Computation of localized flow for steady and unsteady vector fields and its applications

We present, extend, and apply a method to extract the contribution of a sub-region of a data set to the global flow. To isolate this contribution, we decompose the flow in the subregion into a potential flow that is induced by the original flow on the boundary and a localized flow. The localized flow is obtained by subtracting the potential flow from the original flow. Since the potential flow is free of both divergence and rotation, the localized flow retains the original features and captures the region-specific flow that contains the local contribution of the considered subdomain to the global flow. In the remainder of the paper, we describe an implementation on unstructured grids in both two and three dimensions for steady and unsteady flow fields. We discuss the application of some widely used feature extraction methods on the localized flow and describe applications like reverse-flow detection using the potential flow. Finally, we show that our algorithm is robust and scalable by applying it to various flow data sets and giving performance figures.     

Peer-to-peer data structures for cooperative traffic information systems

Cooperative traffic information systems support the driver of a car in selecting a route, based on traffic information collected by other cars. We propose to use a peer-to-peer network based on Internet access via cellular networks to distribute traffic information between the participants of such a system. This approach avoids the well-known limitations of VANET-based communication. Since the data maintained in a cooperative traffic information system has a very specific structure, it is particularly profitable—in terms of bandwidth consumption and latency—to tailor the system to this specific application domain instead of re-using generic peer-to-peer approaches. This realization led us to the development of GraphTIS—a peer-to-peer network specifically designed to manage traffic information. In this paper, we derive, step-by-step, the core mechanisms of GraphTIS, starting with a standard peer-to-peer system, outlining a first solution—named PeerTIS—which is based on a modification of this standard DHT, and then presenting GraphTIS, a novel peer-to-peer system that has been specifically designed to support traffic information systems.     

Automatic Detection and Visualization of Distinctive Structures in 3D Unsteady Multi‐fields

Current unsteady multi‐field simulation data‐sets consist of millions of data‐points. To efficiently reduce this enormous amount of information, local statistical complexity was recently introduced as a method that identifies distinctive structures using concepts from information theory. Due to high computational costs this method was so far limited to 2D data. In this paper we propose a new strategy for the computation that is substantially faster and allows for a more precise analysis. The bottleneck of the original method is the division of spatio‐temporal configurations in the field (light‐cones) into different classes of behavior. The new algorithm uses a density‐driven Voronoi tessellation for this task that more accurately captures the distribution of configurations in the sparsely sampled high‐dimensional space. The efficient computation is achieved using structures and algorithms from graph theory. The ability of the method to detect distinctive regions in 3D is illustrated using flow and weather simulations.     

Multidimensional B-trees for associative searching in database systems

A new method for multiple attribute indexing, the Multidimensional B-Tree (MBDT), is developed. This method is well suited for dynamic databases, since it handles several types of associative queries efficiently and requires low-cost maintenance. Algorithms and search strategies for exact match, partial match, and range queries are presented and statistical procedures are given to estimate the average and worst case retrieval times. The applicability of our organization to practical databases is discussed and analytical tradeoffs with regard to index organizations based on k-d trees are established.     

Performance analysis of a concurrent file reorganization algorithmfor record clustering

Presents a simulation-based performance analysis of a concurrent file reorganization algorithm. We examine the effect on throughput of (a) buffer size, (b) degree of reorganization, (c) write probability of transactions, (d) multiprogramming level, and (e) degree of clustered transactions. The problem of file reorganization that we consider involves altering the placement of records on pages of a secondary storage device. In addition, we want this reorganization to be done in place, i.e. using the file's original storage space for the newly reorganized file. Our approach is appropriate for a non-in-place reorganization as well. The motivation for such a physical change, i.e. record clustering, is to improve the database system's performance, i.e. minimizing the number of page accesses made in answering a set of queries. There are numerous record clustering algorithms, but they usually do not solve the entire problem, i.e., they do not specify how to efficiently reorganize the file to reflect the clustering assignment that they determine. In previous work, we have presented an algorithm that is a companion to general record clustering algorithms, i.e. it actually transforms the file. In this work we show through simulation that our algorithm, when run concurrently with user transactions, provides an acceptable level of overall database system performance     

Topographical distribution and immunocytochemical features of colonic neurons that project to the cranial mesenteric ganglion in the pig

Using the retrograde neuronal tracers Fast blue and Fluorogold, the topographical distribution and morphological features of porcine colonic neurons projecting to the cranial (superior) mesenteric ganglion have been investigated. Two to four weeks after injection of the tracer into the cranial mesenteric ganglion of immature pigs, labelled neurons were found throughout the colon. In the myenteric and outer submucous plexuses, they were present in ganglia situated to the side of the mesenteric attachment. The highest density of labelled neurons was observed at the end of the ascending colon, which in the pig represents 78-80% of the total colon length. The viscerofugal neurons had a multidendritic appearance and part of them were immunoreactive for calcitonin gene-related peptide or serotonin. This study has revealed similarities but also significant differences in the colono-sympathico-colonic pathways between the pig and small laboratory animals such as the guinea-pig.     

Web++ architecture,design and performance

We describe the design of a system for fast and reliable HTTP service which we call Web++. Web++ achieves high reliability by dynamically replicating web data among multiple web servers. Web++ selects the available server that is expected to provide the fastest response time. Furthermore, Web++ guarantees data delivery given that at least one server containing the requested data is available. After detecting a server failure, Web++ client requests are satisfied transparently to the user by another server. Furthermore, the Web++ architecture is flexible enough for implementing additional performance optimizations. We describe implementation of one such optimization, batch resource transmission, whereby all resources embedded in an HTML page that are not cached by the client are sent to the client in a single response. Web++ is built on top of the standard HTTP protocol and does not require any changes either in existing web browsers or the installation of any software on the client side. In particular, Web++ clients are dynamically downloaded to web browsers as signed Java applets. We implemented a Web++ prototype; performance experiments indicate that the Web++ system with 3 servers improves the response time perceived by clients on average by 36.6%, and in many cases by as much as 59%, when compared with the current web performance. In addition, we show that batch resource transmission can improve the response time on average by 39% for clients with fast network connections and 21% for the clients with 56 Kb modem connections. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ranking continuous nearest neighbors for uncertain trajectories

This article addresses the problem of performing Nearest Neighbor (NN) queries on uncertain trajectories. The answer to an NN query for certain trajectories is time parameterized due to the continuous nature of the motion. As a consequence of uncertainty, there may be several objects that have a non-zero probability of being a nearest neighbor to a given querying object, and the continuous nature further complicates the semantics of the answer. We capture the impact that the uncertainty of the trajectories has on the semantics of the answer to continuous NN queries and we propose a tree structure for representing the answers, along with efficient algorithms to compute them. We also address the issue of performing NN queries when the motion of the objects is restricted to road networks. Finally, we formally define and show how to efficiently execute several variants of continuous NN queries. Our experiments demonstrate that the proposed algorithms yield significant performance improvements when compared with the corresponding naïve approaches.     

2D Vector field approximation using linear neighborhoods

We present a vector field approximation for two-dimensional vector fields that preserves their topology and significantly reduces the memory footprint. This approximation is based on a segmentation. The flow within each segmentation region is approximated by an affine linear function. The implementation is driven by four aims: (1) the approximation preserves the original topology; (2) the maximal approximation error is below a user-defined threshold in all regions; (3) the number of regions is as small as possible; and (4) each point has the minimal approximation error. The generation of an optimal solution is computationally infeasible. We discuss this problem and provide a greedy strategy to efficiently compute a sensible segmentation that considers the four aims. Finally, we use the region-wise affine linear approximation to compute a simplified grid for the vector field.