Large perturbations of ammonium and organic acids content in the Summit-Greenland ice core. Fingerprint from forest fires?

Biomass burning is influencing the atmospheric chemistry by emitting large amounts of reactive species such as hydrocarbons, organic acids and nitrogen compounds [Andreae et al., 1988]. Polar ice cores provide a unique record of precipitation whose chemistry reflects the atmospheric composition at the time of deposition. The analysis of such ice samples therefore allows an estimate to be made of the concentration of atmospheric impurities in the past. During the first season of the deep drill operation (GRIP) at Summit, Central Greenland (72° 34' N, 37° 38'W) continuous ammonium (NH4+) measurements were performed between 100 and 600 m depth covering the time period from 330 to 2500 years B.P. The NH4+ concentrations show seasonal variations between 1–20 ng.g−1 with some sporadic high values up to 600 ng.g−1 in narrow layers. The chemical fingerprint points to biomass burning causing the high ammonium peaks

Using LES to Study Reacting Flows and Instabilities in Annular Combustion Chambers

Great prominence is put on the design of aeronautical gas turbines due to increasingly stringent regulations and the need to tackle rising fuel prices. This drive towards innovation has resulted sometimes in new concepts being prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities often take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. Since one of the most limiting factors in performing Large Eddy Simulation (LES) of real combustors is estimating the adequate grid, the effects of mesh resolution are investigated by computing full annular LES of a realistic helicopter combustion chamber on three grids, respectively made of 38, 93 and 336 million elements. Results are compared in terms of mean and fluctuating fields. LES captures self-established azimuthal modes. The presence and structure of the modes is discussed. This study therefore highlights the potential of LES for studying combustion instabilities in annular gas turbine combustors

Simulation aux grandes échelles: instabilités thermo-acoustiques, combustion diphasique et couplages multi-physiques

La combustion turbulente, que ce soit dans des configurations de laboratoire ou dans des configurations réelles industrielles, met en oeuvre un nombre important de physiques fortement couplées: chimie, turbulence, multi-phasique, thermique, etc. Pour répondre aux demandes de plus en plus exigeantes des concepteurs, qui doivent proposer des solutions concurrentielles tout en respectant les contraintes environnementales de bruit et d'émission de polluants, la simulation numérique est devenue incontournable. Plus précisément, la simulation maintenant utilisée comme outil de conception, doit être fiable et précise. Dans le domaine de la combustion turbulente, à fort caractère instationnaire, la Simulation aux Grandes Echelles (SGE) s'est récemment imposée. Cette technique s'est en effet avérée capable de prédire finement le comportement des brûleurs dans des environnements complexes, et permet aujourd'hui d'aborder des problématiques encore mal maîtrisées telles que les instabilités thermo-acoustiques ou la combustion diphasique. On donne ici quelques exemples de problèmes encore ouverts dans ce domaine

A Mesh Adaptation Strategy to Predict Pressure Losses in LES of Swirled Flows

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP/2007-2013) / ERC Grant Agreement ERC-AdG 319067-INTECOCIS

A detailed analysis of the rapid changes in ice-core parameters during the last ice age

Results from deep Greenland ice cores show rapid changes in several parameters in the deepest part. The most probable explanation for these variations is a fast-changing climate during part of the last glaciation. The question arises, however, of whether the observed changes in the ice cores could also be due to, or at least be influenced by, discontinuities in the stratigraphy. We present new CO2 and δ18O data from the Camp Century and Dye 3 deep ice cores. The data show rapid changes in CO2 and δ18O in both cores. One transition which was investigated in detail seems to be more rapid in the ice core from Dye 3 than in the Camp Century core. The broadening of a sharp δ18O transition due to molecular diffusion is discussed. Since this broadening is larger than the observed width of the transition, we discuss the possibility of a mechanism that can produce stratigraphic disturbances on a small scale. This mechanism is based on a calculation of the compression of horizontal layers which have equal density but different viscosities

CO2 record in the byrd ice core 50,000 - 5,000 years BP

The analysis of air in polar ice cores revealed 30% lower CO2 values during glacial periods than during interglacial periods. At present this is confirmed by results from six different ice cores, two from Greenland and four from Antarctica (1–5). In all cores the CO2 change coincides with the change in the isotopic composition of the ice, expressed as either the δ18O or δD ratio; both are indicators for the mean annual surface temperature6. To investigate the relationship between atmospheric CO2 concentration changes with changes in climate, the atmospheric CO2 concentration during and at the end of the last glaciation has to be known in detail. To achieve this, we have studied a great number of samples from the deep ice core from Byrd station, Westantarctica, drilled in 1968. These measurements allow us to reconstruct the atmospheric CO02 concentration in the time period 50,000–15,000 yr bp in great detail