GEOM Module manual: I User guide

The GEOM module is part of the AMAPmod software and consists of a 3D objects description language. Based on the MTG model, this language provides a simple and flexible mechanism to describe a hierarchical 3D scene as a collection of objects arranged into a graph structure, called Scene Graph. In addition to this module, AMAPmod includes a Viewer, which allow the user to examine the scenes he has created and to export them into various 3D file formats. This way it is possible to perform additional operations on the scenes such as ray tracing, walk through, hemispherical snapshots and so on. Although, this language has been designed to be used by non specialist and do not require strong backgrounds in 3D computer graphics, it is recommended to consult books introducing basic concepts on 3D graphics to have a better understanding. This document contains the following chapters: * The chapter 1 explains how to represent 3D scenes using AMAPmod. * The chapter 2 forms a reference to the GEOM's file formats. * The chapter 3 forms a reference to the objects available within the GEOM module

Diffraction électromagnétique par la surface océanique : influence des nonlinéarités et de l'écume

The first part of this document introduces a simple model based on the resolution of the Lagrangian equations of motion termed “Choppy Wave Model”. It takes into account the hydrodynamic nonlinearities of the surface and makes possible to establish its complete statistical properties. The obtained results emphasize the nongaussian aspect of the ocean surface and the importance of the undressed spectrum. Some samples of nonlinear seas illustrate the hydrodynamic modulation of short waves by the long ones. In the second part, the impact of sea surface nonlinearities on the scattering process is quantified. The obtained results correct the bias due to the Gaussian assumption in meteo-oceanic parameters estimation. A new calculation method for the Kirchhoff integral based on fast radial convolutions is also introduced. Finally, the foam impact on the scattering process in micro-waves is estimated and is shown to become significant at strong winds and mainly in HH polarization.La première partie de ce document présente un modèle basé sur la résolution des équations Lagrangiennes du mouvement nommé “Choppy Wave Model”. Il permet de tenir compte des nonlinéarités hydrodynamiques de la surface et d’établir une description complète des grandeurs statistiques. Les résultats obtenus soulignent leur caractère non Gaussien et l’importance du spectre déshabillé. Des échantillons de surfaces nonlinéaires illustrent la modulation des petites vagues par les grandes. L’étude menée en seconde partie quantifie l’impact des nonlinéarités hydrodynamiques sur le phénomène de diffraction et permet de s’affranchir des erreurs dues à l’hypothèse Gaussienne dans l’estimation de paramètres météo-océaniques. Une nouvelle méthode de calcul de l’intégrale de Kirchhoff basée sur des convolutions radiales rapides est aussi introduite. Enfin, l’impact de l’écume en micro-ondes est détaillé et les principaux résultats montrent son importance à forts vents et notamment en polarisation HH

PlantGL : a Python-based geometric library for 3D plant modelling at different scales

In this paper, we present PlantGL, an open-source graphic toolkit for the creation, simulation and analysis of 3D virtual plants. This C++ geometric library is embedded in the Python language which makes it a powerful user-interactive platform for plant modelling in various biological application domains. PlantGL makes it possible to build and manipulate geometric models of plants or plant parts, ranging from tissues and organs to plant populations. Based on a scene graph augmented with primitives dedicated to plant representation, several methods are provided to create plant architectures from either field measurements or procedural algorithms. Because they reveal particularly useful in plant design and analysis, special attention has been paid to the definition and use of branching system envelopes. Several examples from different modelling applications illustrate how PlantGL can be used to construct, analyse or manipulate geometric models at different scales

Elastic behavior in Contact Dynamics of rigid particles

The systematic errors due to the practical implementation of the Contact Dynamics method for simulation of dense granular media are examined. It is shown that, using the usual iterative solver to simulate a chain of rigid particles, effective elasticity and sound propagation with a finite velocity occur. The characteristics of these phenomena are investigated analytically and numerically in order to assess the limits of applicability of this simulation method and to compare it with soft particle molecular dynamics.Comment: submitted to PRE, 7 pages, 6 figure

Caractérisation et optimisation de l'environnement mécanique tridimensionnel des cellules souches au sein des bioréacteurs d'ingénierie tissulaire osseuse

Les cellules souches mésenchymateuses utilisées en ingénierie tissulaire osseuse sont sensibles aux contraintes mécaniques, particulièrement au cisaillement. Il est donc possible d'optimiser leurs conditions de culture (géométrie des scaffolds, flux) pour stimuler les cellules au sein des bioréacteurs. Pour cela, une étude numérique de dynamique des fluides a été réalisée sur différents types de scaffolds granulaires. Elle a montré que ces scaffolds ont une efficacité équivalente à ceux de la littérature pour convertir le flux de milieu de culture en stimulation mécanique. Ces résultats numériques ont ensuite été mis en ?uvre expérimentalement. Les expériences ont montré le rôle fondamental de la nature du biomatériau utilisé comme scaffold ainsi que de l'environnement tridimensionnel

A benchmark for particle shape dependence

International audienceParticle shape is a major parameter for the space-filling and strength properties of granular materials. For a systematic investigation of shape effect, a numerical benchmark test was set up within a collaborative group using different numerical methods and particles of various shape characteristics such as elongation, angularity and nonconvexity. Extensive 2D shear simulations were performed in this framework and the shear strength and packing fraction were compared for different shapes.We show that the results may be analyzed in terms of a low-order shape parameter h describing the degree of distortion from a perfectly circular shape. In particular, the shear strength is an increasing function of h with nearly the same trend for all shapes, the differences being of second order compared to h. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with h from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as h is further increased. Finally, the analysis of contact forces for the same value of h leads to very similar statistics regardless of our specific particle shapes

Particle shape dependence in 2D granular media

Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter $\eta$ describing the degree of distortion from a perfectly spherical shape. Encompassing most specific shape characteristics such as elongation, angularity and nonconvexity, $\eta$ is a low-order but generic parameter that we used in a numerical benchmark test for a systematic investigation of shape-dependence in sheared granular packings composed of particles of different shapes. We find that the shear strength is an increasing function of $\eta$ with nearly the same trend for all shapes, the differences appearing thus to be of second order compared to $\eta$. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with $\eta$ from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as $\eta$ is further increased. These findings suggest that a low-order description of particle shape accounts for the principal trends of packing fraction and shear strength. Hence, the effect of second-order shape parameters may be investigated by considering different shapes at the same level of $\eta$.Comment: 5 pages, 8 figure

Measuring ocean surface velocities with the KuROS and KaRADOC airborne near-nadir Doppler radars: a multi-scale analysis in preparation of the SKIM mission, Submitted to Ocean SCience, July 2019

Surface currents are poorly known over most of the oceans. Satellite-borne Doppler Waves and Current Scatterom-eters (DWCS) can be used to fill this observation gap. The Sea surface KInematics Multiscale (SKIM) proposal, is the first satellite concept built on a DWCS design at near-nadir angles, and now one of the two candidates to become the 9th mission of the European Space Agency Earth Explorer program. As part of the detailed design and feasibility studies (phase A) funded by ESA, airborne measurements were carried out with both a Ku-Band and a Ka-Band Doppler radars looking at the sea surface at 5 near nadir-incidence in a real-aperture mode, i.e. in a geometry and mode similar to that of SKIM. The airborne radar KuROS was deployed to provide simultaneous measurements of the radar backscatter and Doppler velocity, in a side-looking configuration , with an horizontal resolution of about 5 to 10 m along the line of sight and integrated in the perpendicular direction over the real-aperture 3-dB footprint diameter (about 580 m). The KaRADOC system has a much narrower beam, with a circular footprint only 45 m in diameter. 10 The experiment took place in November 2018 off the French Atlantic coast, with sea states representative of the open ocean and a well known tide-dominated current regime. The data set is analyzed to explore the contribution of non-geophysical velocities to the measurement and how the geophysical part of the measured velocity combines wave-resolved and wave-averaged scales. We find that the measured Doppler velocity contains a characteristic wave phase speed, called here C 0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little 15 variations ∆ C associated to changes in sea state. The Ka-band measurements at an incidence of 12 • are 10% lower than the theoretical estimate C 0 2.4 m/s for typical oceanic conditions defined by a wind speed of 7 m/s and a significant wave height of 2 m. For Ku-band the measured data is 1 https://doi. 30% lower than the theoretical estimate 2.8 m/s. ∆ C is of the order of 0.2 m/s for a 1 m change in wave height, and cannot be confused with a 1 m/s change in tidal current. The actual measurement of the current velocity from an aircraft at 4 to 18 • incidence angle is, however, made difficult by uncertainties on the measurement geometry, which are much reduced in satellite measurements