A critical review of buoyancy-induced flow transitions in horizontal annuli

The main mechanisms of transition of buoyancy-induced flows in the horizontal annulus between circular cylinders are reviewed, based on the available literature. Both experimental and theoretical studies are considered. The different scenarios for the evolution of the flow regimes and temperature patterns are tracked, for increasing values of the Rayleigh number, Ra. The occurrence of various instability and bifurcative phenomena is pointed out, and linked to other relevant parameters, such as the radius ratio R and the Prandtl number, Pr. Although most of the relevant literature is on 2D cases, the effect of the third dimension is considered as far as possible. Studies on the influence of the eccentricity of the inner cylinder on the laminar flow and the thermal asset are also reviewed. Finally, open questions and topics for future research are hinted at.