Double-diffusive convection in a rotating cylindrical annulus with conical caps

TL;DR

This study investigates how combined thermal and compositional buoyancy in a rotating cylindrical annulus can lead to low-Rayleigh number convection, with implications for planetary core dynamics.

Contribution

It demonstrates that double-diffusive effects create unique instability regions and low-Rayleigh number convection, differing from purely thermal convection in planetary core models.

Findings

Double-diffusive convection can reduce the critical Rayleigh number.

Isolated instability islands are formed by a double-diffusive eigenmode.

Flow exhibits stronger time dependence and prograde zonal flow near the inner surface.

Abstract

Double-diffusive convection driven by both thermal and compositional buoyancy in a rotating cylindrical annulus with conical caps is considered with the aim to establish whether a small fraction of compositional buoyancy added to the thermal buoyancy (or vice versa) can significantly reduce the critical Rayleigh number and amplify convection in planetary cores. It is shown that the neutral surface describing the onset of convection in the double-buoyancy case is essentially different from that of the well-studied purely thermal case, and does indeed allow the possibility of low-Rayleigh number convection. In particular, isolated islands of instability are formed by an additional "double-diffusive" eigenmode in certain regions of the parameter space. However, the amplitude of such low-Rayleigh number convection is relatively weak. At similar flow amplitudes purely compositional and double-diffusive cases are characterized by a stronger time dependence compared to purely thermal cases, and by a prograde mean zonal flow near the inner cylindrical surface. Implications of the results for planetary core convection are briefly discussed.