5 research outputs found
ModƩlisation Thermoacoustique de Bas Ordre et Simulation de la Fonction de Transfert d'une Flamme Diphasique
Les instabiliteĢs thermoacoustiques continuent dāeĢtre un obstacle majeur dans le deĢveloppement des systeĢmes de combustion des turbines aĢ gaz. Ces instabiliteĢs sont caracteĢriseĢes par des oscillations de pression de grande amplitude dans la chambre de combustion. Elles sont indĆ©sirables car elles entraiĢnent de fortes vibrations augmentant le bruit et les Ć©missions de polluants, provoquant des contraintes thermiques et meĢcaniques excessives sur les composants de la chambre de combustion, voire menacĢ§ant lāinteĢgriteĢ structurelle du systeĢme complet. La simulation aux grandes eĢchelles (LES) sāest aveĢreĢe eĢtre un outil puissant capable de preĢdire de nombreux pheĢnomeĢnes de combustion instationnaire, y compris les instabiliteĢs. Cependant, les couĢts de calcul eĢleveĢs associeĢs empeĢchent cette approche dāeĢtre utiliseĢe en phase de conception pour analyser toutes les conceptions possibles et les conditions de fonctionnement auxquelles les instabiliteĢs restent extreĢmement sensibles. Cāest pourquoi les modeĢles de bas ordre (LOM) sont preĢcieux et compleĢtent bien les LES, en particulier pendant les eĢtapes de preĢconception de la chambre de combustion. Bien que la plupart des outils LOM disponibles effectuent des simplifications physiques importantes (par exemple, lineĢarisation de lāacoustique, reĢponse aĢ la flamme), ils utilisent eĢgalement geĢneĢralement des geĢomeĢtries trop simplifieĢes. Lāun des principaux objectifs de ce travail est de remeĢdier aĢ cette dernieĢre limitation et dāameĢliorer les techniques LOM existantes pour pouvoir geĢrer des geĢomeĢtries reĢalistes complexes.
Une grande partie du travail sāarticule autour du deĢveloppement et de la validation dāun nouvel outil de modeĢlisation de reĢseaux acoustiques baseĢ sur des ex- pansions modales (Galerkin Series) et des meĢthodes dāespace dāeĢtats (viz. STORM) pour preĢdire et analyser les instabiliteĢs. Dans STORM, un systeĢme complexe aĢ analyser est deĢcomposeĢ et repreĢsenteĢ comme un reĢseau dāeĢleĢments geĢomeĢtriques plus simples (sous-domaines), de connexion (couplage), de flamme et dāeĢleĢments dāimpeĢdance. Les caracteĢristiques uniques de STORM sont la technique dāexpansion modale sur des Frame reĢcemment introduite pour modeĢliser lāacoustique dans les sous-domaines du reĢseau et la meĢthodologie dite des connexions spectrales de surface qui a eĢteĢ deĢveloppeĢe reĢcemment au CERFACS. Ensemble, ils permettent des inter- connexions transparentes entre les sous-domaines avec une acoustique 1D/2D/3D et construisent des reĢseaux repreĢsentant des configurations complexes pertinentes pour lāindustrie. Les meĢthodes dāapproximation rationnelle sont discuteĢes pour incorporer des modeĢles reĢalistes dāinteraction flamme/acoustique (cāest-aĢ-dire, les fonctions de transfert de flamme (FTF) dans les reĢseaux STORM. Lāimportance de quelques contraintes physiques, en particulier la causaliteĢ, dans les algorithmes deĢri- vant ces modeĢles de reĢponse de flamme dāordre infeĢrieur, dans le domaine temporel, dans lāespace dāeĢtats et baseĢs sur les donneĢes aĢ partir de donneĢes de simulation expeĢrimentales ou dāordre eĢleveĢ, est mise en eĢvidence. Un type speĢcial dāeĢleĢment dāimpeĢdance de reĢseau, DECBC (Delayed Entropy Coupled Boundary Condition), est eĢgalement deĢveloppeĢ pour faciliter la preĢdiction des instabiliteĢs mixtes entropie- acoustique. Dans lāensemble, STORM preĢsente un outil efficace, modulaire et flexi- ble pour preĢdire les instabiliteĢs thermoacoustiques et devrait aider aĢ deĢterminer les reĢgimes de stabiliteĢ et les strateĢgies de controĢle passif optimales.
Dans la deuxieĢme partie mineure de la theĢse, le forcĢ§age acoustique de la flamme de pulveĢrisation tourbillonnante turbulente est simuleĢ en utilisant lāapproche Euler- Lagrange (EL) LES. Lāobjectif eĢtait de calculer le FTF et dāeĢvaluer la pertinence du cadre de modeĢlisation de la combustion diphasique EL-LES existant pour un tel probleĢme dāidentification de systeĢme. Des travaux reĢcents ont deĢmontreĢ le potentiel de EL-LES pour preĢdire avec preĢcision lāinstabiliteĢ auto-entretenue. Cependant, les simulations forceĢes preĢsentent certaines difficulteĢs et la FTF obtenue numeĢriquement sāeĢcarte des valeurs de reĢfeĢrence expeĢrimentales dāenviron 20 aĢ 30%. Les reĢsultats restent sensibles, en geĢneĢral, aux parameĢtres de modeĢlisation, si bien que dāautres investigations seront neĢcessaires pour ameĢliorer les modeĢles et la fideĢliteĢ des preĢvisions
Unleashing the full power of LHCb to probe Stealth New Physics
We thank the participants in the \Stealth Physics at LHCb" workshop, held in Santiago
de Compostela in February 2020 [333], for many fruitful discussions and for triggering
many ideas that were developed here. We also thank the LHC Long-Lived Particle
community, and the LPCC LHC Long-lived Particle and Dark Matter working groups for
fostering and nurturing the interest in the ideas here discussed. We kindly acknowledge
Vladimir Gligorov, Stephen Farry and Niels Tuning for kindly reading and providing
comments to this document. This work has been supported by MINECO (Spain) through
the RamĆ³n y Cajal program RYC-2016- 20073 and by XuntaGAL under the ED431F
2018/01 project. It has also received nancial support from XuntaGAL (Centro singular
de investigaciĆ³n de Galicia accreditation 2019-2022), by European Union ERDF, by
the \MarĆa de Maeztu" Units of Excellence program MDM-2016-0692 and the Spanish
Research State Agency, and by the Generalitat Valenciana (Spain) through the plan GenT
program (CIDEGENT/2019/068).In this paper, we describe the potential of the LHCb experiment to detect Stealth
physics. This refers to dynamics beyond the Standard Model that would elude
searches that focus on energetic objects or precision measurements of known processes.
Stealth signatures include long-lived particles and light resonances that are produced
very rarely or together with overwhelming backgrounds. We will discuss why LHCb
is equipped to discover this kind of physics at the Large Hadron Collider and provide
examples of well-motivated theoretical models that can be probed with great detail
at the experiment.MINECO (Spain) through
the Ram on y Cajal program RYC-2016- 20073XuntaGAL under the ED431F
2018/01 projectXuntaGAL (Centro singular
de investigaci on de Galicia accreditation 2019-2022)European Union ERDFMarĆa de Maeztu" Units of Excellence program MDM-2016-0692Spanish
Research State AgencyGeneralitat Valenciana (Spain) through the plan GenT
program (CIDEGENT/2019/068
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The past : a compass for future earth
Antarctic sea ice impacts on the ocean-atmosphere heat and gas fluxes, the formation of deep and intermediate waters, the nutrient distribution and primary productivity, the so-called ‘biological carbon pump’, one of the most active in the global ocean. In this study, we explore the link between sea ice dynamic, biological production and nutrient cycling during the late Holocene (the last 2,000 yrs) in the AdĆ©lie Basin, East Antarctica, from the well-dated sediments of the Ocean Drilling Program (ODP) Site U1357. This archive, composed from ~32 meters of seasonal to annual laminated diatomaceous sequences, allows reconstructions at an unprecedented time resolution (5-10 yrs). Our study combines records of diatom census counts and diatom-specific biomarkers (a ratio (D/T) of di- and tri-unsaturated Highly Branched Isoprenoid lipids (HBI)) as indicators of sea ice and biological production changes, XRF data as markers for terrigenous inputs and bulk nitrogen isotopes (d15N) and d15N on chlorins as proxies for reconstructing nitrogen cycle. The diatom and HBI records reveal five distinct periods. From 0 to 350 yrs AD, decreasing occurrences of sea ice-related diatom species (e.g. Fragilariopsis curta + F. cylindrus) together with low D/T values and increasing open ocean diatom species (large centrics, Chaetoceros Resting Spores (CRS)) document a progressive decline of sea ice presence during the year (>9 months per year) with spring melting occurring earlier in the year and autumn sea ice formation appearing later. In contrast, between 350 and 750 yrs AD, high production of open ocean diatom species and low low D/T values and sea ice related species indicate a short duration of sea ice cover (~10 months per year) is illustrated by a pronounced increase of sea ice-associated diatom species and high D/T values. Between ~1400 and 1850 yrs AD, seasonal sea ice strongly declines (<~7 months per year) as a result of early spring melting (increasing CRS production) and late autumn waxing (high occurrences of Thalassiosira antarctica). Longer growing seasons promoted a substantial development of phytoplankton communities (especially large centric diatoms) that conducted to lower D/T values. Consistent with diatom and HBI reconstructions, XRF data show higher Fe/Al and Zr/Al ratios values during inferred warmer periods and lower ratio values during inferred cooler and icier periods, thus supporting a strong impact of the sea ice seasonal cycle on glacial runoffs. The link between sea ice conditions, biological production and nutrient cycling is still being explored and we will discuss its relationship by combining all the cited records cited above with the d15N records that we are currently generated. Based on our results, we find that sea ice dynamic and associated diatom production in the AdĆ©lie Basin revealed an opposite climatic trend than that identified in the Northern Hemisphere for the last 2000 years. The 'Little Ice Age' (1400-1850 yrs AD) or the 'Dark Ages' (400-750 yrs AD) corresponded to warmer climate conditions in the AdĆ©lie Basin, while the 'Roman Warm Period' (0-350 yrs AD) or the 'Medieval Warm Period' (900-1200 yrs AD) were associated to colder conditions. We therefore emphasize that Northern and Southern Hemisphere climate evolved in anti-phase seesaw pattern during the late Holocene