Identification des coefficients aérodynamiques d'un projectile gyrostabilisé à partir de mesures radar

La maîtrise de la précision des tirs de projectiles est un des enjeux majeurs pour la communauté aérobalistique, pour des raisons de sécurité et d'efficacité en situation opérationnelle. Cela requiert une bonne connaissance de tous les paramètres qui conditionnent le vol du projectile, en particulier les coefficients aérodynamiques. Il existe plusieurs méthodes pour déterminer les coefficients aérodynamiques d'un projectile, ces techniques étant utilisées de manière complémentaire et généralement à des stades différents du développement d'un projectile. Les calculs CFD (Computational Fluid Dynamics) autorisent, en plus de déterminer les coefficients aérodynamiques globaux d'un projectile, une compréhension locale des phénomènes physiques en jeu. Les essais en soufflerie permettent la mesure des efforts exercés sur une maquette du projectile, éventuellement en mouvement, positionnée dans une veine d'écoulement. Les essais sur champ de tir restent quant à eux un passage obligé dans le processus de caractérisation aérodynamique d'un projectile. Dans notre étude, nous utilisons les mesures issues de champ de tir pour identifier les coefficients aérodynamiques. La méthode inverse d'identification se base uniquement sur des mesures obtenues avec un radar Doppler muni de la fonction écartométrie. Il est fait en sorte que le processus d'identification soit automatique, robuste face au bruit de mesure et puisse prendre en compte des coefficients varaiables au cours du vol. En se passant de toute instrumentation embarquée dans le projectile, nous contribuons aussi à l'objectif de réduction des coûts. Une reformulation du modèle balistique du Point Matériel Modifié permet de rendre la méthode d'identification indépendante de toute instrumentation embarquée dans le projectile. En prenant en compte les mesures réelles issues de l'observation de plusieurs tirs, on observe que la précision des résultats obtenus est équivalente à l'existant tout en réduisant le nombre de tirs à analyser

Identification inverse des coefficients aérodynamiques d'un engin hypersonique à partir de données issues de vol

Une méthode d'identification des coefficients aérodynamiques d'une flèche à partir de données prises en vol est proposée. Elle est basée sur l'optimisation d'une fonctionnelle où interviennent les mesures (vitesses, rotations, ), ainsi que les coefficients aérodynamiques. Le vol du projectile est représenté par un modèle à six d.d.l. où interviennent 8 coefficients aérodynamiques. L'optimisation de la fonctionnelle conduit à un système non linéaire qui fournit après résolution les coefficients aérodynamiques du projectile ainsi qu'une trajectoire recalculée s'approchant au mieux des mesures

Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea

Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies

Caractérisation de paramètres aérodynamiques d'un projectile gyrostabilisé à partir de l'observation de son vol

Une méthode de caractérisation des paramètres aérodynamiques d'un projectile gyrostabilisé, à partir de l'observation de son vol, est proposée. Le modèle balistique utilisé est basé sur celui du Point Matériel Modifié (PMM) adapté de sorte à ne pas faire apparaître de manière explicite le vecteur obliquité. La procédure d'identification des paramètres consiste en une suite de problèmes d'optimisation non linéaire sous contraintes

Identification of aerodynamic coefficients of a kinetic energy projectile from flight data

International audienc

Identification des coefficients aérodynamiques d'un projectile gyrostabilisé à partir de données de vol

International audienceUne méthode d'identification des coefficients aérodynamiques d'un projectile gyrostabilisé, à partir de données de vol, est proposée. Le modèle balistique utilisé pour représenter le vol du projectile est celui du Point Matériel Modifié (PMM). La technique d'identification est basée sur des problèmes d'optimisation non linéaire sous contraintes. Les coefficients aérodynamiques variant au cours du vol puisqu'ils dépendent du nombre de Mach et de l'obliquité, un fenêtrage temporel est donc réalisé de sorte à considérer les coefficients constants sur chaque intervalle de temps

Voltage Readjustment Methodology According to Pressure and Temperature Applied to a High Temperature PEM Fuel Cell

The operating conditions can have uncontrolled effects on the voltage of a High-Temperature Proton Exchange Membrane Fuel Cell (HT-PEMFC). For instance, the HT-PEMFC can be used at ambient pressure, i.e., without having a back pressure regulator. In this case, the variation in the atmospheric pressure directly affects pressures inside the fuel cell, which induces voltage variation. Moreover, in transient phases, several coupled phenomena can have an uncontrolled effect on the voltage. For example, following a change in the current operating point, thermal conditions in the fuel cell can vary, and the temperature stabilization then leads to a voltage variation. This article introduces a readjustment method for the fuel cell voltage to compensate for the effects of the pressure and temperature variations that are undergone and to decouple their effects. This methodology is based on the realization of a design of experiments to characterize the voltage sensitivity to pressure ([1; 1.5 bar]) and temperature ([120; 180 °C]) between 0.2 and 1 A/cm2 of an Advent PBI MEA (formerly BASF Celtec®-P 1100 W). The data obtained allowed identifying an empirical model that takes into account the aging caused by the experiment. Finally, the methodology is criticized before proposing an alternative method

Contamination of durum wheat by cadmium : lessons from the field

Contamination of food crops by cadmium (Cd) is a major concern, particularly for durum wheat, which is widely consumed and which accumulates this toxic metal to a greater extent than bread wheat. Following the assessment of the risks to human health related to the presence of cadmium in foodstuffs, the European Commission intended to reduce the maximum content of Cd in wheat from 0.2 to 0.1 mg/kg grain. France is in the top 5 of European countries for the production of durum wheat and the project of a lower regulation limit for Cd stressed the urgent need i) to characterize the risk that harvests could exceed the new limit, ii) to identify, in the field, the major factors controlling the level of contamination. For that purpose, a specific project 'Cadur' was conducted between 2011 and 2014 in France. Samples of grains were collected in 2011, 2012 and 2013 among farmers of the different areas of production. The median value of Cd in durum wheat grain was 0.07 mg/kg with less than 5% of samples above the 0.2 mg/kg limit but 25% above 0.1 mg/kg (n=244). However, there were strong variations depending on the geographical areas and for some of them, up to 60% of the samples exceeded 0.1 mg Cd /kg. In order to understand the source of these variations, trials crossing cultivars and the areas of production were investigated. The cultivars were estimated to be responsible of 10-25% of the variation of the grain Cd concentration against 60-70% for the location of the culture. The effect of the location was mainly due to the concentration of Cd in the soil and to the soil pH. A statistical model with these two explanatory variables was able to explain 57% of the variability of the Cd concentration in the grains. Some of the durum wheat cultivars commonly grown in France were assessed with respect to their ability to accumulate Cd. It was found that the high:low ratio for the Cd content in grains was around 1.5 when cultivars were grown on the same soils. Contrary to Canadian cultivars, in France, no specific genetic selection has yet been performed to lower the contamination by Cd. Nitrogen fertilization was shown to have a significant effect, both by the dose and by the timing of application, presumably by modifying the mobility of Cd in the soil. Increasing N supply from 200 to 250 kg N/ha increased the Cd in grains by 20% and by 25% if the total N reached 300 kg N/ha. However, if the additional N was applied as two applications including a late one instead of a single dose, the increase in the grain contamination was not significant for a total of 250 kg N/ha. These studies confirm that both the soil conditions (Cd content, pH) and the cultivars are key points for controlling the contamination of durum wheat by Cd. French cultivars have the potential to be genetically selected to improve their ability to exclude Cd from the grain. They also have to be carefully chosen depending on the local bioavailability of Cd in soil, which can be in part, managed by controlling the pH and the nitrogen fertilisation. Funded by the projects 'Cadur', Arvalis-Inra and ANR-15-CE21-0001-04 'Cadon