About the onset of the Hopf bifurcation for convective flows in horizontal annuli

Experimental and numerical results cannot yet settle whether, between horizontal coaxial cylinders, if the curvature is large, the first transition for convection is an exchange of stability or rather an Hopf bifurcation. We directly show that if the curvature tends to infinity, no periodic linear perturbation exists when the Rayleigh number is equal to the critical one for nonlinear stability.