Other
Scientific paper
Aug 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005jrasc..99q.144t&link_type=abstract
Journal of the Royal Astronomical Society of Canada, Vol. 99, No. 4, p.144
Other
Scientific paper
Modelling the chemical element transport in radiative zones of stars is one of the main difficulties in stellar evolution theory. Observations of chemical abundances on the surface 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. We are especially 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 will show the results of direct numerical simulations with increasing stratification. To calculate the vertical 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 Lagrangian diffusion coefficient from the vertical displacement of Lagrangian particles with Taylor diffusion theory. This study shows two different states of the transport dependant 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, which has been observed and predicted, is not taken into account in current stellar evolution models.
Lignières François
Toque Nathalie
Vincent Alain
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