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Effect of frontal instabilities on underwater acoustic propagation : study of the Narval 2021 data
No full textInternational audienceThermohaline frontal areas can strongly influence acoustic propagation. Due to their dynamics, they are areas difficult to model. In this preliminary work, we propose to use Gaussian process regression to fit a simple structure model to in-situ data. This then allows to model possible samples of the sound-speed field around the Iceland-Faroe front. The Gaussian process framework helps to find the front position, and capture the large scale variability of the area.We then compare ray-tracing simulations on different soundspeed fields, including Gaussian process samples, to in-situ acoustic data. We show that for some source-receiver configurations, range-dependent simulations using the Gaussian process model show the best fit to real data. The structure model lacks mesoscale and sub-mesoscale contributions, which have a strong acoustic impact, but is a good basis for more advanced environmental modelling.</div
Étude et modélisation des propriétés en fatigue à très grand nombre de cycles à partir de mesures d’auto-échauffement sous sollicitation cyclique
No full textMaterial fatigue is the main cause of failure of parts in service. Taking it into account is therefore essential when dimensioning a system. Fatigue testing is a simple but time-consuming way of obtaining a Wöhler curve. Because of the limited load frequency of conventional machines, it used to be assumed that a material had an endurance limit at 107 cycles. However, some systems in service are subjected to up to 1010 cycles, due to low-amplitude vibrations for example, and show signs of failure. The appearence of ultrasonic fatigue machines operating at 20 kHz in the middle of the 20th century made it possible to study these ranges of service life. The aim of this study is to investigate the value of thermometric measurements for understanding VHCF fatigue behavior.In order to calculate the dissipation of our material in an ultrasonic configuration, a 1D analysis protocol was proposed and validated. This protocol enabled us to obtain a self-heating curve at very high frequencies, and to study the effect of frequency on the material's thermal response. A bimodal fatigue model was then proposed, based on a core/skin distinction of our specimens and an energy criterion. This predictive model uses solely the self-heating curve and material parameters. High-frequency fatigue tests validated the model on ML340 steel up to 1010 cycles which has a bimodal fatigue behaviour.La fatigue des matériaux est le facteur principal de rupture de pièces en service. Sa prise en compte est donc primordiale dans le dimensionnement d’un système. La réalisation d’essais de fatigue est un moyen simple mais chronophage d’obtenir une courbe de Wöhler. A cause de la fréquence de sollicitation limitée des machines conventionnelles, il était considéré qu’un matériau avait une limite d’endurance à 107 cycles. Toutefois, certains systèmes en services subissent jusqu’à 1010 cycles, à cause de vibrations de faibles amplitudes par exemple, et présentent des ruptures. L’apparition des machines de fatigues ultrasoniques sollicitant à 20 kHz au milieu du 20ème siècle a rendu possible l’étude de ces gammes de durées de vie. L’objectif de cette étude est d’étudier l’intérêt des mesures thermométriques pour la compréhension du comportement en fatigue VHCF. Afin de pouvoir calculer la dissipation de notre matériau dans une configuration ultrasonique, un protocole de dépouillement 1D a été proposé puis validé. Ce protocole a permis de pouvoir obtenir une courbe d’auto-échauffement à très haute fréquence et d’étudier l’effet de la fréquence sur la réponse thermique du matériau. Un modèle de fatigue bimodale a ensuite été proposé à l’aide d’une distinction coeur/peau de nos éprouvettes et d’un critère énergétique. Ce modèle prédictif est basé uniquement sur la courbe d’auto-échauffement et des paramètres matériaux. Des essais de fatigue à très haute fréquence ont permis de valider ce modèle sur l’acier ML340 jusqu’à 1010 cycles qui présente un comportement bimodal
Transversely isotropic hyperelastoplastic constitutive model for large deformation analysis and simulation of fiber yarns and stranded ropes
No full textInternational audienceThe mechanical behavior of fibrous structures, such as composite reinforcements and synthetic ropes, is characterized by large deformations and complex anisotropic responses. Traditional models, predominantly based on elastic formulations, fail to adequately capture the deformation history and dissipative mechanisms observed in these systems. This study introduces a novel hyperelastoplastic framework tailored for fibrous media, incorporating a new family of invariants that account for the deformation history in the model, beyond the limitations of classical invariants. These advanced invariants enable a more accurate description of the mechanical response under large deformation, particularly in matrix-free mesoscopic architectures where fiber sliding, friction, and transverse compression dominate. The proposed approach is demonstrated to be equally applicable to the mesoscopic modeling of composite reinforcements and stranded ropes, offering improved predictive capabilities for their structural behavior and long-term performance under complex loading conditions
Study and simulation of micro-Doppler signature in radar theory: application to aircraft
No full textInternational audienceFor several decades, aircraft have been extensively used in both civil and military applications. Currently, we are witnessing a proliferation of Unmanned Aerial Vehicles (UAVs) with various shapes in both civil and military domains for different purposes. For example, drones can be used in the production of cinema movies as well as for precise offensive strikes on the battlefield. However, these UAVs are usually smaller than modern fighter aircraft and have a very low Radar Cross Section (RCS), which prevents radars from reliably detecting them. This implies significant security issues, as inexpensive drones can be used for area surveillance or offensive tactics. Such a threat has led governments, like those in Europe in 2019, to enact new laws to curb the increasing utilization of drones.Nevertheless, it could be possible to enhance radar capability to detect and identify drones using the micro-Doppler effect, as many UAVs use propellers to move. Therefore, our problem lies in how to improve radars' capability to detect and identify drones in various environments (sea, urban area, forests, etc.) using the micro-Doppler signatures of targets. In this perspective, we focus on the detailed modeling and simulation of the micro-Doppler effect produced by drone-type targets and the characterization of the simulated radar signal based on time–frequency representations.A few simulations have been carried out using physical optics methods and with the software FEKO in various simplified configurations. Preliminary results show that characteristic patterns appear in time–frequency representations depending on the parameters of the drone system (tilt, relative speed, blade rotation speed, blade dimensions, etc.) that could help in the detection and identification of drone-type targets
Implémentation d’un contrôleur non-linéaire dans la méthode Boucle de Phase Asservie (PLL) pour l’identification expérimentale de structures non-linéaires
No full textInternational audienceExperimental continuation methods are used to retrieve and identify nonlinear characteristics of vibrating structures. Among the available methods, Phase-Locked Loop (PLL) allows for an easy-to-implement yet efficient method to continue nonlinear solutions such as backbone curves or frequency response functions. The PLL automatically locks onto the prescribed phase and thanks to a linear (proportional-integral) controller, can stabilize unstable periodic orbits. However, the tuning of the different parameters to be used in such a loop are seldomly documented in the literature, which in turn might lead to long duration tests. To ease the tuning effort and reduce the experimenting time, a nonlinear controller is here proposed as a way to improve the efficacy of Phase-Locked Loop testing. Thanks to the proposed design, named NCPLL (Nonlinear Controller PLL), most of the parameters are tuned easily, while a rapid locking to the prescribed state is at hand. The nonlinear gain can be easily adapted to reach a locked state rapidly. The efficacy of the NCPLL is first demonstrated on simple numerical examples including nonlinear oscillators with smooth restoring forces and Coulomb friction, and a finite element beam model with localized nonlinearities. Then the method is deployed on two different experimental test rigs. First, the case of smooth nonlinearity is tackled thanks to a cantilever beam vibrating in the magnetic field created by two magnets. Finally, the case of friction is addressed by considering an assembled beam with friction joints. In all the tested cases, the NCPLL shows excellent performance, requiring minimal tuning efforts whilst leading to fast measurements.Les méthodes de continuation expérimentales sont utilisées pour extraire et identifier les caractéristiques non linéaires des structures vibrantes. Parmi les méthodes disponibles, la méthode de Boucle de Phase Asservie (Phase-Locked Loop - PLL) constitue une approche simple à mettre en œuvre et efficace pour poursuivre des solutions non linéaires telles que des "backbone curves" ou des fonctions de réponse en fréquence, étendant ainsi le champ d’application des méthodes de résonance de phase (PRM). La PLL se verrouille automatiquement sur la phase prescrite et, grâce à un contrôleur linéaire (proportionnel-intégral), elle peut stabiliser des orbites périodiques instables. Cependant, le réglage des différents paramètres à utiliser dans une telle boucle est rarement documenté dans la littérature, ce qui peut entraîner des essais de longue durée. Afin de faciliter le réglage et de réduire le temps d’expérimentation, un contrôleur non linéaire est proposé ici pour améliorer l’efficacité des tests basés sur la PLL. Grâce à ce design, nommée NCPLL (Nonlinear Controller PLL), le nombre de paramètres à ajuster est réduit, tout en assurant un verrouillage rapide sur l’état prescrit. Une borne supérieure pour le gain du contrôleur non linéaire est déduite à partir de développements théoriques. L’efficacité du NCPLL est d’abord démontrée sur des exemples numériques simples, incluant des oscillateurs non linéaires avec des forces de rappel régulières et des forces de frottement de type Coulomb, ainsi qu’un modèle éléments finis de poutre avec des non-linéarités localisées. Ensuite, la méthode est appliquée à deux bancs d’essais expérimentaux distincts. Dans un premier temps, le cas d’une non-linéarité régulière est étudié avec une poutre cantilever vibrant dans le champ magnétique créé par deux aimants. Enfin, le cas du frottement est abordé à travers une poutre assemblée comportant des joints frottants. Dans tous les cas testés, le NCPLL démontre d’excellentes performances, nécessitant un réglage minimal tout en permettant des mesures rapides
Linking deterministic and stochastic aspects of the seafloor's acoustic response in the context of seabed mapping with echosounders
No full textInternational audienceIn the different contexts where the monostatic acoustic seafloor response (ASR) is employed (i.e., measurements with multibeam or singlebeam echosounders, applications in seabed mapping, theoretical models with physical or heuristic parameters), its intrinsic nature can be ambiguous. In this article, an analysis of the discrepancies in semantics is proposed, and then the ASR theoretical and physical definitions are recalled. In theoretical models, and while it is acquired at sea, the ASR is shown to be considered as a deterministic parameter. However, a large variability in its measurements with echosounders is observed, suggesting ASR to be physically a stochastic variable. These two seemingly contradictory observations are linked analytically based on a point scattering model. The theoretical and deterministic ASR is related to its stochastic measurements as the expected value. Based on this connection and under the hypotheses of seafloor homogeneity and large insonified areas, the monostatic ASR measured by echosounders (i.e., the backscattering strength) is derived as twice the parameter of the Rayleigh distribution. Relationships between this result and seabed characteristics are discussed, along with their implications for echosounder operations
A Fast Wavelet-Based Hybrid Method to Model the RCS of Metallic Targets in Maritime Environment
No full textInternational audienceThis article focuses on a hybrid wavelet-based model to compute the radar cross section (RCS) of metallic targets in a maritime environment. The latter is based on the parabolic wave equation, solved in the wavelet domain, for the propagation to the target and the wavelet-based method of moments for the scattered field. Key advantages include its ability to account for terrain relief and refraction effects within the propagation channel, as well as providing accurate RCS calculations. Additionally, wavelet-domain compression enables a flexible trade-off between computational efficiency and precision. Numerical experiments in the VHF-band are performed to validate the method
Numerical hydrodynamic characterization for the design of a lab-scale jet-loop reactor
No full textInternational audienceThe jet-loop reactor is a powerful tool for analyzing the kinetics of heterogeneous catalytic reactions due to its high mixing degree. Indeed, the high momentum gas flow injected within the loop induces a large gas recycling flow, minimizing concentration and temperature gradients. The purpose of this numerical study is to optimize the geometric features of the reactor (injection nozzle diameters and length, and outlet pipe diameter) to improve the gas recycle ratio. Its hydrodynamic behavior is predicted by using multi-fluid solver, with an immersed boundary method to model the injector and easily vary its geometry. The effects of the operating parameters such as the injection flowrate, the pressure and the temperature are also assessed. In addition, a residence time distribution analysis allows for the evaluation of the Péclet number as a function of the geometric and operating parameters of the reactor. Then, a zero-dimensional hydrodynamic model, based on a macroscopic momentum balance, is finally developed. After fitting specific terms thanks to separated numerical CFD simulations, it enables a rapid optimization of the reactor design and provides insights into its behavior
Enhanced Cement Foam Composite with Biochar for Eriochrome Black T Dye Removal
No full textCement-based foam composites have gained attention as innovative and high-performing adsorbents for wastewater treatment due to their lightweight, porous, and structurally robust properties. This study investigates the adsorption of Eriochrome Black T dye onto biochar-modified cement foam, providing a cost-effective solution for industrial wastewater management. The integration of biochar into cement foam enhances its surface area and adsorption capabilities while maintaining structural stability and tunable porosity. The composites were characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy to verify quality and functionality. The adsorption process adhered to the Freundlich isotherm model (R2 = 0.967), indicating multilayer adsorption, with a maximum capacity of 13.33 mg/g under optimal conditions. Kinetic studies showed a pseudo-first-order fit (R2 = 0.981), while thermodynamic analysis revealed a spontaneous and endothermic process, with Delta H degrees = 28.84 KJ/mol and Delta G degrees values ranging from -0.457 to -2.36 KJ/mol. These results demonstrate the composite's exceptional efficiency and scalability, making it a sustainable and practical option for removing persistent dyes such as Eriochrome Black T. This work contributes significantly to the advancement of environmentally friendly wastewater treatment technologies
Calibration of non-local damage models from full-field measurements: Application to discrete element fields
No full textInternational audienceContinuous damage models are increasingly used in numerical simulations to design structures, but their local formulations are sensitive to mesh size and present localization of strains in an infinitely thin region. To overcome these problems non-local damage models and related regularization methods, have been developed introducing a characteristic/internal length, thus avoiding pathological mesh dependence. Those methods make the damage evolution depends on the mechanical quantities at the current material point (local) and its neighborhood (non-local).Existing approaches to calibrate the internal length use global quantities in the calibration process, although local data is now becoming accessible (e.g., using digital image correlation). In this study, we investigate the use of full-field displacement measurements and propose a new methodology for calibrating a non-local damage model based on local field measurements and we apply it to calibrate the Eikonal Non-local Gradient (ENL-G) approach from the measured strain and damage field. After detailing the calibration procedure, we then apply it on a simple ideal case. We illustrate, and analyze the robustness of the calibration procedure with respect to the choice of evolution law and measurement noise of the proposed calibration method. To confront the procedure to a more realistic case, we employed a 2D beam-particle model. This discrete model is first identified with respect to the size and shape effect based on one of the comprehensive experimental data sets available in the literature, including four shapes with three sizes each. Then, it is used to generate a “reference” evolution of the damage and strain fields in beams of different sizes subjected to uniaxial tension.The parameters of the discrete model used have been calibrated to represent the scale and size effects, giving a very good representation of the experiments. We also illustrate the evolution of non-local interactions in the Eikonal approach using Green functions. Finally, the application of the calibration procedure shows that it is possible to determine the internal length of the non-local problem studied as well as the damage evolution law and its parameters. The outcomes of this study contribute to shed light on a new methodology to identify non-local damage models based on full-field measurements, and call for experimental size effect campaign with displacement field