166 research outputs found


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    ABSTRACT The work describes a method to predict the evolution of the wheel profile of a railway vehicle, depending on the load history acting on the wheelset. The method is based on the determination of the wear on the contact area, which is divided into finite elements according to the strip theory. For each element, in presence of slip, the amount of material loss is evaluated depending on the local value of tangential force and creepage (the meaning of creepage is assumed according to the definition given in The wear is calculated for the entire contact area superimposing the contribution of each element. The motion of the wheelset in lateral direction causes a motion of the contact patch along the profile. Sequentially, the contact area will acquire a different contact shape and stress distribution. The shape of the worn profile depends on both the load condition and the motion of the wheelset with respect to the track. This profile can be obtained from the new one by subtracting at each time step the material removed from the contact area. This procedure is simple, but requires variable profiles for each time step, and is not efficient in computational terms. The strategy proposed here by the authors, is to consider finite periods obtained superimposing several revolution of the wheelset. The worn profile is evaluated in a single step from the cumulative of damage of an entire period. The limitation of this method consists in the different behavior of a wheelset with worn profile respect to a wheelset with new ones, and therefore produces different wear. It is necessary to determine an optimal value for the period to be used to re-evaluate the profile shape, in order to minimize the difference in the predicted shape itself. The method is applied to a suspended wheelset, running on a simulated test track, with S1002/UIC60 profiles. Different periods of re-evaluation of the profiles are considered in order to demonstrate the influence of this parameter. INTRODUCTION Wear determines the lifespan of a construction, and in railway applications it affects both rails and wheels, which are the two elements providing guidance and support to the motion of the vehicle. Reduction of wear is therefore one of the important objectives of railway technology; tribology studies are used to develop new design methodologies, in order to reduce wear and increase life of rails and wheels. Considering the wheel wear, it is important to evaluate not only the total amount of wear, but also the shape variation of the profile. The same amount of wear can cause a higher or lower deterioration of the wheelset performance depending on the new shape of the worn profile. In fact in case of high equivalent conicity, the stability of the vehicle can be heavily reduced. In general, it is possible to distinguish if the wear is distributed uniformly along the circumferential direction or not. In the last case, unroundness may arise; the wheel is no longer exactly a body of revolution but exhibits waves in the circumferential direction or flatness, this is a problematic type of wear as the rolling stock is severely and dynamically loaded. This type of wear is usually related to braking operations (wheel flat), or to problems depending on material and loading conditions. Prediction of this type of wear requires a complex numerical investigations involving real time simulations, low time step and precise determination of the loads acting on the wheel. In this work, the study is limited to wear with uniform distribution in the circumferential direction; in this case the wheel remains a body of revolution, but it changes shape in the transversal section

    Cellular Automaton for Realistic Modelling of Landslides

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    A numerical model is developed for the simulation of debris flow in landslides over a complex three dimensional topography. The model is based on a lattice, in which debris can be transferred among nearest neighbors according to established empirical relationships for granular flows. The model is then validated by comparing a simulation with reported field data. Our model is in fact a realistic elaboration of simpler ``sandpile automata'', which have in recent years been studied as supposedly paradigmatic of ``self-organized criticality''. Statistics and scaling properties of the simulation are examined, and show that the model has an intermittent behavior.Comment: Revised version (gramatical and writing style cleanup mainly). Accepted for publication by Nonlinear Processes in Geophysics. 16 pages, 98Kb uuencoded compressed dvi file (that's the way life is easiest). Big (6Mb) postscript figures available upon request from [email protected] / [email protected]

    EDACs and test integration strategies for NAND flash memories

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    Mission-critical applications usually presents several critical issues: the required level of dependability of the whole mission always implies to address different and contrasting dimensions and to evaluate the tradeoffs among them. A mass-memory device is always needed in all mission-critical applications: NAND flash-memories could be used for this goal. Error Detection And Correction (EDAC) techniques are needed to improve dependability of flash-memory devices. However also testing strategies need to be explored in order to provide highly dependable systems. Integrating these two main aspects results in providing a fault-tolerant mass-memory device, but no systematic approach has so far been proposed to consider them as a whole. As a consequence a novel strategy integrating a particular code-based design environment with newly selected testing strategies is presented in this pape

    Multistage Switching Architectures for Software Routers

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    Software routers based on personal computer (PC) architectures are becoming an important alternative to proprietary and expensive network devices. However, software routers suffer from many limitations of the PC architecture, including, among others, limited bus and central processing unit (CPU) bandwidth, high memory access latency, limited scalability in terms of number of network interface cards, and lack of resilience mechanisms. Multistage PC-based architectures can be an interesting alternative since they permit us to i) increase the performance of single software routers, ii) scale router size, iii) distribute packet manipulation and control functionality, iv) recover from single-component failures, and v) incrementally upgrade router performance. We propose a specific multistage architecture, exploiting PC-based routers as switching elements, to build a high-speed, largesize,scalable, and reliable software router. A small-scale prototype of the multistage router is currently up and running in our labs, and performance evaluation is under wa