Dual steady solutions in natural convection between horizontal concentric cylinders

Dual steady solutions in natural convection in an annulus between two horizontal concentric cylinders are numerically investigated for a fluid of Prandtl number 0.7. It is found that, when the Rayleigh number based on the gap width exceeds a certain critical value, dual steady two-dimensional (2-D) flows can be realized: one being the crescent-shaped eddy flow commonly observed and the other the flow consisting of two counter-rotating eddies and their mirror images. The critical Rayleigh number decreases as the inverse relative gap width increases.