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On the loss of symmetry in toughness dominated hydraulic fractures
No full textInternational audienceFracking, or hydraulic fracturing, is a ubiquitous technique for generating fracture networks in rocks for enhanced geothermal systems or hydrocarbon extraction from shales. For decades, models, numerical simulation tools, and practical guidelines have been based on the assumption that this process generates networks of self-similar parallel cracks. Yet, some field and laboratory observations show asymmetric crack growth, and material heterogeneity is routinely attributed for it. Here, we show that simultaneous growth of multiple parallel cracks is impossible and that a single crack typically propagates asymmetrically in toughness dominated hydraulic fracturing, in which viscous dissipation of the fluid is negligible. In other words, loss of symmetry is a fundamental feature of hydraulic fracturing in a toughness dominated regime and not necessary the result of material heterogeneities. Our findings challenge the assumptions of symmetrical growth of hydraulic fractures commonly made in practice, and point to yet another instability other than material heterogeneity
A multimodal Parkinson quantification by fusing eye and gait motion patterns, using covariance descriptors, from non-invasive computer vision
No full textInternational audienceBackground and objective: Parkinson’s disease (PD) is a motor neurodegenerative disease principally manifested by motor disabilities, such as postural instability, bradykinesia, tremor, and stiffness. In clinical practice, there exist several diagnostic rating scales that coarsely allow the measurement, characterization and classification of disease progression. These scales, however, are only based on strong changes in kinematic patterns, and the classification remains subjective, depending on the expertise of physicians. In addition, even for experts, disease analysis based on independent classical motor patterns lacks sufficient sensitivity to establish disease progression. Consequently, the disease diagnosis, stage, and progression could be affected by misinterpretations that lead to incorrect or inefficient treatment plans. This work introduces a multimodal non-invasive strategy based on video descriptors that integrate patterns from gait and eye fixation modalities to assist PD quantification and to support the diagnosis and follow-up of the patient. The multimodal representation is achieved from a compact covariance descriptor that characterizes postural and time changes of both information sources to improve disease classification.Methods: A multimodal approach is introduced as a computational method to capture movement abnormalities associated with PD. Two modalities (gait and eye fixation) are recorded in markerless video sequences. Then, each modality sequence is represented, at each frame, by primitive features composed of (1) kinematic measures extracted from a dense optical flow, and (2) deep features extracted from a convolutional network. The spatial distributions of these characteristics are compactly coded in covariance matrices, making it possible to map each particular dynamic in a Riemannian manifold. The temporal mean covariance is then computed and submitted to a supervised Random Forest algorithm to obtain a disease prediction for a particular patient. The fusion of the covariance descriptors and eye movements integrating deep and kinematic features is evaluated to assess their contribution to disease quantification and prediction. In particular, in this study, the gait quantification is associated with typical patterns observed by the specialist, while ocular fixation, associated with early disease characterization, complements the analysis.Results: In a study conducted with 13 control subjects and 13 PD patients, the fusion of gait and ocular fixation, integrating deep and kinematic features, achieved an average accuracy of 100% for early and late fusion. The classification probabilities show high confidence in the prediction diagnosis, the control subjects probabilities being lower than 0.27 with early fusion and 0.3 with late fusion, and those of the PD patients, being higher than 0.62 with early fusion and 0.51 with late fusion. Furthermore, it is observed that higher probability outputs are correlated with more advanced stages of the disease, according to the H&Y scale.Conclusions: A novel approach for fusing motion modalities captured in markerless video sequences was introduced. This multimodal integration had a remarkable discrimination performance in a study conducted with PD and control patients. The representation of compact covariance descriptors from kinematic and deep features suggests that the proposed strategy is a potential tool to support diagnosis and subsequent monitoring of the disease. During fusion it was observed that devoting major attention to eye fixational patterns may contribute to a better quantification of the disease, especially at stage 2
The Underwater Vision Profiler 6: an imaging sensor of particle size spectra and plankton, for autonomous and cabled platforms
No full textInternational audienceAutonomous and cabled platforms are revolutionizing our understanding of ocean systems by providing 4D monitoring of the water column, thus going beyond the reach of ship-based surveys and increasing the depth of remotely sensed observations. However, very few commercially available sensors for such platforms are capable of monitoring large particulate matter (100-2000 μm) and plankton despite their important roles in the biological carbon pump and as trophic links from phytoplankton to fish. Here, we provide details of a new, commercially available scientific camera-based particle counter, specifically designed to be deployed on autonomous and cabled platforms: the Underwater Vision Profiler 6 (UVP6). Indeed, the UVP6 camera-and-lighting and processing system, while small in size and requiring low power, provides data of quality comparable to that of previous much larger UVPs deployed from ships. We detail the UVP6 camera settings, its performance when acquiring data on aquatic particles and plankton, their quality control, analysis of its recordings, and streaming from in situ acquisition to users. In addition, we explain how the UVP6 has already been integrated into platforms such as BGC-Argo floats, gliders and long-term mooring systems (autonomous platforms). Finally, we use results from actual deployments to illustrate how UVP6 data can contribute to addressing longstanding questions in marine science, and also suggest new avenues that can be explored using UVP6-equipped autonomous platforms
Second and third harmonic generation from simultaneous high peak- and high average-power thin disk laser
No full textInternational audienceWe report Second Harmonic Generation (SHG) and Third Harmonic Generation (THG) energy conversion efficiencies up to 59% and 27%, respectively, for laser pulses simultaneously delivering high peak power in the sub-TW range and average powers in the sub-kW range. No damage or efficiency decrease is observed after more than 100 h operation time. The resulting high-energy visible and near-UV pulses are suitable for applications, such as lightning control, material analysis and machining, or OPCPA pumping
Finding the polygon hull of a network without conditions on the starting vertex
No full textInternational audienceMany real‐life problems arising within the fields of wireless communication, image processing, combinatorial optimization, etc, can be modeled by means of Euclidean graphs. In the case of wireless sensor networks, the overall topology of the graph is not known because sensor nodes are often randomly deployed. One of the significant problems in this field is the search for boundary nodes. This problem is important in cases such as the surveillance of an area of interest, image contour reconstruction, graph matching problems, routing or clustering data, etc. In the literature, many algorithms are proposed to solve this problem, a recent one of which is the least polar‐angle connected node (LPCN) algorithm and its distributed version D‐LPCN, which are both based on the concept of a polar angle visit. An inconvenience of these algorithms is the determination of the starting vertex. In effect, the point with the minimum x‐coordinate is a possible starting point, but it has to be known at the beginning, which considerably increases the algorithms' complexity. In this article, we propose a new method called RRLPCN (reset and restart with least polar‐angle connected node), which is based on the LPCN algorithm to find the boundary vertices of a Euclidean graph. The main idea is to start the LPCN algorithm from an arbitrary vertex, and whenever it finds a vertex with an x‐coordinate smaller than that of the starting one, LPCN is reset and restarted from this new vertex. The algorithm stops as soon as it visits the same edge for the second time in the same direction. In addition to finding the boundary vertices, RRLPCN also finds the vertex with minimum x‐coordinate, which is the last starting point of our algorithm. The distributed version of the proposed algorithm, called D‐RRLPCN, is then applied to boundary node detection in the wireless sensor network. It has been implemented using real sensor nodes (Arduino/XBee and TelosB). The simulation results have shown our algorithm to be very effective in comparison to other algorithms
Caractérisation thermomécanique de laminés en fibres de carbone unidirectionnelles/résine époxy sous chargement cyclique de compression
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Scouring of a granular bed by dam break: Experimental study and numerical simulation by a VOF-LPT coupling
No full textInternational audienceThe first part of the research reported here consists of an experimental campaign to study the scouring of a granular bed (glass beads, sand) induced by a dam break in an open channel. Two configurations are considered: with and without cylinders. In the second part of this study, the volume of fluid method coupled with the shear stress transport turbulent model and the lagrangian particle tracking method is used to simulate the local scour processes. The four-way coupling is realized by considering the drag force and collisions between particles. Using a vortex map and velocity profiles, the flow dynamics and scour pattern can be studied. The current research also focuses on the profiles of the erodible particle bed and the sediment transport rate to study the effects of a cylinder on local scour. The presence of the cylinder increases the experimental erosion depth by up to 42%. It was found that for relatively close particle diameters (2 and 3 mm), a granular bed with larger porosity (larger diameter) erodes the most. Experimentally and numerically it was found that the center of the granular bed experiences more erosion compared to the channel sides. In the last part of the research, comparisons between the numerical results and the experimental data allowed risk areas to be identified and the roles of porosity, sediment density, and flow dynamics on erosion to be identified
Suivi cinétique de la cristallisation des hydrates de CO2 par mesure thermique – Effet de l’agitation
No full textNational audienceLes coulis d'hydrates de CO2 constituent un matériau à changement de phase prometteur pour la distribution du froid en réfrigération secondaire [1], en raison de leur chaleur latente élevée (environ 500 kJ.kg-1 d'eau, supérieure à celle de la glace: 333 kJ.kg-1) et de leur large plage de température de fusion adaptée aux applications de climatisation. Ce sont des structures cristallines proche de la glace, au sein desquelles sont enclathrées des molécules de dioxyde de carbone. Si les propriétés thermodynamiques des hydrates de CO2 sont désormais bien établies, la connaissance des phénomènes cinétiques de cristallisation reste parcellaire [2]. Comprendre et contrôler la formation des hydrates de gaz est un défi depuis leur découverte dans les bouchons de pipeline par Hammerschmidt [3]. Mais contrairement à ce cas, où l'on cherche à retarder l'apparition des hydrates, l'objectif ici est de favoriser leur cristallisation. La plupart des réacteurs dans lesquels est étudiée la cristallisation des hydrates à l'échelle du laboratoire sont équipés d'enregistreurs de pression et de température, et c'est à partir d'un bilan matière sur le CO2, en faisant des hypothèses sur la concentration de CO2 en phase liquide et le nombre d'hydratation, que le suivi de la cinétique est réalisé [4]. L'objet des travaux présentés ici est l'étude de la cinétique de cristallisation des hydrates de CO2 pour différents types d'agitateurs et différentes conditions de vitesse d'agitation dans un réacteur batch agité à double enveloppe. Le bilan thermique est obtenu expérimentalement en utilisant un capteur spécialement développé et installé sur la double enveloppe du réacteur. La fraction massique cristallisée est déterminée directement à partir du bilan thermique sur la double-enveloppe. Des essais sur les étapes de chauffage et de refroidissement de l'eau et la cristallisation de la glace ont permis de valider la méthode avant de l'appliquer à la cristallisation des hydrates de CO2. Les résultats expérimentaux sont décrits à partir d'un modèle empirique et comparés à d'autres déterminations cinétiques basées sur le bilan de matière. Enfin, le développement d'un modèle thermique simplifié du réacteur a permis d'améliorer cette détermination cinétique en estimant les flux de chaleur au sein du réacteur et avec l'environnement
Free-Surface Effects on Two-Dimensional Hydrofoils by RANS-VOF Simulations
No full textInternational audienceFoiling yachts and crafts are both very sensitive to the flying height in terms of stability and performance, raising the scientific issue of the influence of the free-surface when the foil is at low submergence. This work presents numerical simulations of a 2D hydrofoil section NACA0012 at 5° angle of attack in the vicinity of the free surface, for different values of the submergence depth, for a chord-based Froude number of 0.571 and a Reynolds number of 159,000. URANS equations are solved with a mixture model to capture the free surface, and using an automatic grid refinement. Verification of the numerical model and validation with data from the literature are presented. Deformation of the free surface and alteration of the hydrodynamic forces compared to the deep immersion case are observed for a submergence depth-to-chord ratio h/c lower than 2. The foil drag increases up to more than three times the infinite-depth value at h/c ≈ 0.5. The lift force slightly increases until h/c around 1, and then decreases sharply. For h/c < 0.5, the pressure field around the foil is totally modified and the lift is swapped to downward. The study highlights the importance of considering the effect of finite submergence to compute foils’ hydrodynamic forces, for example to be used in Velocity Prediction Programs (VPP) of foiling crafts
