Water saturation variation in a fire-heated limestone gallery monitored by ERT

SUMMARY Water migration within the rock mass during a fire has not yet been studied. Here, an innovative high-resolution micro-ERT method is developed at a centimetric scale to measure this phenomenon. The method is two-step based: first, a correction of the electrode effect, and second, the correction of resistivity for rapid variations in temperature with high values. Water migration is successfully described within the wall of an underground limestone quarry with three phases: from fire ignition to 36 min, the surface dries to a 3-cm depth and a higher saturated layer forms deeper than 0.10 m; from 36 min to 46 min (fire supply stops), the rock dries out and water migrates from deeper parts of the rock wall, deeper than 20 cm, towards the surface; finally, after fire extinction, the water migrates through a readjustment phase until it returns to its initial state

Simulation of an experimental fire in an underground limestone quarry for the study of Paleolithic fires

International audienceNumerous fire marks occur on the walls of the Chauvet-Pont d’Arc cave. Dating indicated that some of the fires were contemporary to the Aurignacian. Violent thermal shocks were observed in surprisingly narrow areas of the cave. This raises numerous archaeological questions about the function of the fires; the answers depend on the location of the hearths, and the intensity of the fires. Numerical simulation was used here to provide information about the behaviour of fires in such confined spaces. An underground non-archaeological site, in a limestone quarry, was equipped to monitor fires in an environment similar to that of the Megaceros gallery of the Chauvet-Pont d’Arc cave. The fire and the movement of heat and smoke in the quarry were simulated by the open source code “Fire Dynamics Simulator (FDS)”. Results were validated on wall temperatures recorded behind and above the fire. The thermo-mechanical impact of the fire on the rock was simulated with CAST3M software, providing the most probable zones for limestone spalling due to thermal gradients. The validated approach will, in a forthcoming study, be applied to the Chauvet-Pont d’Arc cave, in which coupled simulations in the air and in the rock should indicate the location of the hearths and the intensity of the fires that generated the marks

Heat transfer in rock masses: Application to the Lascaux Cave (France)

The conservation of prehistorical painted caves is a major issue for the history of mankind. Alterations, which can appear on walls, mainly result from the thermal convection taking place in caves. The convec-tion in caves depends on the difference between the temperatures of the walls which mainly stem from the heat conduction in rock masses. In literature, this problem is currently solved with a uni-dimensional homogeneous model where the outside air temperature is the only source of energy. Here, based on the literature in other fields, we show that the ground surface temperature cannot be considered as equal to the outside temperature. Radiation, convection and water evaporation indeed figure prominently in the process. The mathematical guideline giving the topsoil temperature is detailed. In addition, we propose a mathematical assumption to ensure energy conservation. A simple experimental setup allowing the cir-cumvention of this complex procedure is also suggested. Then, we apply the widely used uni-dimensional homogeneous thermal diffusion model, with the proposed ground surface temperature calculation, to the Lascaux Cave. The comparison between the measurements in the first meter below the ground surface and the calculated temperatures shows a good agreement. Within the cave, the model successfully pre-dicts the temperature measured by a few thermocouples. For others, there are significant discrepancies, which seem to indicate that the homogeneous hypothesis is not precise enough to describe rock masses. The heterogeneity of the Lascaux Cave massif will thus need to be thermally investigated

La 3D au service de la conservation des grottes ornées, l’exemple de Lascaux et du simulateur Lascaux

Numerical simulation in fluid mechanics is applied here to the conservation of the patrimony and more particularly to the Lascaux cave, in Dordogne, France. A very accurate 3D laser survey of the cavity of more than 150 million points was used as a base of the Lascaux simulator, a numerical tool allowing to test in laboratory, on a computer, the modifications to bring eventually to the cave, and to check the way it reacts before achieving these modifications in situ. The simulator is an innovative conserving tool for preventive conservation

Simulation numérique de l'essorage et du refroidissement d'un film liquide en mouvement par un jet plan turbulent

Les procédés d'enduction sont largement utilisés dans l'industrie, notamment en sidérugie. Pour protéger les tôles de la corrosion, celles-ci sont enduites de métal liquide par immersion dans un bain. Lorsqu'elles remontent verticalement dans l'air environnant, un jet d'air plan turbulent les impacte perpendiculairement, induisant la retombée dans le bain d'une partie du métal. Le métal resté sur l'acier se solidifie produisant de la tôle galvanisée. L'essorage pneumatique tient son nom de l'utilisation d'un jet d'air pour diminuer l'épaisseur de métal liquide. Parmi les difféntes techniques d'enduction, l'essorage pneumatique par un jet plan est l'une des plus efficaces mais aussi des plus difficles à maîtriser. En effet, au cours de ce processus, l'impact du jet plan turbulent sur le film liquide fait intervenir l'interaction entre les écoulements diphasiques et turbulents, phénomène physique complexe. relativement peu de références bibliographiques font écho de la simulation numérique du couplage diphasique / turbulent en général, et aucune n'a été consacrée à la simulation de l'essorage pneumatique en particulier. Cette thèse s'inscrit dans ce contexte industriel. Elle présente une revue des différents modèles utilisés pour décrire les écoulements diphasiques d'une part et turbulents d'autre part, ainsi que la modélisation du couplage entre les écoulements diphasiques et turbulents. Les méthodes numériques employées pour résoudre le système des équations constitutives sont abordées puis validées. L'outil numérique est alors utilisé pour répondre aux problématiques industrielles d'enduction et d'essorage pneumatique, permettant ainsi d'améliorer la compréhension des phénomènes et d'optimiser le procédé d'essorage pneumatique.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

A laboratory cave for the study of wall degradation in rock art caves: An implementation in the Vézère area

The aim of this proposal is to present an original approach to the study and preservation of rock art caves. A multidisciplinary study of cave wall alteration will be performed to understand the impact of environmental context on the evolution of wall surfaces. The approach involves the choice of a cave with characteristics similar to painted caves in the studied area (Vézère Valley in Dordogne, France): e.g., cave wall alteration, lithology, morphology, etc. This selected cave is intended to become a laboratory cave, monitored for the acquisition of chemical, physical and biological environmental data on bedrock, air and fluids along with their characteristics. A cave without art or archaeological interest has been chosen specifically to conduct experiments, to make in situ analysis and to obtain samples. The results are to be completed by specific observations in several other caves and compiled in a database.Peer Reviewe

Étude des transferts de masse et de chaleur dans la grotte de Lascaux : le suivi climatique et le simulateur

Mass and heat transfer in Lascaux cave : cumatological observations and simulations The cave of Lascaux, discovered in 1940 and located in the Dordogne area in France, is inscribed in the Unesco World Heritage List. It is considered as one of the major prehistoric caves in the world. Since its discovery, several problems have occurred, due to the huge amount of visitors, and their release of vapour and carbon dioxide by their breath, causing the formation of calcite and the apparition of green algae and mosses. The Ministry of Cultural Affairs had the cave closed in 1963. Since then, prehistorians, archaeologists, geologists, and hydrogeologists have tried hard to maintain the cavity in the most stable state possible, using remote monitoring to record the variations in temperature, hygrometry, and carbon dioxide gas pressure. The biological equilibrium remained fragile, and in 2001, colonies of micro-organisms, mushrooms and bacteria developed on the rock edges and on the floor. This attack made the authorities and the Minister of Culture and Communication create the scientific international committee of Lascaux cave, a multidisciplinary committee (composed of archaeologists, physicists, geologists, hydrogeologists, and conservators working together) to understand the mechanisms of apparition of the micro-organisms in order to stop their propagation. Among the measures taken by the committee, a better understanding of the flows in the cave was considered very important, and has induced the creation of a simulation tool, the «Lascaux Simulator». The non-intrusive character of simulation is one of the major assets of this method. The numerical simulation in fluid mechanics is here dedicated to the conservation of the cave of Lascaux. The simulator is based on a computational fluid dynamics code named Aquilon. A three dimensional survey has been done in the cave using laser scanning. An accurate topology of the environment is incorporated in the simulator. Starting from this point, governing equations of the fluid mechanics are solved and parameters such as temperature, velocity or moisture content are known in every point of the cavity. Thermal conditions are chosen based on the analysis of the calculated and measured temperature data for more than 50 years. In this article, two configurations are presented, the first one in September 1981, a period during which the cave remained in a stable state regarding condensation, and the other one in December 1999 ; at this time, temperature was reversed, the ground of the cavity was colder than the ceiling. This phenomenon implied an inversion of the air flow in the cave. Finally, the removal of the dividing wall of the «Bauer airlock» has been simulated, and it has been shown that the impact on the cavity would be negligible.La grotte de Lascaux, découverte en 1940 et située en Dordogne dans la vallée de la Vézère est inscrite au Patrimoine Mondial de l'Unesco. Elle est considérée comme l'une des grottes ornées les plus importantes au monde. Les visites qui ont suivi sa découverte ont perturbé l'équilibre de la cavité et induit sa fermeture en 1963. Depuis, elle a retrouvé un état à peu près stable, jusqu'en 2001 où une attaque de micro-organismes a eu lieu. Suite à cette prolifération, un comité scientifique international pluridisciplinaire a été créé par le ministre de la Culture et de la Communication. Ce comité a d'emblée souhaité disposer d'une meilleure compréhension des écoulements présents dans la cavité. Pour répondre à cette demande un nouvel outil a été créé, le «Simulateur Lascaux», basé sur un code de mécanique des fluides numérique. Un des principaux atouts de cette méthode est son caractère non intrusif. Deux configurations climatiques ont été choisies dans cet article. En septembre 1981, la grotte était dans des conditions «stables» : les températures dans la roche entourant la grotte suivaient le schéma classique augmentant avec la profondeur. La seconde configuration (en décembre 1999) concerne une période précédant des travaux dans la cavité : les températures se sont inversées se trouvant être plus élevées à la voûte qu'au sol. Cette inversion a conduit au confinement de la grotte. Finalement, les écoulements résultant du retrait d'une cloison ont été appréciés grâce au simulateur, et il a été montré un impact négligeable de ce retrait sur l'équilibre de la cavité.Lacanette Delphine, Malaurent Philippe, Caltagirone Jean-Paul, Brunet Jacques. Étude des transferts de masse et de chaleur dans la grotte de Lascaux : le suivi climatique et le simulateur. In: Karstologia : revue de karstologie et de spéléologie physique, n°50, 2e semestre 2007. Suivi climatique dans la grotte de Lascaux. pp. 19-30