  This study offers an explanation of a recently observed effect of
destabilization of free convective flows by weak rotation. After studying
several models where flows are driven by a simultaneous action of convection
and rotation, it is concluded that the destabilization is observed in the cases
where centrifugal force acts against main convective circulation. At relatively
low Prandtl numbers this counter action can split the main vortex into two
counter rotating vortices, where the interaction leads to instability. At
larger Prandtl numbers, the counter action of the centrifugal force steepens an
unstable thermal stratification, which triggers Rayleigh-B\'enard instability
mechanism. Both cases can be enhanced by advection of azimuthal velocity
disturbances towards the axis, where they grow and excite perturbations of the
radial velocity. The effect was studied considering a combined
convective/rotating flow in a cylinder with a rotating lid and a parabolic
temperature profile at the sidewall. Next, explanations of the destabilization
effect for rotating magnetic field driven flow and melt flow in a Czochralski
crystal growth model were derived.
