Anti-plane shear Lamb''s problem treated by gradient elasticity with surface energy

The consideration of higher-order gradient effects in a classical elastodynamic problem is explored in this paper. The problem is the anti-plane shear analogue of the well-known Lamb's problem. It involves the time-harmonic loading of a half-space by a single concentrated anti-plane shear line force applied on the half-space surface. The classical solution of this problem based on standard linear elasticity was first given by J.D. Achenbach and predicts a logarithmically unbounded displacement at the point of application of the load. The latter formulation involves a Helmholtz equation for the out-of-plane displacement subjected to a traction boundary condition. Here, the generalized continuum theory of gradient elasticity with surface energy leads to a fourth-order PDE under traction and double-traction boundary conditions. This theory assumes a form of the strain-energy density containing, in addition to the standard linear-elasticity terms, strain-gradient and surface-energy terms. The present solution, in some contrast with the classical one, predicts bounded displacements everywhere. This may have important implications for more general contact problems and the Boundary-Integral-Equation Method.     

Products from furans. VI. Synthesis of dihydro-2-disubstituted-2H-pyran-3(4H)-ones,tetrahydro-3-(aminomethyl)-2-(p-methoxyphenyl)-2-methyl-2H-pyran-3-ol derivatives and 6-(p-methoxyphenyl -6-methyl-7-oxa-1,3-diazaspiro[4,5]decane-2,4-dione

Hydrogenolysis of 5,6-dihydro-6-(p-methoxyphenyl)-6-methyl-5-oxo-2H-pyran-2-yl ethyl carbonate yielded dihydro-2-(p-methoxyphenyl)-2-methyl-2H-pyran-3(4H)-one, 3 . Subsequently cyanohydrin 4 , derived from 3 , on reduction afforded 3-(aminomethyl)tetrahydro-2-(p-methoxyphenyl)-2-methyl-2H-pyran-3-ol, 5 . The synthesis of N-dimethyl,N-isopropyl,N-imidazolyl as well as N-oxazolinyl derivatives of 5 is presented. The synthesis of 6-(p-methoxyphenyl)-6-methyl-7-oxa-1,3-diazaspiro[4,5]decane-2,4-dione 10 , a spiro hydantoin prepared from ketone 3 is also reported.     

Improved fairness algorithms for rings with spatial reuse

Ring network architectures that employ spatial reuse permit concurrent transmissions of messages over different links. While spatial reuse increases network throughput, it may also cause starvation of nodes. To alleviate this problem, various policies have been suggested in the literature. In this paper, we concentrate on a class of such policies that achieves fairness by allocating transmission quotas to nodes. For such policies, we provide mechanisms for improving delays and increasing overall throughput without compromising fairness     

Single-trial dynamical estimation of event-related potentials: a Kalman filter-based approach

A method for single-trial dynamical estimation of event-related potentials (ERPs) is presented. The method is based on recursive Bayesian mean square estimation and the estimators are obtained with a Kalman filtering procedure. We especially focus on the case that previous trials contain prior information of relevance to the trial being analyzed. The potentials are estimated sequentially using the previous estimates as prior information. The performance of the method is evaluated with simulations and with real P300 responses measured using auditory stimuli. Our approach is shown to have excellent capability of estimating dynamic changes form stimulus to stimulus present in the parameters of the ERPs, even in poor signal-to-noise ratio (SNR) conditions.     

Visualization of Taylor-Couette and spiral Poiseuille flows using a snapshot FLASH spatial tagging sequence

A new magnetic resonance imaging technique was applied to the Taylor-Couette and spiral Poiseuille (Taylor-Couette with superposed mean axial flux) flows for the first time. The experimental technique is a combination of spatial tagging methods and a snapshot FLASH imaging sequence, which allows the full-field visualization of 2-D slices of the flow field, with image acquisition times approximately half a second. By acquiring images every few seconds, direct visualization of flow patterns can be obtained in the form of cinematography. Tagged images of the Taylor-Couette flow were acquired in both the axial and transverse planes and confirmed previously reported numerical predictions of Taylor cell size. Tagged images of the spiral Poiseuille flows verified that the cells in this flow propagate at a higher velocity than the mean axial flow. In addition, intermittent cell formation was observed as the axial flow was increased.     

Minimum-Energy Broadcasting in Multi-hop Wireless Networks Using a Single Broadcast Tree

In this paper we address the minimum-energy broadcast problem in multi-hop wireless networks, so that all broadcast requests initiated by different source nodes take place on the same broadcast tree. Our approach differs from the most commonly used one where the determination of the broadcast tree depends on the source node, thus resulting in different tree construction processes for different source nodes. Using a single broadcast tree simplifies considerably the tree maintenance problem and allows scaling to larger networks. We first show that, using the same broadcast tree, the total power consumed for broadcasting from a given source node is at most twice the total power consumed for broadcasting from any other source node. We next develop a polynomial-time approximation algorithm for the construction of a single broadcast tree. The performance analysis of the algorithm indicates that the total power consumed for broadcasting from any source node is within 2H(n−1) from the optimal, where n is the number of nodes in the network and H(n) is the harmonic function. This approximation ratio is close to the best achievable bound in polynomial time. We also provide a useful relation between the minimum-energy broadcast problem and the minimum spanning tree, which shows that a minimum spanning tree may be a good candidate in sparsely connected networks. The performance of our algorithm is also evaluated numerically with simulations. A preliminary version of this work appeared in the Proceedings of WiOpt’04: Modeling and Optimization in Mobile, Ad hoc and Wireless Networks, University of Cambridge, UK, March 2004. Ioannis Papdimitriou was fully supported for this work by the Public Benefit Foundation “ALEXANDER S. ONASSIS”, Athens, Greece. Ioannis Papadimitriou was born in Veria, Greece, in 1976. He received his five year Diploma from the Department of Electronic and Computer Engineering, Technical University of Crete (Chania), Greece, in 1999 (graduating 2nd in class). He is currently a postgraduate student - Ph.D. candidate at the Telecommunications division, Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, Greece. His doctoral thesis deals with the design of wireless ad hoc networks. His research interests include broadcast and multicast communication, energy conservation, routing and topology control protocols, MAC layer and QoS issues. During his studies he has been honored with awards and scholarships by the Technical University of Crete, the Hellenic Telecommunications Organization S.A.(OTE S.A.) and Ericsson Hellas S.A. Mr. Papadimitriou has been a member of the Technical Chamber of Greece (TEE) since March 2000, and he has been supported by the Public Benefit Foundation ALEXANDER S. ONASSIS, Athens, Greece, with a scholarship for his doctoral studies from October 2001 to March 2005. Leonidas Georgiadis received the Diploma degree in electrical engineering from Aristotle University, Thessaloniki, Greece, in 1979, and his M.S. and Ph.D. degrees both in electrical engineering from the University of Connecticut, in 1981 and 1986, respectively. From 1981 to 1983 he was with the Greek army. From 1986 to 1987 he was Research Assistant Professor at the University of Virginia, Charlottesville. In 1987 he joined IBM T.J. Watson Research Center, Yorktown Heights, as a Research Staff Member. Since October 1995, he has been with the Telecommunications Department of Aristotle University, Thessaloniki, Greece. His interests are in the area of wireless networks, high speed networks, distributed systems, routing,scheduling, congestion control, modeling and performance analysis. Prof. Georgiadis is a senior member of IEEE Communications Society. In 1992 he received the IBM Outstanding Innovation Award for his work on goal-oriented workload management for multi-class systems.x     

Dispersive Rayleigh-Wave Propagation in Microstructured Solids Characterized by Dipolar Gradient Elasticity

It is well known that the classical theory of elasticity predicts Rayleigh-wave motions, which are not dispersive at any frequency. Of course, at high frequencies, this is a result that contradicts experimental data and also does not agree with results of the discrete particle theory (atomic-lattice approach). To remedy this shortcoming, the Mindlin–Green–Rivlin theory of dipolar gradient elasticity is employed here to analyze waves of the Rayleigh type propagating along the surface of a half-space. The analysis shows that these waves are indeed dispersive at high frequencies, a result that can be useful in applications of high-frequency surface waves, where the wavelength is often on the micron order. Provided that certain relations hold between the various microstructure parameters entering the theory employed here, the dispersion curves of these waves have the same form as that given by previous analyses based on the atomic-lattice theory. In this way, the present analysis gives also means to obtain estimates for microstructure parameters of the gradient theory.     

A Reliable Enantioselective Route to Mono-Protected N1-Cbz Piperazic Acid Building Block

The chiral N1-Cbz, N2-H derivative of the piperazic acid monomer is a valuable building block in the total synthesis of natural products, comprising this nonproteinogenic amino acid. In that context, we wish to report an improved synthetic protocol for the synthesis of both (3R)- and (3S)-piperazic acids bearing the carboxybenzyl protecting group (Cbz) selectively at the N1 position. Our method builds on previously reported protocols, circumventing their potential shortcomings, and optimizing the ultimate selective deprotection at the N2 position, thus, offering an efficient and reproducible pathway to suitably modified piperazates in high optical purity.     

Optimal buffer sharing

We address the problem of designing optimal buffer management policies in shared memory switches when packets already accepted in the switch can be dropped (pushed-out). Our goal is to maximize the overall throughput, or equivalently to minimize the overall loss probability in the system. For a system with two output ports, we prove that the optimal policy is of push-out with threshold type (POT). The same result holds if the optimality criterion is the weighted sum of the port loss probabilities. For this system, we also give an approximate method for the calculation of the optimal threshold, which we conjecture to be asymptotically correct. For the N-ported system, the optimal policy is not known in general, but we show that for a symmetric system (equal traffic on all ports) it consists of always accepting arrivals when the buffer is not full, and dropping one from the longest queue to accommodate the new arrival when the buffer is full. Numerical results are provided which reveal an interesting and somewhat unexpected phenomenon. While the overall improvement in loss probability of the optimal POT policy over the optimal coordinate-convex policy is not very significant, the loss probability of an individual output port remains approximately constant as the load on the other port varies and the optimal POT policy is applied, a property not shared by the optimal coordinate-convex policy     

Bottleneck multicast trees in linear time

On a directed graph with arc costs and a given source node, s, we consider the problem of computing multicast (Steiner) trees spanning any given node subset, V, so that the maximum of the tree arc costs is minimized. We show that this problem can be solved by simply solving the bottleneck path problem, i.e., the problem of determining for each node, t/spl ne/s, a path from s to t so that the maximum of path arc costs is minimized. For the latter problem we provide an implementation of Dijkstra's algorithm that runs in linear time under mild assumptions on arc costs.