用植物油作为绝缘液降低变压器失火风险

每年,总有一些变压器发生故障导致火灾。由于变压器的绝缘液是可燃物,火灾可以持续数小时或数天。火、烟以及可能发生的液体泄漏,不仅会危及人体和动物健康,破坏环境,也会损毁邻近的设备。而且,此类事故会影响相关方的声誉和资产负债情况。因此,业内长期高度重视失火风险较低的变压器。ABB引进了高油酸、高燃点的天然酯绝缘液BIOTEMP,相比于传统使用的矿物油,BIOTEMP可极大地降低起火和火灾蔓延的风险。作为市场上最安全的变压器绝缘液之一,使用BIOTEMP的变压器究竟有何与众不同呢？     

Midinfrared laser source with high power and beam quality

A simple scheme for generation of high power in the midinfrared is demonstrated. By using a 15 W thulium-doped fiber laser emitting at 1907 nm to pump a Q-switched Ho:YAG laser, we obtained 9.8 W at 2096 nm at a 20 kHz pulse repetition rate with excellent beam quality. The output of this laser was used to pump a doubly resonant zinc germanium phosphide based optical parametric oscillator, and we obtained 5.1 W average power in the 3-5 microm range with M2 approximately = 1.8.     

Highly nonlinear solitary waves in chains of hollow spherical particles

We study the dynamic response of one- dimensional granular chains composed of uniform hollow spheres excited by an impulse, and we observe the formation and propagation of highly nonlinear solitary waves. We find that the dynamics of these solitary waves are different from the solitary waves forming in chains composed of uniform solid spheres, because of the changes in the contact interaction between particles. We study the quasi-static contact interaction between two hollows spheres using finite element (FE) simulations, and approximate their response as a power-law type function in the range of forces of interest for this work. The experimental data obtained by testing a chain of particles shows good agreement with theoretical predictions obtained using a long wavelength approximation, and with numerical simulations based on discrete particle and FE models. We also investigate the effect of hollow spheres’ wall thickness on the dynamic response of the chains.     

Flash photolysis and stopped flow studies of the 2′-methoxyflavylium network in aq. acidic and alkaline solution

The rate and equilibrium constants of the network of chemical species involving the dye 2′-methoxyflavylium tetrafluoroborate were characterized using stopped flow and flash photolysis in both acidic and alkaline aqueous solution. The trans-chalcone is the thermodynamic stable species in acidic solutions 2 < pH < 7; irradiation of trans-chalcone at low pH leads to the corresponding coloured flavylium cation. The system reverts to its initial state in a few hours, in the dark. The kinetics of the various steps in the system were determined; flash photolysis revealed that the rate determining process of flavylium formation is the ring closure to give the corresponding hemiketal. In alkaline medium the ionized trans-chalcone is the stable form and clear evidence for the existence of a hemiketal species was obtained. A write-read-erase cycle can be performed with this compound.     

Tightening the upper bound for the minimum energy broadcasting

In this paper we present a new upper bound on the approximation ratio of the Minimum Spanning Tree heuristic for the basic problem on Ad-Hoc Networks given by the Minimum Energy Broadcast Routing (MEBR). We introduce a new analysis allowing to establish a 6.33-approximation ratio in the 2-dimensional case, thus decreasing the previously known 7.6 upper bound [4], almost closing the gap with the lower bound of 6 [12]. Preliminary results concerning this paper appeared in [9]. Michele Flammini received the degree in Computer Science at the University of L’Aquila in 1990 and the Ph.D. degree in Computer Science at the University of Rome “La Sapienza” in 1995. He is full professor at the Computer Science Department of the University of L’Aquila since March 2005. His research interests include algorithms and computational complexity, game theory, communication problems in interconnection networks and routing. He has authored and co-authored more than 70 papers in his fields of interest published in the most reputed international conferences and journals. Ralf Klasing received the PhD degree from the University of Paderborn in 1995. From 1995 to 1997, he was an Assistant Professor at the University of Kiel. From 1997 to 1998, he was a Research Fellow at the University of Warwick. From 1998 to 2000, he was an Assistant Professor at RWTH Aachen. From 2000 to 2002, he was a Lecturer at King’s College London. In 2002, he joined the CNRS as a permanent researcher. From 2002 to 2005, he was affiliated to the laboratory I3S in Sophia Antipolis. Currently, he is affiliated to the laboratory LaBRI in Bordeaux. He has co-authored a Springer Monograph, a book chapter, and has published more than 40 papers in refereed international periodicals. His research interests include Communication Algorithms in Networks, Approximation Algorithms for Combinatorially Hard Problems, Web Graphs and Web Algorithms, and Optimization Problems in Ad-Hoc Wireless Networks. Alfredo Navarra received the degree in Computer Science at the University of L’Aquila in 2000 and the Ph.D. degree in Computer Science at the University of Rome “La Sapienza” in 2004. From 2003 to 2004, he joined the MASCOTTE project team at the INRIA institute of Sophia Antipolis as PhD student and PostDoc for almost two years. In 2005, he was a temporary researcher at the University of L’Aquila. In 2006, he joined the laboratory LaBRI in Bordeaux as PostDoc. His research interests include, algorithms and computational complexity, communication, modelling as well as analysis and experimentation problems on protocols and routing algorithms for interconnaction networks such as Ad Hoc, Wireless, Mobile and Sensor Networks. He has authored and co-authored more than 25 papers in his fields of interest published in the most reputed international conferences and journals. Stéphane Pérennes is a permanent researcher of the French Centre National de la Recherche Scientifique (CNRS). He is affiliated to the MASCOTTE project team at the Institut National de Recherche en Informatique et Automatique (INRIA) of Sophia Antipolis. He has authored and co-authored more than 70 papers in his fields of interest that vary from pure theoretical to applied issues on algorithms and complexity, networking and routing.     

A method for the visual analysis of early‐stage Parkinson's disease based on virtual MRI‐derived SPECT images

Objective: Nuclear medicine imaging of dopamine transporter (DAT) binding is used in the differential diagnosis of neurodegenerative Parkinsonism but DAT receptor density in the posterior part of the putamens is of difficult evaluation because it is strongly affected by the partial‐volume effect. Since its quantitative correction is often difficult, we propose here a qualitative method to compare the real study to virtual images of the patient, obtained as if he/she was healthy. Method: The virtual images are obtained coregistering the magnetic resonance images to SPECT's; segmenting magnetic resonance images to obtain the real shape and position of caudates and putamens; digitally filling these volumes with an activity concentration close to normal condition; blurring and convolving images with the measured point‐spread function of the SPECT system. Result: The method was applied to phantom and, retrospectively, to a small cohort of 20 patients, that underwent SPECT examination more than 3 years ago, to prove the feasibility of the method in a clinical environment. Conclusions: This method enhances the accuracy of the nuclear medicine interpretation by the inclusion of the proper morphological information of each patient, helping the physician to differentiate between partial‐volume effects and real hypofixation of the tracer. The method proved to be feasible but is applicability is fostered by the need of a concomitant MR. © 2012 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 22, 172–176, 2012     

Numerical investigation of flutter stability in subsonic space turbine blisk with emphasis on cut-on/cut-off modes and interblade phase angles

The so-called blisks,i.e.integrally bladed disks,are characterized by very low viscous material damping and make the flutter prediction much more critical.In that framework,a two-dimensional numerical study of a space turbine blisk featuring complex deformation of blades and high eigenfrequency(>40kHz)is performed.The simulations are based on unsteady Reynolds Averaged Navier Stokes computations linearized in the frequency domain and consist in the superposition of an unsteady linear(in time)pressure field,generated by a harmonic perturbation,upon a steady nonlinear(in space)flow.The aerodynamic damping coefficient is calculated over a range of nodal diameters,and the blades are predicted aeroelastically stable.However,violent changes occur and are rather critical since sudden and large deviations in stability appear.In that context,the nature of the waves propagating from the cascade are evaluated.Such an approach provides fundamental knowledge about the perturbations which can either propagate to the far-field(cut-on mode)or decay(cut-off mode).It is expected that the ability of the flow to damp or to amplify the blade motion is strongly affected by the way unsteady perturbations are transferred from the cascade to the far-field.The nature of the waves are first assessed from the aforementioned linearized results,then they are evaluated analytically and finally compared.A good agreement is found despite the strong assumptions of the analytical model.The results show a clear correlation between the cut-on/cut-off conditions and stability.The least stable configuration corresponds to cut-off mode at the inlet and no wave at the outlet.Without outgoing waves from the cascade,the blade is prone to be less stable:the energy from the blades vibration is necessarily dissipated or sent out by the cascade.     

On the equivalence and rewriting of aggregate queries

We introduce a first-order language with real polynomial arithmetic and aggregation operators (count, iterated sum and multiply), which is well suited for the definition of aggregate queries involving complex statistical functions. It offers a good trade-off between expressive power and complexity, with a tractable data complexity. Interestingly, some fundamental properties of first-order with real arithmetic are preserved in the presence of aggregates. In particular, there is an effective quantifier elimination for formulae with aggregation. We then consider the problem of querying data that has already been aggregated in aggregate views, and focus on queries with an aggregation over a conjunctive query (namely single-block aggregate group-by queries without having clause). Our main conceptual contribution is the introduction of a new equivalence relation among conjunctive queries, the isomorphism modulo a product. We prove that the equivalence of aggregate queries such as for instance averages reduces to it. Deciding if two queries are isomorphic modulo a product is shown to be NP-complete. We then analyze the equivalence problem in the case of aggregate conjunctive queries with comparisons. We introduce the concept of cross isomorphic linear expansions, which generalizes isomorphim modulo a product, and we show that equivalence reduces to it and that it can be decided in PSPACE. Finally, we show that the problem of complete rewriting of count queries using count views is NP-complete, and we introduce new rewriting techniques based on the isomorphism modulo a product. to recover the values of counts by complex arithmetical computation from the views.Received: 10 July 2003, Revised: 25 April 2004, Published online: 8 June 2004     

Flight operations recovery: New approaches considering passenger recovery

The sources of disruption to airline schedules are many, including crew absences, mechanical failures, and bad weather. When these unexpected events occur, airlines recover by replanning their operations. In this paper, we present airline schedule recovery models and algorithms that simultaneously develop recovery plans for aircraft, crews, and passengers by determining which flight leg departures to postpone and which to cancel. The objective is to minimize jointly airline operating costs and estimated passenger delay and disruption costs. This objective works to balance these costs, potentially increasing customer retention and loyalty, and improving airline profitability. Using an Airline Operations Control simulator that we have developed, we simulate several days of operations, using passenger and flight information from a major US airline. We demonstrate that our decision models can be applied in a real-time decision-making environment, and that decisions from our models can potentially reduce passenger arrival delays noticeably, without increasing operating costs.     

An efficient, error-bounded approximation algorithm for simulating quasi-statics of complex linkages

Design and analysis of articulated mechanical structures, commonly referred to as linkages, is an integral part of any CAD/CAM system. The most common approaches formulate the problem as purely geometric in nature, though dynamics or quasi-statics of linkages should also be considered. Existing optimal algorithms that compute forward dynamics or quasi-statics of linkages have a linear runtime dependence on the number of joints in the linkage. When forces are applied to a linkage, these techniques need to compute the accelerations of all the joints and can become impractical for rapid prototyping of highly complex linkages with a large number of joints.We introduce a novel algorithm that enables adaptive refinement of the forward quasi-statics simulation of complex linkages. This algorithm can cull away joints whose contribution to the overall linkage motion is below a given user-defined threshold, thus limiting the computation of the joint accelerations and forces to those that contribute most to the overall motion. It also allows a natural trade-off between the precision of the resulting simulation and the time required to compute it. We have implemented our algorithm and tested its performance on complex benchmarks consisting of up to 50,000 joints. We demonstrate that in some cases our algorithm is able to achieve up to two orders of magnitude of performance improvement, while providing a high precision, error-bounded approximation of the quasi-statics of the simulated linkage.