The threshold of averaged thermal convection excitation in a rotating horizontal annulus is studied. The theoretical researches are devoted to the study of convection in a plane layer rotating about an axis located parallel to the layer at large distance from it. This problem corresponds to the limiting case of convection in a horizontal thin annulus rotating about its axis. The action of two mechanisms of convection, thermovibrational and centrifugal, are taken into account. In experiments, the mean thermal convection in a relatively thick annulus is studied. The case of more heated inner boundary is considered. The results of linear stability theory are in a good agreement with the experimental data in a wide interval of dimensionless parameters: centrifugal and vibrational Rayleigh numbers. It is found that in a thick annulus the subcritical convective flows in a form of toroid vortexes affect the threshold of mean convection excitation. It is supposed that these relatively weak subcritical flows are generated by the inertial waves propagating from the cavity ends.
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