ASYMMETRIC MODES AND THE TRANSITION TO VORTEX STRUCTURES IN ROTATING RAYLEIGH-BENARD CONVECTION

We report heat transport measurements and optical shadowgraph visualization of rotating Rayleigh-Benard convection. For dimensionless rotation rates 140 < OMEGA < 4300, the initial transition to convection, occurring at a Rayleigh number R much less than the linear-stability value for roll or vortex states, is a forward Hopf bifurcation to an azimuthally asymmetric state with mode number n. States with n = 3, 4, 5, 6, and 7 exist at low to moderate R and precess with frequencies that depend on R and OMEGA. At higher R there is a continuous transition to a state with noisy, time-dependent heat transport, a distinct array of vortices in the central region, and a modulation of the precession speed of the outer structures.