CONVECTION FOR PRARADTL NUMBERS NEAR 1 - DYNAMICS OF TEXTURED PATTERNS

Rayleigh-Bénard convection in a cylindrical geometry with radius-to-height ratio Γ=40 was studied with the shadowgraph imaging method. The working fluid was CO2 at 32 bars and at temperatures near 34°C, with a Prandtl number σ=0.98. The onset pattern of largely straight, parallel rolls went through successive qualitative changes as εΔT/ΔTc-1 was increased. Quantitative measurements of wave numbers, of spatially averaged roll curvature, and of sidewall roll orientation as functions of ε are presented. As ε was increased, pattern dynamics induced by the skewed-varicose instability were first observed at ε0.09, and roll-nucleating sidewall foci were seen for ε0.15. Spiral defects appeared intermittently at ε0.55. The number of spirals fluctuated with time, but the average number increased with ε until, at ε0.8, spirals were present at all times. Coincident with the increase in spiral-defect activity was a decrease in the average wave number, a marked increase in the sidewall-foci roll-nucleation frequency and average roll curvature, and a distinct shape change in the structure factor S(k). The oscillatory instability was observed at ε3.0, in agreement with the stability analysis for straight rolls. © 1995 The American Physical Society.