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Formation of spanwise vorticy in oblique turbulent bands of transitional plane Couette flow, part 1: Numerical experiments
No full textInternational audienceThis article investigates the formation of spanwise vorticity in the velocity streaks of the oblique laminar-turbulent bands of plane Couette flow (PCF) by mean of Direct Numerical Simulations (DNS). The spanwise vorticity is created by a roll–up type development of the streamwise-wall normal shear layer of the velocity streaks. It is advected by the large scale flow along the bands. We propose a criterion on spanwise vorticity which detects these events in order to perform systematic measurements. Beside of the streamwise and spanwise correlation lengths of the rolls, their advection velocity is measured and shown to match the large scale flow along the band near the turbulent region. Eventually, we discuss the possible relation between ejection of vorticity away from the bands near the laminar region and the size of said laminar region
Inverse Cauchy method with conformal mapping : application to latent flatness defect detection during rolling process
No full textInternational audienceIn this paper a semi-analytical inverse Cauchy problem is presented for a finite cylinder with a cylindrical hole under the surface called the detecting roll. Applications to latent flatness defect detection during rolling process are considered. A steel strip wraps at a known angle around the detecting roll. The presented inverse method aims at evaluating the residual stress profile of the strip. The inputs are the displacements and free surface traction at the hole surface and the outputs are the resultant forces per unit length in the contact between the strip and the roll, thus by equilibrium of the strip the residual stress profile is inferred. This paper is based on plane complex elasticity with conformal mapping techniques. Several 2D-computations are performed at several axial positions. This work is theoretical, the measurement device is not available and technical issues are not broached. Some numerical examples with synthetic data are provided in order to evaluate accuracy and noise sensitivity. Results encourage further investigations and technical developments
LA POLITIQUE CLIMATIQUE ENTRE CHOIX NATIONAUX ET SCENARIOS MONDIAUX Implications des positionnements cognitifs et éthiques
No full textCet ouvrage analyse la manière dont des stratégies nationales de lutte contre l’effet de serre peuvent se déterminer en fonction des positionnements cognitifs et éthiques des États nationaux au regard de scénarios climatiques mondiaux. Sont ainsi distinguées les approches cognitives de type « prédictif », « thomiste » ou « symétrique », croisées avec des positionnements éthiques désignés comme « égocentrique », « altruiste-dynastique, « solidariste intragénérationnel », « altruiste cosmopolitique » et « universaliste kanto-millien ». Pour évaluer ces configurations on suppose que les choix faits par un État s’appuient sur une évaluation des dommages climatiques associés à différents scénarios de concentration de gaz à effet de serre. Les indicateurs centraux utilisés sont la valeur actuelle du dommage entraîné par l’émission d’une tonne de CO2 et le niveau du taux d’actualisation. Dans ce cadre classique où se met à l’épreuve le concept de dommages pour appréhender des phénomènes de long terme, l’évaluation détermine quelles configurations cognitivo-éthiques justifient la cible du « Facteur 4 » en 2050 : il en existe un nombre réduit, relevant soit d’un « altruisme cosmopolitique (international et intergénérationnel) », soit d’un « universalisme kanto-millien », si cette cible est choisie en fonction du seul problème climatique. En postulant qu’une valeur de 100 € / tCO2 en 2030 est un point de passage obligé vers le « Facteur 4 »en 2050, de tels choix éthiques impliquent la reconnaissance d’une valeur actuelle 2010 d’un niveau élevé pour le dommage entraîné par l’émission d’une tonne de CO2e (au moins 53 € / tCO2, soit un ordre de grandeur supérieur au prix 2013 du carbone sur le marché ETS) et l’adoption d’un taux d’actualisation caractéristique qui, dépendant des configurations étudiées, ne saurait être plus élevé que 3,25 %. De telles valeurs doivent être considérées comme des conditions logiques à respecter pour tous les choix dérivés et en particulier lorsqu’il s’agit de déterminer la meilleure trajectoire intertemporelle de réduction des émissions. Autre résultat : en prenant en compte l’incidence de la date d’émission sur le dommage climatique, il s’avère qu’« altruisme cosmopolitique » et « universalisme kanto-millien » conduisent à des recommandations strictement opposées quant au profil de la meilleure trajectoire temporelle de « consommation » d’un budget d’émissions fixé pour la période 2011-2050 : le premier demande de concentrer les émissions en début de période et le second de débuter par un « choc de réduction ». Au total aucune des configurations justifiant la cible du « Facteur 4 » ne s’accorde avec l’application simple d’une règle de Hotelling qui ferait progresser la valeur de la tonne de CO2 au taux d’actualisation standard pour l’investissement public
On the fan associated to a linear code
No full textInternational audienceWe will show how one can compute all reduced Gröbner bases with re-spect to a degree compatible ordering for code ideals -even though these binomial ideals are not toric. To this end, the correspondence of linear codes and binomial ideals will be briefly described as well as their resemblance to toric ideals. Finally, we will hint at applications of the degree compatible Gröbner fan to the code equivalence problem
The atmospheric component of the Mediterranean Sea water budget in a WRF multi-physics ensemble and observations
No full textInternational audienceThe use of high resolution atmosphere-ocean coupled regional climate models to study possible future climate changes in the Mediterranean Sea requires an accurate simulation of the atmospheric component of the water budget (i.e., evaporation, precipitation and runoff). A specific configuration of the version 3.1 of the weather research and forecasting (WRF) regional climate model was shown to systematically overestimate the Mediterranean Sea water budget mainly due to an excess of evaporation (~1,450 mm yr-1) compared with observed estimations (~1,150 mm yr-1). In this article, a 70-member multi-physics ensemble is used to try to understand the relative importance of various sub-grid scale processes in the Mediterranean Sea water budget and to evaluate its representation by comparing simulated results with observed-based estimates. The physics ensemble was constructed by performing 70 1-year long simulations using version 3.3 of the WRF model by combining six cumulus, four surface/planetary boundary layer and three radiation schemes. Results show that evaporation variability across the multi-physics ensemble (~10 % of the mean evaporation) is dominated by the choice of the surface layer scheme that explains more than ~70 % of the total variance and that the overestimation of evaporation in WRF simulations is generally related with an overestimation of surface exchange coefficients due to too large values of the surface roughness parameter and/or the simulation of too unstable surface conditions. Although the influence of radiation schemes on evaporation variability is small (~13 % of the total variance), radiation schemes strongly influence exchange coefficients and vertical humidity gradients near the surface due to modifications of temperature lapse rates. The precipitation variability across the physics ensemble (~35 % of the mean precipitation) is dominated by the choice of both cumulus (~55 % of the total variance) and planetary boundary layer (~32 % of the total variance) schemes with a strong regional dependence. Most members of the ensemble underestimate total precipitation amounts with biases as large as 250 mm yr-1 over the whole Mediterranean Sea compared with ERA Interim reanalysis mainly due to an underestimation of the number of wet days. The larger number of dry days in simulations is associated with a deficit in the activation of cumulus schemes. Both radiation and planetary boundary layer schemes influence precipitation through modifications on the available water vapor in the boundary layer generally tied with changes in evaporation. © 2014 Springer-Verlag Berlin Heidelberg
Spread in model climate sensitivity traced to atmospheric convective mixing
No full textInternational audienceEquilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming. © 2014 Macmillan Publishers Limited
Simulated air quality and pollutant budgets over Europe in 2008
No full textInternational audienceMajor gaseous and particulate pollutant levels over Europe in 2008 have been simulated using the offline-coupled WRFCMAQ chemistry and transport modeling system. The simulations are compared with surface observations from the EMEP stations, ozone (O3) soundings, ship-borne O3 and nitrogen dioxide (NO2) observations in the western Mediterranean, tropospheric NO2 vertical column densities from the SCIAMACHY instrument, and aerosol optical depths (AOD) from the AERONET. The results show that on average, surface O3 levels are underestimated by 4 to 7% over the northern European EMEP stations while they are overestimated by 7-10% over the southern European EMEP stations and underestimated in the tropospheric column (by 10-20%). Particulate matter (PM) mass concentrations are underestimated by up to 60%, particularly in southern and eastern Europe, suggesting underestimated PM sources. Larger differences are calculated for individual aerosol components, particularly for organic and elemental carbon than for the total PM mass, indicating uncertainty in the combustion sources. Better agreement has been obtained for aerosol species over urban areas of the eastern Mediterranean, particularly for nss-SO4 2, attributed to the implementation of higher quality emission inventories for that area. Simulated AOD levels are lower than the AERONET observations by 10% on average, with average underestimations of 3% north of 40°N, attributed to the low anthropogenic emissions in the model and 22% south of 40°N, suggesting underestimated natural and resuspended dust emissions. Overall, the results reveal differences in the model performance between northern and southern Europe, suggesting significant differences in the representation of both anthropogenic and natural emissions in these regions. Budget analyses indicate that O3 and peroxyacetyl nitrate (PAN) are transported from the free troposphere (FT) to the planetary boundary layer over Europe, while other species follow the reverse path and are then advected away from the source region. © 2013 Elsevier B.V
The impact of random initialization on the runtime of randomized search heuristics
No full textInternational audienceIt has often been observed that the expected runtime of an evolutionary algorithm with random initialization does not deviate much from the expected runtime when starting in an initial solution of average fitness. Having this information a priori would greatly simplify the runtime analysis for the algorithm using random initialization. We prove such a result for the optimization of the OneMax test function via the two randomized search heuristics Randomized Local Search (RLS) and the (1+1) Evolutionary Algorithm. For both algorithms, we show that the expected runtime from a random initial solution deviates at most by a constant number of iterations from the expected runtime when starting with a solution having exactly n/2 ones.For RLS we can precisely compute that this constant is -1/2 ± o(1). This leads to an extremely precise bound for the expected runtime. The expected number of fitness evaluations until an optimal search point is found, is n Hn/2 - 1/2 ± o(1), where Hn/2 denotes the (n/2)th harmonic number when n is even, and Hn/2:= (H⌊ n/2 ⌋ + H⌈ n/2 ⌉)/2 when n is odd.The main technique to obtain these results is a coupling of the optimization process starting from different fitness levels. We believe this technique to be interesting also much beyond the specific results mentioned above; e.g., for the study of other optimization problems
Multicolor two-photon light-sheet microscopy
No full textInternational audienceTwo-photon microscopy is the most effective approach for deep-tissue fluorescence cellular imaging; however, its application to high-throughput or high-content imaging is often hampered by low pixel rates, challenging multicolor excitation and potential cumulative photodamage. To overcome these limitations, we extended our prior work and combined two-photon scanned light-sheet..
Exposure to ambient black carbon derived from a unique inventory and high-resolution model
No full textInternational audienceBlack carbon (BC) is increasingly recognized as a significant air pollutant with harmful effects on human health, either in its own right or as a carrier of other chemicals. The adverse impact is of particular concern in those developing regions with high emissions and a growing population density. The results of recent studies indicate that BC emissions could be underestimated by a factor of 2–3 and this is particularly true for the hot-spot Asian region. Here we present a unique inventory at 10-km resolution based on a recently published global fuel consumption data product and updated emission factor measurements. The unique inventory is coupled to an Asia-nested (∼50 km) atmospheric model and used to calculate the global population exposure to BC with fully quantified uncertainty. Evaluating the modeled surface BC concentrations against observations reveals great improvement. The bias is reduced from −88% to −35% in Asia when the unique inventory and higher-resolution model replace a previous inventory combined with a coarse-resolution model. The bias can be further reduced to −12% by downscaling to 10 km using emission as a proxy. Our estimated global population-weighted BC exposure concentration constrained by observations is 2.14 μg⋅m−3; 130% higher than that obtained using less detailed inventories and low-resolution models