HR: 1340h
AN: T33A-1355    [Abstracts]
TI: A Numerical Simulation of Longitudinal Rolls and Its Implications to Small Scale Convection in the Mantle
AU: * Bondar, F
EM: bondar@uiuc.edu
AF: Department of Theoretical and Applied Mechanics, University of Illinois, 216 Talbot Lab, 104 S. Wright, Urbana, IL 61801 United States
AU: Hsui, A T
EM: hsui@uiuc.edu
AF: Department of Geology, University of Illinois, 1301 W. Green, 245 NHB, Urbana, IL 61801 United States
AU: Riahi, D N
EM: d-riahi@uiuc.edu
AF: Department of Theoretical and Applied Mechanics, University of Illinois, 216 Talbot Lab, 104 S. Wright, Urbana, IL 61801 United States
AB: The possible existence of longitudinal rolls in the Earth's mantle was first proposed by Frank Richter over three decades ago. Subsequently, many investigators have presented various observations to suggest their existence with varying degrees of success. In recent years, however, this feature has received renewed interests. Based on seismic anisotropy studies, such a structure has been suggested to exist beneath the Pacific Plate as well as within the D" layer above the CMB. Although the onset of longitudinal rolls has been investigated and laboratory experiments have been conducted to study their structures, direct numerical simulations have received relatively little attention. In this paper, we shall present some of our simulation results to gain improved understanding of longitudinal rolls and their possible implications to the structure of the mantle. Our simulations are carried out using FLUENT 6, a commercial computational fluid dynamic package available at the NCSA of the University of Illinois. We have examined flow structures over a range of Rayleigh numbers varying from 5000 to 10$^{6}$, and Peclet numbers ranging from 10 to 10$^{3}$. Our results show that longitudinal rolls can exist only within a range of specific combinations of Rayleigh and Peclet numbers, consistent with the stability analysis result of Koranaga and Jordan (2003). Particle motions of the longitudinal rolls appear to follow a helical path. This may have implications to the seismic anisotropy investigations. In addition, we have also examined the effect of a stress-free bottom boundary to study if such structure in the D" layer can exist. Our results show that longitudinal rolls can exist with a stress free bottom boundary. However, horizontal scale of the rolls in the transverse direction becomes larger. Core temperature of the rolls is also higher than that of a no-slip bottom boundary. This is probably because heat can be brought into the system from below more easily when the boundary is stress-free.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting