper, we present an adoptive-parent-based framework for a ZigBee cluster-Tree Network to increase bandwidth utilization without generating any extra message exchange. To optimize the throughput in the framework, we model the process as a vertex-constraint maximum flow problem, and develop a distributed algorithm that is fully compatible with the ZigBee standard. The optimality and convergence property of the algorithm are proved theoretically. Finally, the results of simulation experiments demonstrate the significant performance improvement achieved by the proposed framework and algorithm over existing approaches.
Yu-kai Huang - One of the best experts on this subject based on the ideXlab platform.
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Distributed throughput optimization for ZigBee cluster-Tree Networks
IEEE Transactions on Parallel and Distributed Systems, 2012Co-Authors: Yu-kai Huang, Pi-cheng Hsiu, Ai Chun Pang, Pangfeng LiuAbstract:ZigBee, a unique communication standard designed for low-rate wireless personal area Networks, has extremely low complexity, cost, and power consumption for wireless connectivity in inexpensive, portable, and mobile devices. Among the well-known ZigBee topologies, ZigBee cluster-Tree is especially suitable for low-power and low-cost wireless sensor Networks because it supports power saving operations and light-weight routing. In a constructed wireless sensor Network, the information about some area of interest may require further investigation such that more traffic will be generated. However, the restricted routing of a ZigBee cluster-Tree Network may not be able to provide sufficient bandwidth for the increased traffic load, so the additional information may not be delivered successfully. In this paper, we present an adoptive-parent-based framework for a ZigBee cluster-Tree Network to increase bandwidth utilization without generating any extra message exchange. To optimize the throughput in the framework, we model the process as a vertex-constraint maximum flow problem, and develop a distributed algorithm that is fully compatible with the ZigBee standard. The optimality and convergence property of the algorithm are proved theoretically. Finally, the results of simulation experiments demonstrate the significant performance improvement achieved by the proposed framework and algorithm over existing approaches.
Mario Alves - One of the best experts on this subject based on the ideXlab platform.
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a time division beacon scheduling mechanism for ieee 802 15 4 zigbee cluster Tree wireless sensor Networks
Euromicro Conference on Real-Time Systems, 2007Co-Authors: Anis Koubaa, Andre Cunha, Mario AlvesAbstract:While the IEEE 802.15.4/Zigbee protocol stack is being considered as a promising technology for low-cost low-power wireless sensor Networks (WSNs), several issues in their specifications are still open. One of those ambiguous issues is how to build a synchronized cluster-Tree Network, which is quite suitable for ensuring QoS support in WSNs. In fact, the current IEEE 802.15.4/Zigbee specifications restrict the synchronization in the beacon-enabled mode (by the generation of periodic beacon frames) to star-based Networks, while they support multi-hop Networking using the peer-to-peer mesh topology, but with no synchronization. Even though both specifications mention the possible use of cluster-Tree topologies, which combine multi-hop and synchronization features, the description on how to effectively construct such a Network topology is missing. This paper tackles this problem, unveiling the ambiguities regarding the use of the cluster-Tree topology and proposing a synchronization mechanism based on Time Division Beacon Scheduling to construct cluster-Tree WSNs. We also propose a methodology for an efficient duty-cycle management in each router (cluster-head) of a cluster-Tree WSN that ensures the fairest use of bandwidth resources. The feasibility of the proposal is clearly demonstrated through an experimental test bed based on our own implementation of the IEEE 802.15.4/Zigbee protocols.
P R Kumar - One of the best experts on this subject based on the ideXlab platform.
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optimal computation of symmetric boolean functions in Tree Networks
International Symposium on Information Theory, 2010Co-Authors: Hemant Kowshik, P R KumarAbstract:In this paper, we address the scenario where nodes with sensor data are connected in a Tree Network, and every node wants to compute a given symmetric Boolean function of the sensor data. We first consider the problem of computing a function of two nodes with integer measurements. We allow for block computation to enhance data fusion efficiency, and determine the minimum worst-case total number of bits to be exchanged to perform the desired computation. We establish lower bounds using fooling sets, and provide a novel scheme which attains the lower bounds, using information theoretic tools. For a class of functions called sum-threshold functions, this scheme is shown to be optimal.
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optimal computation of symmetric boolean functions in Tree Networks
arXiv: Information Theory, 2010Co-Authors: Hemant Kowshik, P R KumarAbstract:In this paper, we address the scenario where nodes with sensor data are connected in a Tree Network, and every node wants to compute a given symmetric Boolean function of the sensor data. We first consider the problem of computing a function of two nodes with integer measurements. We allow for block computation to enhance data fusion efficiency, and determine the minimum worst-case total number of bits to be exchanged to perform the desired computation. We establish lower bounds using fooling sets, and provide a novel scheme which attains the lower bounds, using information theoretic tools. For a class of functions called sum-threshold functions, this scheme is shown to be optimal. We then turn to Tree Networks and derive a lower bound for the number of bits exchanged on each link by viewing it as a two node problem. We show that the protocol of recursive inNetwork aggregation achieves this lower bound in the case of sumthreshold functions. Thus we have provided a communication and in-Network computation strategy that is optimal for each link. All the results can be extended to the case of non-binary alphabets. In the case of general graphs, we present a cut-set lower bound, and an achievable scheme based on aggregation along Trees. For complete graphs, the complexity of this scheme is no more than twice that of the optimal scheme.
