Dynamique des panaches thermiques laminaires : application aux panaches mantelliques

In Earth Sciences, hot upwelling plumes are thought to develop from the base of the 2900 kmthick solid mantle of our planet and to generate hotspots, i. E. Intraplate volcanic islands such as Hawaii and La Reunion. Although generated through chaotic Rayleigh-B´enard instabilities at a Prandtl number (Pr) around 1023, they can be modelled with the simpler case of starting plumes out of a finite-size heater, either numerically for infinite Prandtl number, or in the laboratory with fluids with Pr ~ 103 - 106. Hence, the question is to find simple scaling laws for isolated rising plumes and apply them to the Earth’s mantle case. In this thesis we study the characteristics of an isolated plume growing in a viscous fluid with constant viscosity. We use both laboratory experiments and numerical models : the visualization techniques give us access to the growing plume temperature and velocity fields, on a 2-D section of the tank, whereas the numerical simulations are axisymmetric finite element simulations that attempt to reproduce the laboratory conditions as closely as possible. We find excellent quantitative agreement between the two fully independent approaches. This is used to derive scaling laws for the dynamics of the plume head and stem, and apply them to the Earth’s case. We further show that for Pr larger than 1000, confinement effects are more important than inertial effects and that plumes dynamics are well described by the approximation if infinite Prandlt number. This is especially true for planetary mantles