Diffusion turbulente anisotrope dans les zones radiatives d'etoiles (French text)

Modelling the chemical element transport in radiative zones of stars is the one of the main difficulties in stellar evolution theory. Observations of chemical abundances on top of stars show that microscopic diffusion cannot explain the abundances anomalies. Other processes such as turbulent transport caused by instability in radiative zones must be taken into account.; In this thesis, we are interested in turbulent diffusion generated by shear instabilities and we study a highly simplified version of flows in stellar radiative zones. The flow is under Boussinesq approximation, it is two-dimensional and stably stratified. The tracer representing a non reactive chemical element is released in the flow. We did different direct numerical simulations by increasing stratification. To calculate the vertical turbulent diffusion coefficient, we use two different approaches. First, the tracer mean flux is linked to the mean vertical gradients, which enables us to define a turbulent diffusion coefficient. Then, we calculate the diffusion coefficient from the vertical displacement of Lagrangian particles with Taylor diffusion theory.; This study shows two different states of the transport dependent on stratification. For a moderate stratification, the vertical diffusion coefficient decreases faster than what is predicted by mixing length theories. This seems to result from high anisotropy between vertical and horizontal motions. At highest stratifications, we observe that mean vertical displacement of Lagrangian particles is under-diffusive and approaches an asymptotic limit. The hindrance of vertical transport of chemical elements is not taken into account in current stellar evolution models.