Extreme Lagrangian acceleration in confined turbulent flow

A Lagrangian study of two-dimensional turbulence for two different geometries, a periodic and a confined circular geometry, is presented to investigate the influence of solid boundaries on the Lagrangian dynamics. It is found that the Lagrangian acceleration is even more intermittent in the confined domain than in the periodic domain. The flatness of the Lagrangian acceleration as a function of the radius shows that the influence of the wall on the Lagrangian dynamics becomes negligible in the center of the domain and it also reveals that the wall is responsible for the increased intermittency. The transition in the Lagrangian statistics between this region, not directly influenced by the walls, and a critical radius which defines a Lagrangian boundary layer, is shown to be very sharp with a sudden increase of the acceleration flatness from about 5 to about 20

Conditional vorticity budget of coherent and incoherent flow contributions in fully developed homogeneous isotropic turbulence

We investigate the conditional vorticity budget of fully developed three-dimensional homogeneous isotropic turbulence with respect to coherent and incoherent flow contributions. The Coherent Vorticity Extraction based on orthogonal wavelets allows to decompose the vorticity field into coherent and incoherent contributions, of which the latter are noise-like. The impact of the vortex structures observed in fully developed turbulence on statistical balance equations is quantified considering the conditional vorticity budget. The connection between the basic structures present in the flow and their statistical implications is thereby assessed. The results are compared to those obtained for large- and small-scale contributions using a Fourier decomposition, which reveals pronounced differences

Étude d'un feu dans un ensemble de locaux ventilés mécaniquement

La compréhension des incendies dans des milieux confinés et ventilés mécaniquement est un enjeu important pour la prévention et la sécurité incendie. L’incendie fait intervenir des problèmes multi-physique et multi-échelle, ce qui rend complexe leur modélisation. La propagation des produits de combustion et des fumées d’un local à un autre nécessite une attention particulière car on retrouve ces structures multi-compartimentées dans de nombreux bâtiments, en particulier dans les installations nucléaires ou les navires. Cette étude présente les dernières avancées de l’outil de simulation SAFIR développé conjointement par la DGA et l’IUSTI et sa validation par confrontation avec les résultats expérimentaux obtenus dans le cadre du projet PRISME INTEGRAL de l’OCDE/NEA. De type volumes finis, le code SAFIR décrit l'ensemble des phénomènes de base qui régissent le développement et la propagation du feu: transport de gaz, turbulence, combustion turbulente et rayonnement. L'extension, dans SAFIR, de la technique des régions bloquées au cas d'écoulements complexes a permis une meilleure description des échanges thermiques avec les régions solides présentes dans le domaine de calcul. La configuration étudiée est celle de trois locaux séparés par des portes et d’un corridor, ventilés mécaniquement. Les murs sont en béton, certaines parois et le plafond ont été isolés avec de la laine de roche. Deux essais ont été réalisés avec des combustibles différents, un combustible liquide le dodécane 〖(C〗_12 H_26) et un combustible très hétérogène une armoire électrique. La validation, en termes de température et vitesse aux ouvertures, montre un très bon accord entre le calcul et l’expérience. Compte tenu des écarts observés sur les concentrations de O_2 et CO_2, la pertinence des modèles de combustion est discutée

Evaluation des flux de chaleur mis en jeu lors de la combustion stationnaire de plaques thermiquement épaisses de PMMA

Dans la communauté scientifique, un élément qui fait consensus est la difficulté à modéliser et à évaluer la source combustible, en termes de puissance et donc de débit de pyrolyse. Les modèles de pyrolyse existants, reposent le plus souvent sur des hypothèses simplificatrices. Les études expérimentales dédiées à ce problème montrent une grande dispersion des résultats. L’objectif de la présente étude est double : améliorer notre connaissance des mécanismes thermiques impliqués dans le processus de dégradation thermique des matériaux solides et acquérir des données utiles à la validation des modèles de pyrolyse. Pour cela, des expérimentations ont été conduites pour quantifier l’ensemble des flux de chaleur mis en jeu lors de la combustion stationnaire d’une plaque verticale de PMMA thermiquement épaisse. Les résultats obtenus montrent une décroissance du flux total de la flamme de 30.5 à 23.7kW/m² quand la hauteur augmente de 2.5 à 20cm alors que le flux radiatif est quasiment constant, autour de 7kW/m². La réémission de la plaque a été estimée en utilisant une procédure de retournement de la plaque, pour laquelle on observe l’extinction naturelle de la flamme. A mi-hauteur de la plaque, le flux réémis par la plaque a été estimé à 11.5kW/m², ce qui correspond à une température équivalente corps noir de 671K. Les débits massiques surfaciques ont été déduits du bilan d’énergie à l’interface et comparés à ceux obtenus par des mesures directes. L’accord est excellent. D’autres expérimentations ont par ailleurs été développées afin de déterminer les flux radiatifs à l’arrière de la plaque combustible. A partir de ces données et de la résolution numérique du problème de transferts couplés faisant intervenir le rayonnement et la conduction dans le matériau semi-transparent solide, les contributions radiatives internes provenant de la flamme, de la surface bullée, et du matériau vierge chauffé ont été estimées autour de 2.6kW/m². Références 1.Orloff et al., Proc. Combust. Inst., 15 (1974), 183–192. 2.Beaulieu et al., Fire Safety J. 43 (2008), 410–428. 3.Gollner et al., 7th US Combust. Meeting, (2011). 4.Tsai et al., Proc. 8th Symp. IAFSS (2005), 409–419. 5.Pizzo et al., Combust. Flame, 152-3 (2008), 451-460. 6.Modest M.F., Radiative Heat Transfer, Mc Graw-Hill, NY, 1993

The CD4+ T-cell transcriptome and serum IgE in asthma: IL17RB and the role of sex

<p>Abstract</p> <p>Background</p> <p>The relationships between total serum IgE levels and gene expression patterns in peripheral blood CD4+ T cells (in all subjects and within each sex specifically) are not known.</p> <p>Methods</p> <p>Peripheral blood CD4+ T cells from 223 participants from the Childhood Asthma Management Program (CAMP) with simultaneous measurement of IgE. Total RNA was isolated, and expression profiles were generated with Illumina HumanRef8 v2 BeadChip arrays. Modeling of the relationship between genome-wide gene transcript levels and IgE levels was performed in all subjects, and stratified by sex.</p> <p>Results</p> <p>Among all subjects, significant evidence for association between gene transcript abundance and IgE was identified for a single gene, the interleukin 17 receptor B (IL17RB), explaining 12% of the variance (r²) in IgE measurement (p value = 7 × 10^-7, 9 × 10^-3after adjustment for multiple testing). Sex stratified analyses revealed that the correlation between IL17RB and IgE was restricted to males only (r²= 0.19, p value = 8 × 10^-8; test for sex-interaction p < 0.05). Significant correlation between gene transcript abundance and IgE level was not found in females. Additionally we demonstrated substantial sex-specific differences in IgE when considering multi-gene models, and in canonical pathway analyses of IgE level.</p> <p>Conclusions</p> <p>Our results indicate that IL17RB may be the only gene expressed in CD4+ T cells whose transcript measurement is correlated with the variation in IgE level in asthmatics. These results provide further evidence sex may play a role in the genomic regulation of IgE.</p

The influence of walls on Lagrangian statistics in two-dimensional turbulence

International audienceThe influence of solid walls on the Lagrangian statistics of statistically stationary two-dimensional turbulence is investigated by comparing the flow in a circular wall-bounded and in an unbounded periodic domain. A Fourier pseudo-spectral method is used, which is combined in the wall bounded case with a volume penalization technique to impose no-slip conditions. A particular emphasis is put on the acceleration of fluid particles. It is investigated to what extent the impact of the boundaries influences the shape of the probability density functions of Lagrangian velocity increments. It is shown that the influence of walls is not confined to a small near-wall region but alters the statistics in the entire flow domain. This can be explained by the vorticity generation in the turbulent boundary layer which destabilizes and leads to the formation of vortices that subsequently detach and travel into the bulk flow. The enstrophy level is thus increased with respect to the one in the unbounded periodic domain

On the Assessment of the Chilean Solar Thermal Regulation Using a Modular Simulation Model Coupled to a Multiobjective Optimization Algorithm

International audienceIn Chile, the solar thermal regulation DS331, which utilizes a global modeling approach, governs the deployment of solar thermal systems (STSs) across highly variable climatic zones. This regulation’s one‐size‐fits‐all approach often misrepresents the solar potential and economic feasibility in different regions. To address these limitations, we introduce a refined modular energy model that incorporates a 1D multinode stratification technique for hot water storage. This model is associated with a multiobjective optimization process using the NSGA‐II algorithm, focusing on optimizing the solar collector area and storage volume to maximize solar fraction and life cycle savings (LCSs) across 20 major Chilean cities. Our results demonstrated that the optimized systems achieve solar fractions ranging from 0.92 to 1.00, significantly improving upon the current regulation’s performance, particularly in southern regions where solar radiation is lower. Notably, the optimized configurations suggested a potential reduction in collector areas by up to 20% and storage volumes by up to 15% compared to those recommended by DS331, while still exceeding the legal requirements for the solar fraction. This optimization made it possible to increase LCS by ~25%–30% across various scenarios, indicating a substantial improvement in cost‐effectiveness. Based on these findings, existing solar thermal regulations should be revised to take into account local climatic and consumption data. Such adjustments would ensure more accurate sizing of STS, enhanced economic viability, and greater incentive alignment for widespread adoption. This study underlines the critical role of detailed, location‐specific energy modeling in shaping effective energy policies and advancing the deployment of renewable technologies in diverse environmental contexts

Lagrangian statistics of two–dimensional turbulence in a square container

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Efficiency of laminar and turbulent mixing in wall-bounded flows

International audienceA turbulent flow mixes in general more rapidly a passive scalar than a laminar flow does. From an energetic point of view, for statistically homogeneous or periodic flows, the laminar regime is more efficient. However, the presence of walls may change this picture. We consider in this investigation mixing in two-dimensional laminar and turbulent wall-bounded flows using direct numerical simulation. We show that for sufficiently large Schmidt number, turbulent flows more efficiently mix a wall-bounded scalar field than a chaotic or laminar flow does. The mixing efficiency is shown to be a function of the Péclet number, and a phenomenological explanation yields a scaling law, consistent with the observations