Observations of convection at rayleigh numbers up to 760,000 in a fluid with large prandtl number

Observations are reported on the stability and structure of Rayleigh-Benard convection in a fluid with a Prandtl number of 8,600, and Rayleigh numbers between 50,000 and 760,000. Under carefully initiated and controlled conditions, stationary bimodal convection was observed, even at the highest Rayleigh number. Unstable bimodal patterns broke down in a way that tends to reduce the wavelength of the cells. Oscillating spoke-shaped convection was observed in convection started from random initial conditions. The gradual increase in the occurrence of oscillations and the dependence on the initial conditions probably accounts for previous disagreements about the onset of time dependence. A square convection planform was found which was stable at the highest Rayleigh numbers but unstable for lower Rayleigh numbers. The observations demonstrate the existence of different possible solutions at a given Rayleigh number.