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

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

Thermophysical behaviour of green bodies

International audienc

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 modeling 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 are 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 ranging from tissues to plant communities

ESA EE-9 SKIM airborne demonstrator: first results

International audienceSKIM is an ESA Earth Explorer-9 candidate mission designed to measure directly and simultaneously, for the first time, directional ocean surface current vector (OSCV) and ocean wave spectra. SKIM will measure at 6° and 12° incidence angle using the pulse-pair radar Doppler technique to observe the sea surface Doppler signal and then derive the ocean surface current. The concept has been first demonstrated using opportunity data from the Ka-band AirSWOT instrument showing unambiguous estimations of the wave spectrum and surface current [Nouguier et al., 2016, IEEE TGRSL].The DRIFT4SKIM campaign is a dedicated airborne campaign to validate the SKIM concept. In order to assess this capability, airborne Doppler measurement will be compared with a wide range of observations that cover all instruments generally used as reference for the measurement of both currents and the driving forces of currents (wind, temperature gradient, waves). The DRIFT4SKIM campaign, takes place in the Iroise sea (4 to 6°W, 48 to 49°N) end of November 2018 (after the deadline for the abstract).Observations will be performed in two 4-km side squares located in the field-of-view of a shore-based HF radar system (measuring surface current vectors):one where currents are spatially homogeneous; and one where strong horizontal current gradients are present. Two microwave radars will observed the sea surface: KuROS is a near nadir Ku-band conical scanning fan-beam real aperture pulse-coherent radar (Caudal et al., 2014, JAOT)KaRADOC is a near nadir Ka-band narrow-beam real aperture pulse-coherent radar. Additional observations will be carried out from optical cameras with an additional airborne platform in order to confirm the homogeneity of the experiment based on the sea surface temperature, and provide additional observations of wave spectrum and current. Ocean surface processes will be characterized using in situ instrumentation with a large number (100) of surface drifters (current in the top meter); drogue drifters (10 for each drogue depth) at 15, 5, 2 and 1 m; 10 drifting wave measuring buoys; wind measurement and a buoy carrying current profiler.The paper will present a first analysis of the microwave radar Doppler measurements and assessed these measurements against the geophysical conditions which will be well monitored during the campaign with comparison with in situ measurements

Wind-Wave Attenuation in Arctic Sea Ice: A Discussion of Remote Sensing Capabilities

Wind-generated waves strongly interact with sea ice and impact air-sea exchanges, operations at sea, and marine life. Unfortunately, the dissipation of wave energy is not well quantified and its possible effect on upper ocean mixing and ice drift is still mysterious. As the Arctic is opening up and wave energy increases, the limited amount of in situ observations is a clear limitation to our scientific understanding. Both radar and optical remote sensing has revealed the frequent presence of waves in ice, and could be used more systematically to investigate wave-ice interactions. Here we show that, in cloud-free conditions, Sentinel-2 images exhibit brightness modulations in ice-covered water, consistent with the presence of waves measured a few hours later by the ICESat-2 laser altimeter. We show that a full-focus SAR processing of Sentinel-3 radar altimeter data also reveals the presence and wavelengths of waves in sea ice, within minutes of Sentinel-2 imagery. The SWIM instrument on CFOSAT is another source of quantitative evidence for the direction and wavelengths of waves in ice, when ice conditions are spatially homogeneous. In the presence of sea ice, a quantitative wave height measurement method is not yet available for all-weather near-nadir radar instruments such as altimeters and SWIM. However, their systematic colocation with optical instruments on Sentinel-2 and ICESat-2, which are less frequently able to observe waves in sea ice, may provide the empirical transfer functions needed to interpret and calibrate the radar data, greatly expanding the available data on wave-ice interactions.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Mathematical Geodesy and PositioningPhysical and Space Geodes