Natural convection in air-filled horizontal annuli is studied numerically for radius ratio and Rayleigh number in the ranges
and
Three complementary approaches are used to investigate stability and multiplicity of flow states. A systematic investigation performed for two-dimensional flows leads first to establish a stability diagram of flow regimes as a function of Ra and R. Flow transitions as a function of R, from narrow to large gap annuli of infinite length, are then examined by using 3D-linear stability computations. New stable and unstable regions are highlighted and are strengthened thanks to direct 3D-numerical experiments for finite length annuli. The influences of both the axial aspect ratio on the onset of instabilities and the initial conditions on the asymptotic flow states are also examined.
