François Cauneau

Spatial resolution improvement by merging MERIS-ETM images for coastal water monitoring

The Medium Resolution Imaging Spectrometer (MERIS) launched in March 2002 and has been providing images since June 2002. Before its launch, we had implemented a method to improve its resolution by merging its images with Landsat Enhanced Thematic Mapper images in order to preserve the best characteristics of the two images (spatial, spectral, temporal). We now present the results of this method for real MERIS images (level 1b and 2) in a coastal area. The robustness of the method is studied as well as the influence of the delay between the acquisitions of the two images.

Optical Remote Sensing of Shallow-Water Environmental Parameters: A Feasibility Study

Abstract A review of methods used for deriving seafloor reflectance and water diffuse attenuation form remotely sensed data is conducted. Emphasis is made on inverse methods applied to remote sensing of ocean properties. Then an inverse scheme for the retrieval of optically active components in coastal shallow waters is described and validated through simulation. The proposed method is based on a synthetic reflectance database and an optimization scheme. A sensitivity analysis is performed. It shows that the method is robust, even when applied to extreme environmental conditions, given that the database covers the optical properties of the water body under consideration.

Diffusion During the Supercritical Drying of Silica Gels

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO2 drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO2. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.

3D Unsteady State MHD Modeling of a 3-Phase AC Hot Graphite Electrodes Plasma Torch

We present, in this paper, the magnetohydrodynamic (MHD) modeling of a three-phase plasma torch. The MHD equations are solved using CFD software Code Saturne®, a computational fluid dynamics software which is based on colocated finite volume. The model developed is 3-D, time dependent, and assumes Local Thermodynamic Equilibrium (LTE). Regarding numerical issues, the modeling of the three-phase AC discharge is particularly tricky since the arcs ignition, by the rotating electrical potential, is relative to the electron density of the electrode gap middle. However, despite these challenging difficulties, the numerical model has been successfully implemented by a LTE assumption. After a detailed description of the model, the results are presented, analyzed, and discussed. The influence of current and nitrogen flow rate over the arc characteristics are studied in terms of temperature, arc behavior (position and motion), velocity and electrical potential. The model gave significant information on parameters that could hardly be obtained experimentally. This study has shown the strong influence of the electrode jets on the overall arc and flow behavior. This work is likely to open the way toward a better understanding of three-phase discharges, which technologies are currently encountering an important development in many application fields.

Three-Dimensional Unsteady MHD Modeling of a Low-Current High-Voltage Nontransferred DC Plasma Torch Operating With Air

We present, in this paper, the MHD modeling of a dc plasma torch operating with air under very peculiar high-voltage low-current conditions. The model developed is 3-D, is time dependent, and assumes local thermodynamic equilibrium (LTE). The study has been carried out considering an axial injection of air with flow rates varying in the range of 0.16-0.5 g/s and currents varying in the range of 300-600 mA. The numerical modeling has been developed using Code_Saturne, a computational fluid dynamics software developed by EDF R&D which is based on colocated finite volume. After a detailed description of the model, the results are presented, analyzed, and discussed. The influence of current and that of air flow rate over the arc characteristics are studied in terms of temperature, velocity, electrical potential, Joule heating, and arc root motion. Regarding numerical issues, the MHD modeling of low-current high-voltage arc discharge is particularly tricky since, below 1 A, the self-induced magnetic field becomes negligible and the convection effects induce a highly irregular and unstable motion of the arc column. However, despite these difficulties, the numerical model has been successfully implemented. Numerical results have shown good correlation and good trends with experimental ones despite a discrepancy which is probably due to the LTE assumption. The model gave fruitful and significant information on parameters that could hardly be obtained experimentally. This preliminary work is likely to open the way toward a better understanding of low-current arc discharges, which technologies are currently encountering an important development in many application fields.

3D MHD modelling of low current?high voltage dc plasma torch under restrike mode

We present in this paper a magnetohydrodynamic (MHD) modelling of the gliding arc behaviour of a dc plasma torch operating with air under low current and high voltage conditions. The low current leads to instabilities and difficulties with simulating the process because the magnetic field is not sufficient to constrict the arc. The model is 3D, time dependent and the MHD equations are solved using CFD software Code_Saturne?. Although the arc is definitively non-local thermodynamic equilibrium (LTE), the LTE assumption is considered as a first approach. The injection of air is tangential. A hot gas channel reattachment model has been used to simulate the restriking process of the arc root. After the description of the model, the most appropriate electrical voltage breakdown parameter has been selected in comparing with experimental results. A typical operating point is then studied in detail and shows the helical shape of the arc discharge in the nozzle. Finally, the mass flow rate and the current have been varied in the range 0.16?0.5?g?s?1 and 100?300?mA, respectively, corresponding to typical glidarc operating points of our experimental plasma torch. The model shows good consistency with experimental data in terms of global behaviour, arc length, mean voltage and glidarc frequency.

Bathymetric Estimation Using MERIS Images in Coastal Sea Waters

Bathymetric estimation using remote sensing images has previously been applied to high spatial resolution imagery such as CASI, Ikonos, or SPOT but not on medium spatial resolution images (i.e., MERIS). This choice can be justified when there is a need to map the bathymetry on large areas. In this letter, we present the results of the bathymetry estimation over a large known area, the Gulf of Lion (France), expanding over 270times180 km

A Comparison Between MHD Modeling and Experimental Results in a 3-Phase AC Arc Plasma Torch: Influence of the Electrode Tip Geometry

Arc behavior in 3-Phase AC plasma technology remains poorly explored. This system noticeably differs from the classical DC plasma torches and aims to overcome certain limitations, such as efficiency, equipment cost and reliability. A MHD model of a 3-Phase AC plasma torch was recently developed at Mines-ParisTech. The model does not include the electrodes in the computational domain. In parallel, experiments were conducted using a high-speed video camera shooting 100,000 frames per second. In this paper, the comparison between MHD modeling and experimental results shows that the arc behavior is in line with the results from the MHD model. Particularly, the strong influences of both the electrode jets and Lorentz forces on the arc motion are confirmed. However, some differences between experimental and numerical electrical waveforms are observed and particularly in the current–voltage phase shift. A new model was then developed by integrating the electrodes into the computational domain and adjusting the electrode tip geometry. With this simulation, we were able to reproduce the phase shift, power and voltage values with a good accuracy showing the strong influence of electrode tip geometry on the 3-Phase arc plasma discharge.

Fusion of MeRIS and ETM images for coastal water monitoring

Ocean colour monitoring is usually based on optical remote sensing with spatial resolutions around 1 km. This spatial scale is available with such sensors as NOAA-AVHRR,POLDER or SeaWiFS, which cover one to several thousand kilometers in a single swath and providing very short revisit periods. The MeRIS sensor, launched on board ENVISAT in 2002, was designed for sea colour observation, with a 300 meter spatial resolution, 15 programmable spectral bands and a 3 day revisit period. Tree hundred meter is a high resolution for an oceanographic sensors, but it is still too rough for coastal water monitoring, where physical and biological phenomena require better spatial resolution [1]. On the opposite, multispectral Landsat ETM images offer a suitable spatial resolution, but have only 4 spectral bands in the visible and near infrared spectrum, allowing poor spectral characterization. A few years ago, in order to combine the spectral resolution of MeRIS and the spatial resolution of Landsat ETM,we had implemented a merging method proposed by Zhukov et al. [2]. Before the launch of ENVISAT, we applied this method to simulated MeRIS images [3]. MeRIS was launched in March 2002 and has been providing images since June 2002. This method is now applied to real MeRIS images. Two product levels are considered. Level 1b contains radiance measurements at the top of the atmosphere for the calibrated and geocoded fifteen (15) MeRIS bands. Level 2 contains normalized surface reflectance and several geophysical and biophysical parameters such as algal pigment index, suspended sediment,Rayleigh-corrected vegetation indices, aerosol type, cloud albedo. The method was tested for radiance (level lb) and reflectance (level 2) over a coastal area of approximately 30times30 km2 located around the Thau lagoon (southern France).

3-D Flow Modeling of a Three-Phase AC Plasma Torch Working With Air Using a Stationary Source Domain With Gas Radiation

This computational fluid dynamics work is dedicated to the study of heat and mass transfer in a new 100-kW three-phase ac/50 Hz plasma torch operating with air. The transient behavior of the arc is simplified using time-averaged heat and momentum source inputs in a stationary source volume whose form was chosen according to the results from a separate magnetohydrodynamic calculation. A parametric study of source volume and momentum intensity is conducted and shows a little influence on the overall temperature and velocity fields. Radiation from gas and walls is taken into account using the discrete ordinate model. Air radiation data consider both atomic and molecular contributions in the temperature range between 300 and 30 000 K and for the wavelengths from 0.209 μm to the far infrared. The results show a huge impact of radiation on wall temperature and heat losses. Two different methods, Planck and Rosseland, are used for the calculation of mean absorption coefficients. Although Plancks average is recommended in this case, the absolute temperature difference between both averages is below 13%. Finally, this paper allows checking that the temperature of different components of plasma torch remains below the physical limits of the selected materials.