Simulation aéro-thermo-mécanique des effets du feu sur les parois d'un milieu confiné : application à l'étude des thermo-altérations de la grotte Chauvet-Pont d'Arc

Abstract : In 1994, the discovery of the Chauvet-Pont d'Arc cave (Ardèche, France) revealed singularanthropogenic thermal marks on walls. They are the witnesses of high intensity prehistorical firescarried out deep in the cavity. The thermoluminescence evaluation of the heating ages is consistentwith the earlier period of human occupation between 37,000 and 33,500 years ago. The archaeologistsidentified two kinds of thermo-alterations : colour changes and spallings. The colour changes resultfrom high-temperature chemical reactions in limestone, turning rock red or grey. Ex situ tests showedthat red colour happens after heating at 250oC for ten minutes while at least 350oC is necessary forgrey. Spalling stems from high stresses in rock due to restrained thermal expansion and thermohydricprocesses. In addition, part of the walls near thermo-altérations is still covered with soot. From theseclues, this investigation aims to characterize the fires of the Megaloceros Gallery which is located inthe deep part of the cave. Estimating the amounts of wood, the fires number and the ability tosupply the hearths could help make assumptions about the function of these fires.For the sake of conservation, only simulation can reproduce fires in the cave geometry withoutrequiring any reconstruction. This study is to set up a numerical modelling of fires in confinedgeometries and the induced thermal impacts on walls. A fluid-structure coupling is then developedfrom two free open source codes : OpenFOAM and Cast3m. The former manages the simulation offire scenarios through the FireFOAM tool. The latter handles the thermo-mechanical calculations inthe rock mass. To extend the initial scope of FireFOAM, some numerical models have beenimplemented in the code. This relates to soot deposit, danger assessment, thermocouple correctionand a thermal boundary condition. In addition, some modelling requirements improving the qualityof the results are detailed in the manuscript. The advanced model is then validated on experimentalfires in a former limestone quarry which has dimensions close to the Megaloceros Gallery ones. Thesame fuel (pinus sylvestris) as the one identified in the cave is burnt. The combustion led to similarthermo-alterations as those observed in the Chauvet-Pont d'Arc cave. Spallings and colour changesoccurred on the ceiling and walls of the quarry. The comparison with simulation is carried out thanksto the measurement of temperatures, velocities, soot deposits, gases and particles concentrations.The numerical model is then applied to the simulation of fires in the Megaloceros Gallery geometry.All the impacted areas of this gallery are considered and the scenarios that may have occurred arespecified. This investigation then provides an overview of the fires locations and intensities in thispart of the cave. Moreover, the compatibility with living conditions is indicated for the most powerfulfires. These information could help for archaeologists in the understanding of the functions of these fires.
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Fabien Salmon. Simulation aéro-thermo-mécanique des effets du feu sur les parois d'un milieu confiné : application à l'étude des thermo-altérations de la grotte Chauvet-Pont d'Arc. Mécanique [physics]. Université de Bordeaux, 2019. Français. ⟨NNT : 2019BORD0041⟩. ⟨tel-02303048⟩



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