Approches simultanées et séquentielles de la mise en correspondance par propagation

National audienceLa mise en correspondance de pixels est une étape importante de la reconstruction 3D. Parmi les méthodes existantes, nous nous intéressons plus particulièrement à celles par propagation de germes qui s'appuient sur un ensemble d'appariements fiables (germes). Le principe utilisé pour la propagation est que le correspondant d'un voisin d'un germe est recherché au voisinage du correspondant de ce germe. Il existe deux types d'approches pour ces méthodes : l'approche séquentielle et l'approche simultanée. Dans cet article, nous proposons un état de l'art de ces méthodes ainsi qu'une amélioration de l'approche simultanée. Notre méthode se déroule en deux phases faisant intervenir deux mesures de corrélation dont une est robuste aux occultations dans les zones proches des discontinuités de profondeur. Nous proposons également une étude comparative des différentes approches

An SPH study on viscoplastic surges overriding mobile beds: The many regimes of entrainment

Flow-type landslides entrain mobile bed material, but the processes involved are diverse and require systematic study. We perform direct numerical simulations using the open-source SPH package DualSPHysics with a regularized Herschel–Bulkley rheology. We then compare model output with physical test data, and hence investigate the effects of varying the bed yield stress $_{,b}$ and bed depth ℎ$_b$, interpreting the results using a newly-identified set of dimensionless numbers. Results reveal diverse interaction regimes between surges and mobile beds, including ‘‘rigid bed’’, ‘‘lubrication’’, ‘‘shallow ploughing’’, ‘‘surfing’’, ‘‘plunging’’, and ‘‘deep ploughing’’. Shallow, borderline-stable beds ‘‘lubricate’’ the surge: once destabilized, these beds cause strong acceleration of the combined flow front. Deeper borderline-stable beds allow the surge material to ‘‘plunge’’ downward, massively displacing bed material upward and downstream. For stabler beds, ‘‘ploughing’’ and ‘‘surfing’’ are associated with intermediate and high values of $_{,b}$, respectively. In both cases, beds retard the surge, with mobile dams forming for ‘‘ploughing’’ regimes. Across all regimes identified, the influence of $_{,b}$ is non-monotonic, with intermediate values decelerating the combined flow fronts the most. Furthermore, the different interaction regimes exhibit unique velocity profiles. We develop phase diagrams based on three dimensionless numbers, demarcating these regimes

Tensorial rheological model for concentrated non-colloidal suspensions: normal-stress differences

Most existing rheological models for non-colloidal suspensions fail to simultaneously capture the two main non-Newtonian trends of these systems, namely finite normal stress differences and transient effects. We address this issue by extending a previously-proposed minimal model accounting for microstructure anisotropy through a conformation tensor, and which was shown to correctly predict transient effects (Ozenda et al. 2018). The new model is compared to a large experimental dataset involving varying volume fractions, from dilute to concentrated cases. Both transient evolution of apparent viscosity during shear reversal, and normal stress differences in steady state, are quantitatively reproduced in the whole range of volume fraction. Furthermore, the model is validated against particle pressure measurements that were not used for parameter identification. Even if the proposed constitutive equation for the Cauchy stress tensor is more difficult to interpret than in the minimal model, this study opens way for the use of conformation tensor rheological models in applications where the effect of the second normal stress difference is prominent, like elongational flows or migration phenomenon

Modèles de couche mince pour les fluides à seuil : formulations alternatives et confrontation avec des résultats expérimentaux

Nous présentons des résultats expérimentaux originaux concernant la forme et la dynamique interne de coulées gravitaires à surface libre de fluides viscoplastiques. Ces résultats sont ensuite exploités dans le but de tester et améliorer les modèles hydrauliques de type couche mince développés pour simuler ces écoulements. Nous discutons notamment la validité des relations de fermeture classiquement utilisées pour les fluides à seuil et la contribution de termes additionnels prédits par des modèles de nouvelle génération développés récemment

Mass exchange between geophysical flows and beds: Idealised computational modelling using a Herschel-Bulkley rheology

A key mechanism by which geophysical flows evolve is mass exchange with the underlying bed, either by entraining material from the bed, or by depositing material. Although it is known that some consequences of these mass exchange processes include changes in the volume, momentum and local rheology of the flow, the circumstances under which specific changes occur are not well-established. Given the enormous number of competing mechanisms present in geophysical flows, it is not surprising that the state of the art for modelling entrainment is essentially still empirical. In this study, we implement a Herschel-Bulkley (non-Newtonian) rheology into an existing open-source Smoothed Particle Hydrodynamics solver (DualSPHysics). This rheology can reasonably represent clay-rich flows, typical of those observed in the French Prealps. We hence undertake a highly-idealised, quantitative investigation of entrainment mechanisms for flows overriding non-fixed beds. For the beds, we vary the yield stress and the depth. Preliminary results reveal a rich variety of behaviours that can be obtained for different bed properties, including both acceleration and deceleration of the flow material. These mechanisms are reminiscent (but not identical) of observations from other studies where geo-materials were used

Fracture et déclenchement des avalanches de plaque : conditions critiques pour la propagation de la fracture

International audienceThe failure of a weak snow layer buried below cohesive slab layers is a necessary, but insufficient, condition for the release of a dry-snow slab avalanche. The size of the crack in the weak layer must also exceed a critical length to propagate across a slope. In contrast to pioneering shear-based approaches, recent developments account for weak layer collapse and allow for better explaining typical observations of remote triggering from low-angle terrain. However, these new models predict a critical length for crack propagation that is almost independent of slope angle, a rather surprising and counterintuitive result. Based on discrete element simulations we propose a new analytical expression for the critical crack length. This new model reconciles past approaches by considering for the first time the complex interplay between slab elasticity and the mechanical behavior of the weak layer including its structural collapse. The crack begins to propagate when the stress induced by slab loading and deformation at the crack tip exceeds the limit given by the failure envelope of the weak layer. The model can reproduce crack propagation on low-angle terrain and the decrease in critical length with increasing slope angle as modeled in numerical experiments. The good agreement of our new model with extensive field data and the ease of implementation in the snow cover model SNOWPACK opens a promising prospect for improving avalanche forecasting

Prédétermination des hauteurs de départ d'avalanches. Modélisation combinée statistique-mécanique

La prédétermination de la hauteur de départ des avalanches représente un défi majeur pour l'évaluation du risque en montagne. Cette hauteur constitue en effet un ingrédient d'entrée important des procédures de zonage et de cartographie du risque. Dans cette thèse, nous présentons un formalisme rigoureux dans lequel les distributions de hauteur de départ d'avalanche sont exprimées à travers un couplage des facteurs mécaniques et météorologiques. Le critère de stabilité du système plaque - couche fragile est étudié en utilisant une analyse mécanique par éléments finis prenant en compte l'hétérogénéité spatiale des propriétés mécaniques. Considérant qu'une avalanche ne peut se produire que si la hauteur de chute de neige dépasse une hauteur critique correspondant au critère de stabilité, les distributions de hauteur de départ obtenues à partir du modèle mécanique sont couplées avec la distribution des chutes de neige extrêmes sur 3 jours. Nous montrons que ce modèle couplé est capable de reproduire des données de terrain de 369 avalanches naturelles de plaque à La Plagne (France). Non seulement la queue de la distribution en loi puissance, correspondant à des épaisseurs de plaque élevées, mais aussi le corps de la distribution pour les plaques moins épaisses, sont bien reproduits par le modèle. Les avalanches petites à moyennes semblent être essentiellement contrôlées par la mécanique, tandis que les grosses avalanches et l'exposant de la loi puissance associé, sont influencés par un couplage mécanique - météorologique fort. Par ailleurs, nous démontrons que la distribution obtenue est fortement dépendante de l'espace, et, en utilisant les processus max-stables permettant une interpolation spatiale rigoureuse, notre modèle couplé est utilisé pour obtenir des cartes de hauteur de départ d'avalanche pour différentes périodes de retour sur l'ensemble des Alpes françaises.The evaluation of avalanche release depth distributions represents a major challenge for hazard management in mountaineous regions. This depth constitutes an important input ingredient of hazard mapping procedures. This PhD thesis presents a rigorous formalism in which these distributions are expressed through a coupling of mechanical and meteorological factors. The stability criterion of a layered snowpack is investigated using a finite-element analysis accounting for the spatial heterogeneity of weak-layer mechanical properties. Considering that an avalanche can occur only if the snowfall depth exceeds a critical value corresponding to a stability criterion, release depth distributions obtained from the mechanical model are coupled with the distribution of 3-day extreme snowfalls. We show that this coupled model is able to reproduce field data from 369 natural slab avalanches in La Plagne (France). Not only the power-law tail of the distribution, corresponding to large slab depths, but also the core of the distribution for shallow slab depths, are well represented. Small to medium-sized avalanches appear to be controlled mainly by mechanics, whereas large avalanches and the associated power-law exponent, are influenced by a strong mechanical-meteorological coupling. Finally, we demonstrate that the obtained distribution is strongly space dependent, and, using max-stables processes allowing a rigorous spatial interpolation, our coupled model is used to obtain release depth maps for given return periods in the whole French Alps.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Quasistatic to inertial transition in granular materials and the role of fluctuations

On the basis of discrete element numerical simulations of a Couette cell, we revisit the rheology of granular materials in the quasistatic and inertial regimes, and discuss the origin of the transition between these two regimes. We show that quasistatic zones are the seat of a creep process whose rate is directly related to the existence and magnitude of velocity fluctuations. The mechanical behavior in the quasistatic regime is characterized by a three-variable constitutive law relating the friction coefficient (normalized stress), the inertial number (normalized shear rate), and the normalized velocity fluctuations. Importantly, this constitutive law appears to remain also valid in the inertial regime, where it can account for the one-to-one relationship observed between the friction coefficient and the inertial number. The abrupt transition between the quasistatic and inertial regimes is then related to the mode of production of the fluctuations within the material, from nonlocal and artificially sustained by the boundary conditions in the quasistatic regime, to purely local and self-sustained in the inertial regime. This quasistatic-to-inertial transition occurs at a critical inertial number or, equivalently, at a critical level of fluctuations

Influence of weak-layer heterogeneity on snow slab avalanche release: application to the evaluation of avalanche release depths

The evaluation of avalanche release depths constitutes a great challenge for risk assessment in mountainous areas. This study focuses on slab avalanches, which generally result from the rupture of a weak layer underlying a cohesive slab. We use the finite-element code Cast3M to build a mechanical model of the slab/weak-layer system, taking into account two key ingredients for the description of avalanche release: weak-layer heterogeneity and stress redistribution via slab elasticity. The system is loaded by increasing the slope angle until rupture. We first examine the cases of one single and two interacting weak spots in the weak layer, in order to validate the model. We then study the case of heterogeneous weak layers represented through Gaussian distributions of the cohesion with a spherical spatial covariance. Several simulations for different realizations of weak-layer heterogeneity are carried out and the influence of slab depth and heterogeneity correlation length on avalanche release angle distributions is analyzed. We show, in particular, a heterogeneity smoothing effect caused by slab elasticity. Finally, this mechanically based probabilistic model is coupled with extreme snowfall distributions. A sensitivity analysis of the predicted distributions enables us to determine the values of mechanical parameters that provide the best fit to field data