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A Decomposition Method by Interaction Prediction for the Optimization of Maintenance Scheduling
No full textInternational audienceOptimizing maintenance scheduling is a major issue to improve the performance of hydropower plants. We study a system of several physical components of the same family: either a set of turbines, a set of transformers or a set of generators. The components share a common stock of spare parts and experience random failures that occur according to known failure distributions. We seek a deterministic preventive maintenance strategy that minimizes an expected cost depending on maintenance and forced outages of the system. The Auxiliary Problem Principle is used to decompose the original large-scale optimization problem into a sequence of independent subproblems of smaller dimension while ensuring their coordination. Each subproblem consists in optimizing the maintenance on a single component. Decomposition-coordination techniques are based on variational techniques but the maintenance optimization problem is a mixed-integer problem. Therefore, we relax the dynamics and the cost functions of the system. The resulting algorithm iteratively solves the subproblems on the relaxed system with a blackbox method and coordinates the components. Relaxation parameters have an important influence on the optimization and must be appropriately chosen. An admissible maintenance strategy is then derived from the resolution of the relaxed problem. We apply the decomposition algorithm on a system with 80 components. It outperforms the reference blackbox method applied directly on the original problem
Local transparent boundary conditions for wave propagation in fractal trees (ii): error and complexity analysis
No full textInternational audienceThis work is dedicated to a refined error analysis of the high-order transparent boundary conditions introduced in the companion work [8] for the weighted wave equation on a fractal tree. The construction of such boundary conditions relies on truncating the meromorphic series that represents the symbol of the Dirichlet-to-Neumann operator. The error induced by the truncation depends on the behaviour of the eigenvalues and the eigenfunctions of the weighted Laplacian on a self-similar metric tree. In this work we quantify this error by computing asymptotics of the eigenvalues and bounds for Neumann traces of the eigenfunctions. We prove the sharpness of the obtained bounds for a class of self-similar trees
A pressure-based diffuse interface method for low-Mach multiphase flows with mass transfer
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Analyse Mathématique du modèle de Goldstein pour l' acoustique harmonique dans un écoulement
No full textInternational audienceGoldstein’s equations have been introduced in 1978 as an alternative model to linearized Euler equations to model acoustic waves in moving fluids. This new model is particularly attractive since it appears as a perturbation of a simple scalar model: the potential model. In this work we propose a mathematical analysis of boundary value problems associated with Goldstein’s equations in the time-harmonic regime.Les équations de Goldstein ont été introduites in 1978 comme alternative aux équations d'Euler linéarisées, modélisant la propagation acoustique en écoulement. Ce nouveau modèle est particulièrement attractif car il apparaît comme une perturbation d'un modèle scalaire simple: le modèle potentiel. Dans ce travail, nous proposons une analyse mathématique des problèmes aux limites associés aux équations de Goldstein en régime harmonique
Axiparabola: a new tool for high-intensity optics
No full textInternational audienceAbstract An axiparabola is a reflective aspherical optics that focuses a light beam into an extended focal line. The light intensity and group velocity profiles along the focus are adjustable through the proper design. The on-axis light velocity can be controlled, for instance, by adding spatio-temporal couplings via chromatic optics on the incoming beam. Therefore the energy deposition along the axis can be either subluminal or superluminal as required in various applications. This article first explores how the axiparabola design defines its properties in the geometric optics approximation. Then the obtained description is considered in numerical simulations for two cases of interest for laser-plasma acceleration. We show that the axiparabola can be used either to generate a plasma waveguide to overcome diffraction or for driving a dephasingless wakefield accelerator
Investigation of shipborne GNSS ZTD retrieval processing parameters by simulation
No full textInternational audienceThe aim of this work is to study the impact of the processing parameterization on the estimation of the zenith total delay (ZTD) from a shipborne GNSS antenna measurement. For this purpose, we used a simplified observation model, and simulated a realistic configuration of measurements (ephemerids, troposphere, motion of the shipborne antenna). Different sources of error that could affect the measurement were also simulated. The impact of these errors was then evaluated on the estimation by Kalman filtering, using different parameterizations (multi-constellation, solution sampling, random walk process noise for the ZTD estimates, observation weighting, cut-off angle). As it could have been expected, low cut-off angle (in the range of 3 to 7 degrees) and multi-constellation provide more accurate results. The choice of the data weighting is shown to significantly impact the difference on the estimates, and the use of a square-root of sine function, or uniform weighting of elevation gives the most conclusive results. High value of random walk process noise for ZTD estimates should also be avoided. Globally, the accuracy of the ZTD estimation can be improved up to more than 90% according to the configuration.The results of this work will be helpful to set up an optimal parameterization for the processing of massive dataset of GNSS measurements acquired from shipborne antennas
Automatic Drones Detection using Micro Doppler Analysis, a Preliminary Modelling
No full textInternational audienceThe study described herein focuses on the automatic detection of drones utilizing micro Doppler effects. Drones are often employed for both military and commercial applications. As a result of their small size, reasonable flight speed, and low flying altitude, they are difficult to spot. Because of the increasing density of drone deployment in the near future, a comprehensive detection and monitoring system is essential for safety. Preliminary analytical study has been conducted in this regard to examine the micro Doppler signature of drones caused by their rotating blades. Following that, such micro Doppler effects were investigated for possible target detection. In this context, rotational blades are presumed to have flat, rectangular point scatter reflections. The relevant signal model was then created by integrating geometrical and analytical functions. In fact, Chen's model was used as the basis and was updated for multi-rotor, multi-target scenarios. Finally, for target detection, a detection algorithm based on the Doppler filter has been constructed. As a result, the findings can be seen below
Determination of the ion collection efficiency of the Razor Nano Chamber for ultra‐high dose‐rate electron beams
No full textInternational audienceBackground: Ultra-high dose-rate (UHDR) irradiations (> 50 Gy/s) have recently garnered interest in radiotherapy as they can trigger the so-called “FLASH” effect. To transfer this to clinical radiotherapy treatments, practical tools for online dosimetry need to be developed. Small volume size in ionization chambers may be an interesting characteristic to limit the charge recombination effects.Aims: In that context, we have investigated the charge collection behavior of the recent IBA RazorTM Nano Chamber (RNC) in UHDR pulses to evaluate its potential interest for FLASH radiotherapy.Methods: The RNC Ion Collection Efficiency (ICE) was quantified by performing simultaneous dose measurements under UHDR electron beams with dose-rate independent Gafchromic™ EBT3 films, that were used as the dose reference. A dose-per-pulse (DPP) range from 0.01 to 30 Gy and two pulse duration (1 and 3 µs) were investigated. An empirical logistic model was used to fit the RNC collection efficiency measurements and the results were compared to the Advanced Markus ion chamber. Results: The RNC ICE was found to decrease as the DPP increases, starting from doses above 0.2 Gy/pulse and down to 40% of efficiency at 30 Gy/pulse (Figure). The RNC resulted in a higher ICE for a given DPP in comparison to the Markus chamber, with a measured efficiency found higher than 55% for 10 Gy/pulse versus 25% for the Markus. However, the RNC shows a higher sensitivity to the pulse duration, with a lower efficiency found at 1 µs than at 3 µs, suggesting that this chamber could be more sensitive to the dose-rate within the pulse [1]. Conclusions: The results confirmed that the small sensitive volume of the RNC ensures higher ICE compared to larger chambers, which makes it attractive for online dosimetry of FLASH radiotherapy. [1] Cavallone et al., Med Phys. 2022;1–12. https://doi.org/10.1002/mp.1567
