Joint stereo-PIV and NO-LIF in turbulent premixed hydrogen-air flames

A new technique to simultaneously and instantaneously resolve 3D velocity/2D strain rate fields and scalar/scalar gradient fields was developed and evaluated in this study. This technique combines Planar Laser Induced Fluorescence of the NO radical (NO-PLIF) and Stereoscopic Particle Image Velocimetry (SPIV). It was found that the NO-PLIF technique allowed the determination of various iso-c contours and as such would, in principle, allow the study of the influence of the heat release on various properties, provided a calibration of the NO-PLIF signal as a function of temperature is achieved. It was also shown that the NO-PLIF technique may not be unambiguous at detecting flame extinction. The SPIV technique allowed the determination of the velocities in 3D and of the strain rates in 2D from which the most extensive and the most compressive strain rates but not the intermediate strain rate could be extracted. Information on strain rates and progress variable gradients were of particular interest in this study as they were needed to study the turbulence-scalar interaction which appears explicitly in the transport equation for the scalar dissipation rate which was derived recently. Using the technique above mentioned, this work also aimed at gathering and analysing data such as flame normal orientation, progress variable gradients, velocity change across the flame front and strain rates along the flame contours in turbulent premixed hydrogen/air flames with added nitrogen. The flame normal orientation was found to be consistent with the regime of the flames studied. A new method was designed and presented to infer from the progress variable gradients the component of the flame normal in the third dimension. The velocity change across the flame front, inferred from the SPIV data, was found to be extremely small. It is thought that the (low) heat release of the flames studied contributed more to corrugation of the flame front than acceleration of the gases across the flame front. The strain rates were studied along apparently non-wrinkled and clearly wrinkled flame contours. Their variation could not successfully be linked to curvature solely. Their values were mostly below the value expected for extinction strain rates. Last, this study aimed at investigating the turbulence-scalar interactions in turbulent premixed hydrogen/air flames with added nitrogen via the characteristics of the alignment of the flame normal vectors with the principal strain rates. The results of this study are quite different from earlier experimental results obtained for turbulent premixed ethylene/air flames. The strong preferential alignment of the flame front normal with the most extensive strain rate observed for ethylene/air flames could not be observed for the hydrogen/air flames with added nitrogen studied in the present work. The key outcome of this study was that no preferential alignment could be observed for most of the flames. A slight preferential alignment of the flame front normal with the most compressive strain rate was observed for the flames with very low adiabatic flame temperature. The differences observed were attributed partly to Lewis number effects and partly to the low heat release superimposed on the hydrodynamic fields in the flames studied

Joint stereo-PIV and NO-LIF in turbulent premixed hydrogen-air flames

A new technique to simultaneously and instantaneously resolve 3D velocity/2D strain rate fields and scalar/scalar gradient fields was developed and evaluated in this study. This technique combines Planar Laser Induced Fluorescence of the NO radical (NO-PLIF) and Stereoscopic Particle Image Velocimetry (SPIV). It was found that the NO-PLIF technique allowed the determination of various iso-c contours and as such would, in principle, allow the study of the influence of the heat release on various properties, provided a calibration of the NO-PLIF signal as a function of temperature is achieved. It was also shown that the NO-PLIF technique may not be unambiguous at detecting flame extinction. The SPIV technique allowed the determination of the velocities in 3D and of the strain rates in 2D from which the most extensive and the most compressive strain rates but not the intermediate strain rate could be extracted. Information on strain rates and progress variable gradients were of particular interest in this study as they were needed to study the turbulence-scalar interaction which appears explicitly in the transport equation for the scalar dissipation rate which was derived recently. Using the technique above mentioned, this work also aimed at gathering and analysing data such as flame normal orientation, progress variable gradients, velocity change across the flame front and strain rates along the flame contours in turbulent premixed hydrogen/air flames with added nitrogen. The flame normal orientation was found to be consistent with the regime of the flames studied. A new method was designed and presented to infer from the progress variable gradients the component of the flame normal in the third dimension. The velocity change across the flame front, inferred from the SPIV data, was found to be extremely small. It is thought that the (low) heat release of the flames studied contributed more to corrugation of the flame front than acceleration of the gases across the flame front. The strain rates were studied along apparently non-wrinkled and clearly wrinkled flame contours. Their variation could not successfully be linked to curvature solely. Their values were mostly below the value expected for extinction strain rates. Last, this study aimed at investigating the turbulence-scalar interactions in turbulent premixed hydrogen/air flames with added nitrogen via the characteristics of the alignment of the flame normal vectors with the principal strain rates. The results of this study are quite different from earlier experimental results obtained for turbulent premixed ethylene/air flames. The strong preferential alignment of the flame front normal with the most extensive strain rate observed for ethylene/air flames could not be observed for the hydrogen/air flames with added nitrogen studied in the present work. The key outcome of this study was that no preferential alignment could be observed for most of the flames. A slight preferential alignment of the flame front normal with the most compressive strain rate was observed for the flames with very low adiabatic flame temperature. The differences observed were attributed partly to Lewis number effects and partly to the low heat release superimposed on the hydrodynamic fields in the flames studied.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Does global warming favour the occurrence of extreme floods in European Alps? First evidences from a NW Alps proglacial lake sediment record

Flood hazard is expected to increase in the context of global warming. However, long time-series of climate and gauge data at high-elevation are too sparse to assess reliably the rate of recurrence of such events in mountain areas. Here paleolimnological techniques were used to assess the evolution of frequency and magnitude of flash flood events in the North-western European Alps since the Little Ice Age (LIA). The aim was to document a possible effect of the post-19(th) century global warming on torrential floods frequency and magnitude. Altogether 56 flood deposits were detected from grain size and geochemical measurements performed on gravity cores taken in the proglacial Lake Blanc (2170 m a.s.l., Belledonne Massif, NW French Alps). The age model relies on radiometric dating (Cs-137 and Am-241), historic lead contamination and the correlation of major flood- and earthquake-triggered deposits, with recognized occurrences in historical written archives. The resulting flood calendar spans the last ca 270 years (AD 1740-AD 2007). The magnitude of flood events was inferred from the accumulated sediment mass per flood event and compared with reconstructed or homogenized datasets of precipitation, temperature and glacier variations. Whereas the decennial flood frequency seems to be independent of seasonal precipitation, a relationship with summer temperature fluctuations can be observed at decadal timescales. Most of the extreme flood events took place since the beginning of the 20(th) century with the strongest occurring in 2005. Our record thus suggests climate warming is favouring the occurrence of high magnitude torrential flood events in high-altitude catchments

Chemical cycling and deposition of atmospheric mercury in Polar Regions: review of recent measurements and comparison with models

Mercury (Hg) is a worldwide contaminant that can cause adverse health effects to wildlife and humans. While atmospheric modeling traces the link from emissions to deposition of Hg onto environmental surfaces, large uncertainties arise from our incomplete understanding of atmospheric processes (oxidation pathways, deposition, and re-emission). Atmospheric Hg reactivity is exacerbated in high latitudes and there is still much to be learned from polar regions in terms of atmospheric processes. This paper provides a synthesis of the atmospheric Hg monitoring data available in recent years (2011–2015) in the Arctic and in Antarctica along with a comparison of these observations with numerical simulations using four cutting-edge global models. The cycle of atmospheric Hg in the Arctic and in Antarctica presents both similarities and differences. Coastal sites in the two regions are both influenced by springtime atmospheric Hg depletion events and by summertime snowpack re-emission and oceanic evasion of Hg. The cycle of atmospheric Hg differs between the two regions primarily because of their different geography. While Arctic sites are significantly influenced by northern hemispheric Hg emissions especially in winter, coastal Antarctic sites are significantly influenced by the reactivity observed on the East Antarctic ice sheet due to katabatic winds. Based on the comparison of multi-model simulations with observations, this paper discusses whether the processes that affect atmospheric Hg seasonality and interannual variability are appropriately represented in the models and identifies research gaps in our understanding of the atmospheric Hg cycling in high latitudes

Intermittent rivers and ephemeral streams: what water managers need to know

Peer reviewe

Le partage de la ressource en eau sur la Durance en 2050 : vers une évolution du mode de gestion des grands ouvrages duranciens ?

Congrès SHF: Water Tensions in Europe and in the Mediterranean: water crisis by 2050?, Paris, FRA, 08-/10/2015 - 09/10/2015International audienceUne vision prospective de la gestion de l'eau du bassin de la Durance et des territoires alimentés par ses eaux à l'horizon 2050 a été élaborée, appuyée par une chaine de modèles incluant des représentations du climat, de la ressource naturelle, des demandes en eau et du fonctionnement des grands ouvrages hydrauliques (Serre-Ponçon, Castillon et Sainte-Croix), sous contraintes de respect des débits réservés, de cotes touristiques dans les retenues et de restitution d'eau stockée pour des usages en aval. Cet ensemble, validé en temps présent, a été alimenté par des projections climatiques et paramétré pour intégrer les évolutions du territoire décrites par des scénarios de développement socio-économique avec une hypothèse de conservation des règles de gestion actuelles. Les résultats suggèrent à l'horizon 2050 : une hausse de la température moyenne de l'air impactant l'hydrologie de montagne ; une évolution incertaine des précipitations ; une réduction des stocks de neige et une fonte avancée dans l'année qui induisent une réduction des débits au printemps ; une diminution de la ressource en eau en période estivale ; une diminution de la demande globale en eau à l'échelle du territoire, cette demande étant fortement conditionnée par les scénarios territoriaux élaborés ici ; la satisfaction des demandes en eau en aval des ouvrages considérées comme prioritaires, au détriment de la production d'énergie en hiver (flexibilité moindre en période de pointe) et du maintien de cotes touristiques en été ;une diminution de la production d'énergie due notamment à la réduction des apports en amont des ouvrages hydroélectriques

Top-down constraints on atmospheric mercury emissions and implications for global biogeochemical cycling

We perform global-scale inverse modeling to constrain present-day atmospheric mercury emissions and relevant physiochemical parameters in the GEOS-Chem chemical transport model. We use Bayesian inversion methods combining simulations with GEOS-Chem and ground-based Hg[superscript 0] observations from regional monitoring networks and individual sites in recent years. Using optimized emissions/parameters, GEOS-Chem better reproduces these ground-based observations and also matches regional over-water Hg[superscript 0] and wet deposition measurements. The optimized global mercury emission to the atmosphere is ~ 5.8 Gg yr[superscript −1]. The ocean accounts for 3.2 Gg yr[superscript −1] (55% of the total), and the terrestrial ecosystem is neither a net source nor a net sink of Hg[superscript 0]. The optimized Asian anthropogenic emission of Hg[superscript 0] (gas elemental mercury) is 650–1770 Mg yr[superscript −1], higher than its bottom-up estimates (550–800 Mg yr[superscript −1]). The ocean parameter inversions suggest that dark oxidation of aqueous elemental mercury is faster, and less mercury is removed from the mixed layer through particle sinking, when compared with current simulations. Parameter changes affect the simulated global ocean mercury budget, particularly mass exchange between the mixed layer and subsurface waters. Based on our inversion results, we re-evaluate the long-term global biogeochemical cycle of mercury, and show that legacy mercury becomes more likely to reside in the terrestrial ecosystem than in the ocean. We estimate that primary anthropogenic mercury contributes up to 23 % of present-day atmospheric deposition.National Science Foundation (U.S.). Atmospheric Chemistry Program (1053648

Science and Management of Intermittent Rivers and Ephemeral Streams (SMIRES)

More than half of the global river network is composed of intermittent rivers and ephemeral streams (IRES), which are expanding in response to climate change and increasing water demands. After years of obscurity, the science of IRES has bloomed recently and it is being recognised that IRES support a unique and high biodiversity, provide essential ecosystem services and are functionally part of river networks and groundwater systems. However, they still lack protective and adequate management, thereby jeopardizing water resources at the global scale. This Action brings together hydrologists, biogeochemists, ecologists, modellers, environmental economists, social researchers and stakeholders from 14 different countries to develop a research network for synthesising the fragmented, recent knowledge on IRES, improving our understanding of IRES and translating this into a science-based, sustainable management of river networks. Deliverables will be provided through i) research workshops synthesising and addressing key challenges in IRES science, supporting research exchange and educating young researchers, and ii) researcher-stakeholder workshops translating improved knowledge into tangible tools and guidelines for protecting IRES and raising awareness of their importance and value in societal and decision-maker spheres. This Action is organized within six Working Groups to address: (i) the occurrence, distribution and hydrological trends of IRES; (ii) the effects of flow alterations on IRES functions and services; (iii) the interaction of aquatic and terrestrial biogeochemical processes at catchment scale; (iv) the biomonitoring of the ecological status of IRES; (v) synergies in IRES research at the European scale, data assemblage and sharing; (vi) IRES management and advocacy training