Flow structure in a Rayleigh-Benard cell upon impulsive spin-up

We investigate convection in a cylindrical Rayleigh-Benard cell with radius -to-height ratio Gamma = 1/2. The cell is subjected to impulsive spin-up ab out its vertical axis. We use TLC (thermochromic liquid crystal) imaging fo r temperature field measurements and PIV (particle image velocimetry) for v elocity reconstruction of the transition in the range of Rayleigh numbers R from 5 X 10(7) to 5 X 10(8) and dimensionless rotation rates Omega from 0 up to 8 X 10(4). The initial (at rest) and the final (in steady rotation) s tates of the system are those of turbulent convection. The most persistent transient feature is a well-defined ring pattern characterized by a decreas e in temperature, axial velocity directed downward and high azimuthal shear . The latter leads to formation of an azimuthally regular structure of Kelv in-Helmholz vortices. During the final stage of the transition, this vortic al structure loses azimuthal regularity and an irregular pattern of vortice s characteristic of turbulent rotating convection forms.