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Étude expérimentale et numérique des écoulements à surface libre en présence d'obstacles émergés et faiblement submergés

Abstract : According to the Ministry of Ecological and Solidarity Transition, floods are the main natural risk in France. As a result of climate change, extreme floods will become more frequent. People and property, such as housing and industrial installations, must therefore be protected against these floods for which data are very scarce or even non-existent. When moving from low to extreme flooding, the spatial extent of flooding in the floodplain varies greatly. The nature of the obstacles encountered varies: low vegetation, trees, houses... These hydraulic obstacles or macro-roughnesses can be emerged, or slightly submerged.The purpose of this thesis work is to analyze the dominant physical processes associated with extreme flood flows in the presence of emerged or slightly submerged obstacles. This thesis is organized in two parts: an experimental part and a numerical part.The experimental part concerns the exploration, in a laboratory channel, of the vertical structure of the velocity profile and the geometric effects related to the studied configuration representing an urbanized floodplain with immersion rates of (dollar symbol) D/h = 0.42, 0.93, 0.98(dollar symbol) and (dollar symbol)1.48(dollar symbol) ((dollar symbol) D(dollar symbol) being the water depth and (dollar symbol)h(dollar symbol) the obstacle height). ADV and PIV measurements of mean velocities and turbulent fluctuations have shown that flow properties change significantly depending on whether the obstacles are emerged or slightly submerged. The interactions between vortex detachment, the free surface, obstacles, and bottom roughness are studied.The numerical part analyses different types of modelling conventionally used to simulate flood events. The experimental case of the thesis serves as a reference.First, an analytical model based on the conservation of momentum of an elementary hydraulic volume allows considering different models of the overall flow resistance to obstacles in a uniform regime. The first simple model integrates the resistance to flow due to bottom friction and obstacles in a single "friction" type term. A second model divides the flow resistance into two parts: a bottom friction term and an obstacle resistance, represented by a drag coefficient. The obstruction to the flow is then represented by a porosity coefficient.The analysis focuses on the ability to use friction or drag coefficients calibrated for the lowest flow rates to simulate high flow rates, and in particular, the transition from emergent to submergent obstacle flows.Then, the obstacles are explicitly represented in a two-dimensional Shallow-Water model. Mesh convergence is studied and a comparative analysis of experimental and simulated results is conducted.Furthermore, the ingluence of the geometric distribution of obstacles on the vertical profile of the average longitudinal velocity is studied with 3D-LES model using Code_Saturne. The experimental results serve as a reference case for validation.Recommendations are made for how to model these flows for operational applications
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Marina Oukacine. Étude expérimentale et numérique des écoulements à surface libre en présence d'obstacles émergés et faiblement submergés. Mécanique des fluides [physics.class-ph]. Université Paris-Est, 2019. Français. ⟨NNT : 2019PESC1019⟩. ⟨tel-02948861⟩

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