Calcul multi-domaines et approches multi-échelles pour la simulation numérique de crashs automobiles

En dynamique explicite, l\u27analyse des structures est souvent très coûteuse à cause du grand nombre de pas de temps nécessaire à la stabilité des modèles. Le but est ici de proposer une méthode originale basée sur les techniques de décomposition de domaine, afin de réduire les temps de calcul. Une méthode de gestion du contact unilatéral entre sous-domaines a tout d\u27abord été introduite (méthode de la peau artificielle). Celle-ci repose sur la superposition d\u27une interface de contact (par pénalité) et d\u27une interface de liaisonelle renvoie ainsi le traitement du contact inter-domaine à un contact interne au domaine. Par ailleurs, une méthode de raffinement a été intégrée à l\u27approche multi-domaines afin d\u27enrichir localement et temporairement le maillage au cours du calcul. La méthode étant rendue particulièrement instable par l\u27utilisation du schéma explicite, l\u27accent a été mis sur la manière de rééquilibrer la solution avant la reprise des calculs sur la nouvelle discrétisation

Pollen use for the spatial high-resolution reconstruction of Plio-Pleistocene Hominini environments in the East African Rift (Ethiopia and Tanzania)

Cette thèse présente l'étude du signal pollinique de sédiments modernes et fossiles pour contribuer à une meilleure connaissance des micro-habitats associés aux Hominini du Plio-Pléistocène du rift africain. Grâce à un jeu de données modernes de 283 spectres polliniques de surface (dont 62 nouveaux) centré sur l’Ethiopie, il a été développé une approche qui permet maintenant d'identifier 24 types de végétations dont 9 ripariennes. Appliquée aux 42 spectres (dont 9 nouveaux ) des formations pliocènes de Hadar et Busidima (basse vallée de l'Awash, Éthiopie), cette approche montre que, entre 3,42 et 2,96 Ma, le Nord Afar a subi d'importants refroidissements (-6,5 à -10,5°C) associés aux évènements globaux des stades isotopiques marins M2, MG2 et MG4. Ceux-ci ont induit d’importants changements de végétation à l’échelle régionale durant cette période, alors qu’au même moment des formations arborées se maintenaient, localement, à proximité de la paléorivière Awash et de résurgences d'eau souterraine offrant ainsi un habitat boisé stable à Australopithecus afarensis. L’analyse pollinique d’un sondage sub-moderne en bordure du Lac d’Eyasi (Tanzanie) éclaire sur la dynamique de végétation associée aux résurgences et au climat régional en zone aride. Enfin, les reconstructions de paléovégétations associées aux Hominini étant limitées par les échantillons sub-stériles, il est développé ici un système de reconnaissance automatique. Basé sur des réseaux de neurones convolutifs, ce système est capable d'identifier avec succès (taux d'erreur <4%) des grains de pollens modernes et fossiles, quel que soit leur état de dégradation, ce qui est une avancée majeureThis thesis presents the study of the pollen signal in modern and fossil sediments to contribute to a better understanding of the micro-habitats associated with the Plio-Pleistocene Hominini of the African Rift. With a modern dataset of 283 surface pollen spectra (including 62 new) centered on Ethiopia, an approach has developed that now allows the identification of 24 vegetation types including 9 riparian. Applied to the 42 spectra (including 9 new) of the Pliocene Hadar and Busidima Formations (Lower Awash Valley, Ethiopia), this approach shows that between 3.42 and 2.96 Ma, North Afar underwent significant cooling (-6.5 to -10.5°C) associated with the global events of the marine isotopic stages M2, MG2 and MG4. These induced important changes in vegetation on a regional scale during this period, while at the same time wooded formations were maintained, locally, near the Awash paleoriver and groundwater springs thus providing a stable wooded habitat for Australopithecus afarensis. Pollen analysis of a sub-modern sediment core near the shores of Lake Eyasi (Tanzania) sheds light on the vegetation dynamics associated with resurgences and the regional climate in arid zones. Finally, as the reconstructions of paleovegetations associated with the Hominini are limited by the sub-sterile samples, an automatic recognition system is developed here. Based on convolutional neural networks, this system is capable of successfully identifying (error rate <4%) modern and fossil pollen grains, whatever their state of degradation, which is a major advanc

Lower to Mid-Pliocene pollen data from East African hominid sites, a review

International audienceThis paper presents fossil pollen data (39 samples) available for the Lower Pliocene outcrops at five hominid sites in East Africa located within the intertropical region between 3° South to 11° North. They are dated from 4.2 to 2.95 millions years (Ma). Most of these data were obtained many years ago, except one additional pollen assemblage recently obtained in the Woranso-Mille area of the Lower Awash valley, Ethiopia. In East Africa, Plio-Pleistocene sedimentary sequences are internationally renowned for the continuous palaeo-anthropological researches pursued for decades, following the discoveries of preserved remains of fossil hominins associated to a rich, diversified and abundant mammalian fauna. As a result of these new discoveries, geological studies have continued, being connected to following field expeditions and exploration of new sites in the Ethiopian Rift. We present here the revised geological context, adding chronological precision to previous age constraints of the original pollen data. For each terrestrial Pliocene site, we also include some comments regarding the significance of pollen counts and briefly discuss their interpretation for reconstructing past vegetation, environment, and inferred climatic parameters at the studied hominin sites

Bilobate Phytolith Size Matters for Taxonomical and Ecological Identification of Chad Grasses: A Case Study on 15 Species

International audienceThe Poaceae family produces diagnostic phytoliths commonly called Grass Silica Short Cell Phytoliths (GSSCP), of which shapes and sizes are very distinguishable from those produced by other plant families. Grass Silica Short Cell Phytoliths are usually classified into four main categories: the rondels, bilobates, crosses, and saddles. Among them, the bilobates have in common a general "dumbbell" shape consisting of two lobes inter-connected by a shank. They represent the dominant morphotypes observed in the leaf tissues of many Panicoideae grass species. Besides, they tend to often be related to this subfamily in the fossil record, especially when they occur in large amounts. Several studies based on modern African grasses have however demonstrated that bilobates were also abundantly produced by a few members of the Aristidoideae, Chloridoideae, and Oryzoideae grass subfamilies which, like Panicoideae, are today mainly distributed in tropical low-altitude areas. The relationship between bilobates and Panicoideae is thus not so direct in the tropics. As a result, it can lead to significant misinterpretations when applied to paleoenvironmental and archaeological surveys. This study investigates variations in the size of bilobate phytoliths in 15 African grass species, including eight Panicoideae, three Aristidoideae, two Chloridoideae, and two Oryzoideae, with the intention to explore new alternatives for refining their identification in the past deposits of tropical Africa. Different statistical analyses (Principal Component Analysis, Mann\textendash Whitney tests, decisional trees) applied to the previous dataset allowed testing whether: (i) bilobate length and width significantly vary between grass species; (ii) these variations (if so) have the potential to be used as taxonomical and/or ecological proxies. Our analyses confirm that bilobate length is significant in discriminating dry-adapted Aristidoideae and Chloridoideae species from the other sub-families. In particular, a bilobate base longer than 25 μm is almost exclusively observed for Aristidoideae and Chloridoideae, which is in agreement with previous studies. Although subject to less variations, the length/width of bilobate shanks and the average length of their lobes appear significantly useful to refine grass taxonomical identification and discriminate < 120 cm-high grasses from taller grasses. Yet, it seems that the key towards a robust identification of bilobate phytoliths involves considering the size parameters

Calcul multi-domaines et approches multi-échelles pour la simulation numérique de crashs automobiles

En dynamique explicite, l analyse des structures est souvent très coûteuse à cause du grand nombre de pas de temps nécessaire à la stabilité des modèles. Le but est ici de proposer une méthode originale basée sur les techniques de décomposition de domaine, afin de réduire les temps de calcul. Une méthode de gestion du contact unilatéral entre sous-domaines a tout d abord été introduite (méthode de la peau artificielle). Celle-ci repose sur la superposition d une interface de contact (par pénalité) et d une interface de liaison ; elle renvoie ainsi le traitement du contact inter-domaine à un contact interne au domaine. Par ailleurs, une méthode de raffinement a été intégrée à l approche multi-domaines afin d enrichir localement et temporairement le maillage au cours du calcul. La méthode étant rendue particulièrement instable par l utilisation du schéma explicite, l accent a été mis sur la manière de rééquilibrer la solution avant la reprise des calculs sur la nouvelle discrétisationIn explicit dynamics, structural analysis is often very expensive because of the large number of time steps necessary to the stability of the models. The goal is here to propose an original method, based on the domain decomposition algorithms, in order to reduce the time of calculation. A method of handling unilateral contact between domains was first introduced. This method rests on the superposition of a interface contact (by penalty) and of a link interface; it thus returns the treatment of the inter-domain contact to an internal contact to the domain. In addition, a refinement method was integrated into the multi-domain approach in order to enrich locally and temporarily the mesh during calculation. The method being made particularly unstable by the use of the explicit scheme, the accent was put on the way of rebalancing the solution before the resumption of calculations on the new discretization.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

A method to handle mesh switches for non-linear structural analysis in explicit dynamics

International audienc

Vegetation dynamics of Kisima Ngeda freshwater spring reflect hydrological changes in northern Tanzania over the past 1200 years: Implications for paleoenvironmental reconstructions at paleoanthropological sites

13 pagesInternational audienceKisima Ngeda (KN), a spring on the northern margin of saline Lake Eyasi, Tanzania, sustains an Acacia-Hyphaene palm woodland and Typha swamps, while the surrounding vegetation is semi-desert. To study the vegetation changes associated with this spring, which represents a plausible modern analog for the fossil springs documented in the nearby paleoanthropological and archaeological sites of Olduvai Gorge, we analyzed the pollen content of a 43 cm-long sediment core that documents vegetation changes since the last ~1200 years (from cal yrs. C.E. 841 to 2011). Our results show that (1) Hyphaene palms, which require meso-halophytic soil conditions were most abundant in the area of the coring site until cal yrs. C.E. ~1150 when the groundwater supplying the KN spring was likely lower than at present, allowing intrusions of saline lake waters. (2) From cal yrs. C.E. ~1200, a peat began to develop, the palm woodland was replaced by a Mimosaceae woodland, and the increased presence of Typha pollen indicates the presence of more wetlands. (3) From cal yrs. C.E. 1600, the groundwater level of the KN spring increased and reached its highest level in the last 1200 years. (4) Peaks of wetland expansion, which reflect increased groundwater flow and level in response to amplified rainfall in the recharge area (Mt Oldeani, Ngorongoro Highlands), occurred at cal yrs. C.E. ~1200–1400 and ~1650–2011. These outflows of groundwater at Kisima Ngeda were linked to the intensity and frequency of positive Indian Ocean Dipole (IOD) events, which trigger heavy rains in eastern Africa. We conclude that the Kisima Ngeda hydrological system, which has been active for more than 1200 years, responds rapidly to regional climate change driven by changes in the sea surface temperatures (SSTs) of the Indian Ocean. Yet, it is also capable of remaining active during dry intervals as inferred from the Kisima Ngeda record prior to cal yrs. C.E. 1200. Our results support the hypothesis that this type of system helped to maintain Plio-Pleistocene hominin populations and activities in the arid lowlands of the rift on a multi-decennial scale

Strength and failure of an aluminum/PA66 self-piercing riveted assembly at low and moderate loading rates: Experiments and modeling

International audienceThe self piercing riveting (SPR) process is increasingly used in the automotive industry due to its ability to connect multi-materials for weight reduction considerations. The strength and failure of such multi-material assemblies need to be characterized and modelled for full-scale structural computations. An aluminum/PA66 composite 2-sheet SPR assembly is characterized in pure and mixed tensile/shear single connection Arcan experiments. The peak force and dissipated energy increase with the loading angle. Neither loading velocity sensitivity in the range 0.016mm/s−100mm/s nor Pa66 composite fiber orientation sensitivity are observed. Some failure modes of aluminum-aluminum SPR are observed. Experiments are also carried out on a single hat component with multiple connections for two loading rates: 5mm/s and 1500 mm/s. The maximum forces and dissipated energies slightly increase at 1500mm/s. Both tensile and shear dominated mode mix ratio values are experienced by the connections. A 13 independent parameter SPR connector model is employed to model the metallic-composite SPR joint. It features three b-norm criteria employed for irreversible deformation, maximum force and failure. It can be generated by combining some elementary behaviors of the general connector model of Abaqus. The calibration procedure decouples the pure tensile/shear contributions from the mixed tensile/shear ones. The parameters are identified based on the Arcan tests and validated on the component experiments. The overall comparison between computations and experiments show satisfactory results

Springs, palm groves, and the record of early hominins in Africa

International audienceHominins evolved in Africa during a period of overall regional cooling, drying, and increasingly variable climate. Despite prevailing regional aridity since the mid-Miocene, data show that early hominins Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus ramidus lived in environments made of mosaics of grasslands, mixed grasslands, woodlands, and forests, where wooded habitats were maintained by edaphic rather than regional (climatic) humidity. Groundwater systems (springs, seeps, shallow aquifers) and surface water (rivers, lakes), locally create wetter and more wooded environments in addition to that supported by precipitation alone. However, edaphically sustained woodlands are rare to missing in most published paleoeclogical interpretations of hominin archeological sites. To explore the importance of groundwater to the record of hominins in Africa, we provide newly acquired field data from spring sites in the Awash Valley, Ethiopia, and Lake Eyasi-Lake Manyara region, Tanzania, and re-evaluate published data from the Ardipithecus-bearing Aramis Member, Ethiopia. Results show that (1) in arid Eastern Africa, a wide variety of microhabitats such as groundwater-fed wetlands, Hyphaene palm woodlands, Phoenix reclinata palm woodlands, and structurally complex and species-rich forest patches exist due to local variability of geologic, topographic, and hydrologic conditions. (2) These microhabitats carry some characteristic pollen and phytolith signals, that may be easily masked by the signal of surrounding grass-dominated shrublands and grasslands. (3) The Aramis Member (Awash Valley, Ethiopia), which is to date, the best documented paleo-groundwater ecosystem, is not a riparian habitat. It is one of N 50 examples (within 22 geographically distinct areas) in Africa and the Middle East where evidence of groundwater systems co-exist with hominin and/or archeological remains. Springs are commonly localized features of limited area within a landscape, but provide ecological continuity through time and diverse microhabitats, some of which may be densely forested. At the local scale, springs create microclimates, distinctive vegetation, and increase soil nutrients, species richness, structural complexity, and provide habitat for animals. At the landscape scale, they represent hydro-refugia favoring increased connectivity among animals and allowing migrations during dry periods. We conclude that in the East African Rift where low, highly seasonal rainfall and high evaporative demand limit vegetation growth in many areas, groundwater-fed zones create diverse microhabitats and play a major role in ecosystem functioning. It is likely that, within a context of increasing aridity and expansion of grass-dominated open habitats during the Mio-Pliocene, early hominins and many other animals viewed edaph-ically sustained woodlands as attractive habitats