Heat transfer in rapidly rotating convection with heterogeneous thermal boundary conditions

TitleHeat transfer in rapidly rotating convection with heterogeneous thermal boundary conditions
Publication TypeJournal Article
Year of Publication2017
AuthorsMound J.E, Davies CJ
JournalJournal of Fluid Mechanics
Volume828
Pagination601-629
Date Published2017/10
Type of ArticleArticle
ISBN Number0022-1120
Accession NumberWOS:000409383700023
KeywordsBenard convection; constrained convection; core-mantle boundary; cores; earths core; geophysical and geological flows; numerical-simulation; planetary; rayleigh-benard convection; rotating flows; rotationally; secular variation; seismic velocity; spherical fluid shell; unifying theory
Abstract

Convection in the metallic cores of terrestrial planets is likely to be subjected to lateral variations in heat flux through the outer boundary imposed by creeping flow in the overlying silicate mantles. Boundary anomalies can significantly influence global diagnostics of core convection when the Rayleigh number, Ra, is weakly supercritical; however, little is known about the strongly supercritical regime appropriate for planets. We perform numerical simulations of rapidly rotating convection in a spherical shell geometry and impose two patterns of boundary heat flow heterogeneity: a hemispherical Y-1(1) spherical harmonic pattern; and one derived from seismic tomography of the Earths lower mantle. We consider Ekman numbers 10(-4) <= E <= 10(-6), flux-based Rayleigh numbers up to similar to 800 times critical, and a Prandtl number of unity. The amplitude of the lateral variation in heat flux is characterised by q*(L) = 0, 2.3, 5.0, the peak-to-peak amplitude of the outer boundary heat flux divided by its mean. We find that the Nusselt number, Nu, can be increased by up to similar to 25% relative to the equivalent homogeneous case due to boundary-induced correlations between the radial velocity and temperature anomalies near the top of the shell. The Nu enhancement tends to become greater as the amplitude and length scale of the boundary heterogeneity are increased and as the system becomes more supercritical. This Ra dependence can steepen the Nu alpha Ra-gamma scaling in the rotationally dominated regime, with gamma for our most extreme case approximately 20% greater than the equivalent homogeneous scaling. Therefore, it may be important to consider boundary heterogeneity when extrapolating numerical results to planetary conditions.

DOI10.1017/jfm.2017.539
Short TitleJ. Fluid Mech.
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