HR: 09:05h
AN: T41H-05    [Abstracts]
TI: Scaling the heat flux through a thermal boundary layer
AU: * Choblet, G
EM: choblet@chimie.univ-nantes.fr
AF: Universite de Nantes - CNRS UMR 6112 - Laboratoire de Planetologie et Geodynamique, 2, rue de la Houssiniere, Nantes, 44322 France
AU: Labrosse, S
EM: labrosse@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, place Jussieu, Cedex 05, Paris, 75252 France
AB: Heat transfer by thermal convection is almost exclusively controlled by the nature of boundary layers: thermal interfaces within the mantle thus act on the cooling of the Earth as preponderant features. Forty years ago, L. N. Howard proposed a simple model describing time-dependent heat transfer at high Rayleigh number with the fast convective destabilization of a thermal boundary layer after a slow conductive growth. The time-averaged thickness of the boundary layer is linked to the duration of the conductive stage and allows the computation of the local Rayleigh number $Ra_{\delta}$, linked to the stability of the boundary layer. A scaling relationship for the heat flux as a function of the Rayleigh number is obtained in the form $Nu=(Ra/Ra_{\delta})^{1/3}$. It must be emphasized that this 'critical' local value is different than the classical critical value controlling the stability of the whole layer. We aim at re-evaluating Howard's model, modifying several aspects of the boundary layer characteristics and testing the predictions on convective flows: the description of Rad, as a function of the globally defined Ra and of thermal and mechanical conditions within the boundary layer, should help to refine the scaling laws for heat transfer. We use a simple numerical procedure to solve the linear stability problem for a cooled layer with various heating modes, boundary conditions and viscosity laws. These results are then compared to 3D calculations for thermal convection, concerning both the onset of convective instabilities and fully-developed, time-dependent, convective motion.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8130 Heat generation and transport
DE: 5418 Heat flow
DE: 5430 Interiors (8147)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting