Onset of thermal convection and influence of rotation in wide spherical gap flow

The onset of thermal convection and the effect of rotation in a high Prandtl number fluid in a wide gap between two concentric spheres with an axial force field are investigated experimentally. Both spheres rotate along the vertical axis with the same angular velocity Ω while the inner one (r₁) is cooled and the outer one (r₂) is heated. The velocity field is investigated by different visualization techniques and Particle Image Velocimetry (PIV). The axisymmetric basic flow is disturbed by local instabilities. At a Rayleigh number of Ra = 6.97 · 10⁶, a pulsing vortex develops in the south polar region. A different, coexisting instability in the outer boundary layer appears at Ra = 1.79 · 10⁷. Rotating with Taylor numbers Ta > 1.4 · 10⁵, this instability vanishes. The instabilities occur mainly in the southern hemisphere where the thermal stratification is unstable.