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Weakly nonlinear mode interactions in spherical Rayleigh–Bénard convection
Published online by Cambridge University Press: 09 July 2019
Abstract
In an annular spherical domain with separation $d$, the onset of convective motion occurs at a critical Rayleigh number
$Ra=Ra_{c}$. Solving the axisymmetric linear stability problem shows that degenerate points
$(d=d_{c},Ra_{c})$ exist where two modes simultaneously become unstable. Considering the weakly nonlinear evolution of these two modes, it is found that spatial resonances play a crucial role in determining the preferred convection pattern for neighbouring modes
$(\ell ,\ell \pm 1)$ and non-neighbouring even modes
$(\ell ,\ell \pm 2)$. Deriving coupled amplitude equations relevant to all degeneracies, we outline the possible solutions and the influence of changes in
$d,Ra$ and Prandtl number
$Pr$. Using direct numerical simulation (DNS) to verify all results, time periodic solutions are also outlined for small
$Pr$. The
$2:1$ periodic signature observed to be general for oscillations in a spherical annulus is explained using the structure of the equations. The relevance of all solutions presented is determined by computing their stability with respect to non-axisymmetric perturbations at large
$Pr$.
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- © 2019 Cambridge University Press
References
Mannix and Mestel supplementary movie 1
DNS of the odd-odd $\ell=1, m=3$ periodic solution for $\mathrm{Ra}_c = 440.852, \epsilon =5.85$ and $\Pran = 0.05$. Even and odd mode components of the solution oscillate with their frequencies in a $2:1$ ratio.
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