Stability of Rayleigh-Benard convection rolls and bimodal flow at moderate Prandtl number

Experimental observations of cellular convection between two rigid, horizontal, conducting boundaries are reported for two different cases. First, the stability of two-dimensional roll convection of various wavenumbers and Rayleigh numbers is investigated in fluids of Prandtl numbers 16 and 2.7. The results qualitatively agree with earlier observations by Busse and Whitehead of fluid with Prandtl number 126 but they differ somewhat quantitatively. Second, the stability of the bimodal flow, consisting of two rolls of differing and perpendicular wavenumber, is observed to be stable for given bandwidths and ranges of Rayleigh numbers when the configuration of the bimodal flow, consisting of two sets of rolls at right angles, is flawless — without any disruptions in the periodic matrix. The stability range differs from earlier experiments and from our own experiments in which the bimodal planform is uncontrolled.