Showing papers by "École normale supérieure de Cachan published in 2021"

Normal mode decomposition and dispersive and nonlinear mixing in stratified fluids

Abstract: Motivated by the analysis of the propagation of internal waves in a stratified ocean, we consider in this article the incompressible Euler equations with variable density in a flat strip, and we study the evolution of perturbations of the hydrostatic equilibrium corresponding to a stable vertical stratification of the density. We show the local well-posedness of the equations in this configuration and provide a detailed study of their linear approximation. Performing a modal decomposition according to a Sturm–Liouville problem associated with the background stratification, we show that the linear approximation can be described by a series of dispersive perturbations of linear wave equations. When the so-called Brunt–Vaisala frequency is not constant, we show that these equations are coupled, hereby exhibiting a phenomenon of dispersive mixing. We then consider more specifically shallow water configurations (when the horizontal scale is much larger than the depth); under the Boussinesq approximation (i.e., neglecting the density variations in the momentum equation), we provide a well-posedness theorem for which we are able to control the existence time in terms of the relevant physical scales. We can then extend the modal decomposition to the nonlinear case and exhibit a nonlinear mixing of different nature than the dispersive mixing mentioned above. Finally, we discuss some perspectives such as the sharp stratification limit that is expected to converge towards two-fluid systems.

Hybrid Excited Synchronous Machines

Abstract: Hybrid excited synchronous machines (HESM) combine permanent-magnet (PM) excitation and wound field (WF) excitation. The goal of hybrid excitation is to combine the advantages of PM excited machines and wound field synchronous machines. HESM have been identified as one of the emerging technologies for modern energy conversion systems. They have been the subject of many review papers. The principle of hybrid excitation allows solving many drawbacks related to permanent magnet electric machines operation: flux weakening, energy efficiency, and permanent magnets price fluctuation. It helps to introduce an additional degree of freedom in the design of synchronous machines, and allows therefore an easier adaptation of PM synchronous machines to a wider applications scope. To this additional degree corresponds the possibility of adjusting the contribution of the two magnetic field sources, PM and WF. The use of this technology for electric traction has been the subject of many scientific and technical contributions. In this contribution, the emphasis will be put on the use of these machines as generators in transportation applications and renewable energy applications. The design and operation of three particular structures will be presented. Two of them have been designed as generators for transportation applications, and the third one has been designed as generator for renewable energy conversion. All of them are flux switching hybrid excited synchronous structures (FSHESM).

A thermal and acoustic co-simulation method for the multi-domain optimization of nearly zero energy buildings

Abstract: In the last years, the Nearly Zero Energy Building (or NZEB) concept has become one of the main targets to drive the built environment towards a sustainable future. Advanced methodologies and tools have been developed to support the complex NZEB design problem considering multiple objectives and the renovated interest for building occupants’ wellbeing has highlighted the need for integrating energy and comfort aspects for a holistic vision of the future NZEB. This work intends to advance in the ongoing discussion about comfort-driven NZEB design and develop a new multi-tool multi-objective simulation-based optimization methodology to integrate energy, cost and acoustic performance in the search for the NZEB optimal design. The proposed approach relies on the coupling between an energy model in TRNSYS®, a sound insulation model developed in Matlab® and the PSO algorithm to drive the optimization through the GenOpt® tool. After theoretical elaboration of the so-defined optimization problem, the method was applied to a French single-family case study. The applied methodology identifies several non-obvious solutions that are near-optimal for all the three objectives and shows that the traditional cost-optimized NZEB design would lead to low sound insulation performance, revealing the potential for a full exploitation of synergies between thermal and acoustic domains in high-performance buildings, reinforcing the need for further investigation in the field.

Multiplex‐multiphoton microscopy and computational strategy for biomedical imaging

Abstract: We demonstrate the benefit of a novel laser strategy in multiphoton microscopy (MPM). The cheap, simple, and turn‐key supercontinuum laser system with its spectral shaping module, constitutes an ideal approach for the one‐shot microscopic imaging of many fluorophores without modification of the excitation parameters: central wavelength, spectral bandwidth, and average power. The polyvalence of the resulting multiplex‐multiphoton microscopy (M‐MPM) device is illustrated by images of many biomedical models from several origins (biological, medical, or vegetal), generated while keeping constant the spectral parameters of excitation. The resolution of the M‐MPM device is quantified by a procedure of point‐spread‐function (PSF) assessment led by an original, robust, and reliable computational approach FIGARO. The estimated values for the PSF width for our M‐MPM system are shown to be comparable to standard values found in optical microscopy. The simplification of the excitation system constitutes a significant instrumental progress in biomedical MPM, paving the way to the imaging of many fluorophores with a single shot of excitation without any modification of the lighting device.

On Flat Lossy Channel Machines.

Abstract: We show that reachability, repeated reachability, nontermination and unboundedness are NP-complete for Lossy Channel Machines that are flat, ie, with no nested cycles in the control graph The upper complexity bound relies on a fine analysis of iterations of lossy channel actions and uses compressed word techniques for efficiently reasoning with paths of exponential lengths The lower bounds already apply to acyclic or single-path machines

Optically Switched Dual-Wavelength Cavity Ring-Down Spectrometer for High-Precision Isotope Ratio Measurements of Methane δD in the Near Infrared.

Abstract: We report a spectrometer employing optically switched dual-wavelength cavity ring-down spectroscopy (OSDW-CRDS) for high-precision measurements of methane isotope ratios. A waveguide optical switch rapidly alternated between two wavelengths to detect absorption by two isotopologues using near-infrared CRDS. This approach alleviated common-mode noise that originated primarily from temperature and frequency fluctuations. We demonstrated the measurement of δD in natural abundance methane to a precision of 2.3 ‰, despite the lack of active temperature or frequency stabilization of the cavity. The ability of alternating OSDW-CRDS to improve the isotope precision in the absence of cavity stabilization were measured by comparing the Allan deviation with that obtained when frequency-stabilizing the cavity length. The system can be extended to a wide variety of applications such as isotope analysis of other species, kinetic isotope effects, ortho-para ratio measurements, and isomer abundance measurements. Furthermore, our technique can be extended to multiple isotope analysis or two species involved in kinetics studies through the use of multiport or high-speed optical switches, respectively.

Non-Darcian effect on double-diffusive natural convection inside aninclined square Dupuit-Darcy porous cavity under a magnetic field

Abstract: This paper presents a numerical study of a double diffusive convection in an inclined square porous cavity filled with an electrically conducting binary mixture. The upper and bottom walls are maintained at a constant temperatures and concentrations whereas the left and right walls are assumed to be adiabatic and impermeable. A uniform and tilted magnetic field is applied at an angle, γ, about the horizontal, it is obvious that this is related to the orientation of the magnetic force that can help or oppose the buoyant force. The Dupuit-Darcy flow model, which includes effects of the inertial parameter, with the Boussinesq approximation, energy and species transport equations are solved numerically using the classical finite difference method. Governing parameters of the problem under study are the thermal Rayleigh number, Rt, Hartmann number, Ha, Lewis number, Le, the buoyancy ratio, φ,inclination angle, Φ and tilting angle of the magnetic field, γ,. The numerical results are reported on the contours of streamline, temperature, and concentration and for the average Nusselt and Sherwood numbers for various parametric conditions. It is demonstrated that both the inertial effect parameter and the magnetic field, have a strong influence on the strength of the natural convection heat and mass transfer within the porous layer.

A topological method for finding invariant sets of continuous systems

Abstract: A usual way to find positive invariant sets of ordinary differential equations is to restrict the search to predefined finitely generated shapes, such as linear templates, or ellipsoids as in classical quadratic Lyapunov function based approaches. One then looks for generators or parameters for which the corresponding shape has the property that the flow of the ODE goes inwards on its border. But for non-linear systems, where the structure of invariant sets may be very complicated, such simple predefined shapes are generally not well suited. The present work proposes a more general approach based on a topological property, namely Wazewski’s property. Even for complicated non-linear dynamics, it is possible to successfully restrict the search for isolating blocks of simple shapes, that are bound to contain non-empty invariant sets. This approach generalizes the Lyapunov-like approaches, by allowing for inwards and outwards flow on the boundary of these shapes, with extra topological conditions. We developed and implemented an algorithm based on Wazewski’s property, SOS optimization and some extra combinatorial and algebraic properties, that shows very nice results on a number of classical polynomial dynamical systems.

Polynomial interrupt timed automata: Verification and expressiveness

Abstract: Interrupt Timed Automata ( ITA ) form a subclass of stopwatch automata where reachability and some variants of timed model checking are decidable even in presence of parameters. They are well suited to model and analyze real-time operating systems. Here we extend ITA with polynomial guards and updates, leading to the class of polynomial ITA ( PolITA ). We prove that reachability is decidable in 2EXPTIME on PolITA , using an adaptation of the cylindrical algebraic decomposition algorithm for the first-order theory of reals. We also obtain decidability for the model checking of a timed version of CTL and for reachability in several extensions of PolITA . In particular, compared to previous approaches, our procedure handles parameters and clocks in a unified way. We also study expressiveness questions for PolITA and show that PolITA are incomparable with stopwatch automata.

On the validation of a priori estimates of standard displacement uncertainties in T3-stereocorrelation

Abstract: Uncertainty quantification is crucial for any measurement technique. The present work aims at validating a priori estimates of displacement uncertainties. Images acquired prior to fourteen thermomechanical tests were analyzed via FE-based stereocorrelation to determine actual displacement uncertainties, which were compared to a priori estimates. For the studied experimental database, a very good agreement was observed between a priori and a posteriori estimates of standard displacement uncertainties.

Thermal Stability of Superconductors

Abstract: A stability criterion is worked out for the superconducting phase. The validity of a prerequisite, established previously for persistent currents, is thereby confirmed. Temperature dependence is given for the specific heat and concentration of superconducting electrons in the vicinity of the critical temperature Tc. The isotope effect, mediated by electron-phonon interaction and hyperfine coupling, is analyzed. Several experiments, intended at validating this analysis, are presented, including one giving access to the specific heat of high-Tc compounds.

SPAWN: An Iterative, Potentials-Based, Dynamic Scheduling and Partitioning Tool

Abstract: Many applications of physics modeling use regular meshes on which computations of highly variable cost over time can occur. Distributing the underlying cells over manycore architectures is a critical load balancing step that should be performed the less frequently possible. Graph partitioning tools are known to be very effective for such problems, but they exhibit scalability problems as the number of cores and the number of cells increase. We introduce a dynamic task scheduling and mesh partitioning approach inspired by physical particle interactions. Our method virtually moves cores over a 2D/3D mesh of tasks and uses a Voronoi domain decomposition to balance workload. Displacements of cores are the result of force computations using a carefully chosen pair potential. We evaluate our method against graph partitioning tools and existing task schedulers with a representative physical application, and demonstrate the relevance of our approach.

Determining the position of a single spin relative to a metallic nanowire

Abstract: The nanoscale localization of individual paramagnetic defects near an electrical circuit is an important step for realizing hybrid quantum devices with strong spin-microwave photon coupling Here, we fabricate an array of individual nitrogen vacancy (NV) centers in diamond near a metallic nanowire deposited on top of the substrate We determine the relative position of each NV center with ∼ 10 nm accuracy, using it as a vector magnetometer to measure the field generated by passing a DC through the wire

Computer-implementated method for identifying mechanical properties by coupled correlation of images and mechanical modelling

Abstract: The general field of the invention is that of computer-implemented methods for identifying mechanical parameters of an object subject to mechanical stress. The method according to the invention comprises a step of acquiring, by an imaging means, images of the object in question, before and during application of the mechanical stress, three calculation steps of the effects due to the stress produced, either cased on the modelling of recorded images, or based on a theoretical mechanical modelling of the stress, a step of defining a function equal to the difference between the two modellings, and a last step of minimising said function so that the experimental model is the closest possible to the theoretical mechanical model. Additional measures allowing the method according to the invention to be refined.

Numerical Study of the Effect of the Location of Baffles on the Flow and Heat Transfer of A Newtonian Fluid in A Ventilated Enclosure

Abstract: Flow and heat transfer analysis in ventilated cavities is one of the most widely studied problems in thermo-fluids area. Two-dimensional mixed convection in a ventilated rectangular cavity with baffles is studied numerically and the fluid considered in this study is hot air (Pr = 0.71). The horizontal walls are maintained at a constant temperature, higher than that imposed on the vertical ones. Two very thin heat-conducting baffles are inserted inside the enclosure, on its horizontal walls, to control the flow of convective fluid. The governing equations are discretized using the finite volume method and the SIMPLER algorithm to treat the coupling velocity–pressure. Line by line method is used to solve iteratively the algebraic equations. The effect of the Richardson number Ri (0.01- 100) and the location of the baffles within the cavity on the isothermal lines, streamlines distributions and the average Nusselt number (Nu) has been investigated. The result shows that the location opposite the baffles, close to the fluid outlet, is the optimal choice to be considered for industrial applications.

Experimental study on the evaporating progress of hexane lens on immiscible liquid：Spreading and receding

Abstract: The change of evaporation liquid on another immiscible liquid has important guiding significance for many applications. In this experiment, the geometric temperature distribution and evaporation rate of n-hexane droplets were observed and recorded by changing the temperature of deionized water. The results show that with the increase of temperature of deionized water-based solution, the maximum diameter of n-hexane droplet spreading after titration increases gradually, while the minimum diameter of n-hexane droplet disappearing decreases gradually. Meanwhile, the evaporation rate of n-hexane droplet is constant during the whole evaporation process. It should also be mentioned that if the base solution is changed from deionized water to a certain concentration of salt solution, the maximum diameter of n-hexane droplet spreading will be reduced, and the evaporation intensity will be relatively reduced. These experimental results will give us a better understanding of the mechanism and characteristics of droplet evaporation.

Study on The Effect of Different Plenum Chamber Coefficients on Frosting of Air-cooler

Abstract: The performance of the air-cooler in the refrigeration system plays a key role in improving the energy efficiency. Here, the plenum chamber coefficient was defined to study a possible way of enhancing the refrigeration system performance. In order to investigate the influence of the plenum chamber coefficient on the frosting and the cooling system of the chillers, three plenum chamber coefficients (0.74,0.97,1.2) were studied under high humidity. The temperature decreasing curve of the environmental chamber, the velocity distribution of the air cooler, and the frost accumulation under and on the air-cooler were analysed. The results show that the frost accumulation on the tube will increase with the increase of the plenum chamber coefficient and the frost accumulation on the fin will increase at first and then decrease with the increase of the plenum chamber coefficient. On the whole, in this study, frosting is the most obvious when the plenum chamber coefficient was 0.97