HR: 0830h
AN: GP11C-0279    [PDF]
TI: Modeling Subgrid Scales in Geodynamo Simulations
AU: * Matsushima, M
EM: mmatsush@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AB: Understanding of geodynamo mechanism is in progress through numerical simulations, although parameters employed are still far from those of the real Earth. Very small diffusivities must be adopted to carry out realistic numerical simulations. In reality, a range of spatial scales of convective motions in the core is very broad, from global core scales to dissipative scales. It is impossible, at the present, to resolve such small scales in global numerical simulations, but they cannot be neglected; large-scale fields are diffused much more effectively by turbulent eddies than by molecular processes. It is hence significant to model physical processes in subgrid scales. We have been performing direct numerical simulations (DNS) of magnetoconvective turbulence in a rapidly rotating system to understand its anisotropy and to model it for use in global geodynamo simulations. We have derived expressions of the turbulent flux for heat and momentum by second moment closure. Comparison of the turbulent flux estimated in the second-moment closure model with that obtained through DNS suggests that the model represents anisotropic eddy diffusivities well. It should be noted that values of Reynolds stress obtained through DNS are used to compute the turbulent heat flux in the model, for example. When we determine the turbulent flux for heat and momentum simultaneously in the model, we find that the realizability is not always satisfied. It is necessary to improve the model. We examine the scale similarity for results of DNS. We here apply a filter function to smaller regions into which the computational box in DNS is divided. Using values thus obtained, the turbulent heat flux is computed for the whole computational region. It turns out that there is a linear relationship between the turbulent flux and the relative length scale of smaller regions. By applying this relationship to larger scales, it is possible to model subgrid scales in numerical simulations.
DE: 1507 Core processes (8115)
DE: 1510 Dynamo theories
DE: 3210 Modeling
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting