Convection for Prandtl numbers near 1: Dynamics of textured patterns.

Abstract

Rayleigh-Benard convection in a cylindrical geometry with radius-to-height ratio {Gamma}=40 was studied with the shadowgraph imaging method. The working fluid was CO{sub 2} at 32 bars and at temperatures near 34 {degree}C, with a Prandtl number {sigma}=0.98. The onset pattern of largely straight, parallel rolls went through successive qualitative changes as {epsilon}{equivalent_to}{Delta}{ital T}/{Delta}{ital T}{sub {ital c}}{minus}1 was increased. Quantitative measurements of wave numbers, of spatially averaged roll curvature, and of sidewall roll orientation as functions of {epsilon} are presented. As {epsilon} was increased, pattern dynamics induced by the skewed-varicose instability were first observed at {epsilon}{approx}0.09, and roll-nucleating sidewall foci were seen for {epsilon}{approx_gt}0.15. Spiral defects appeared intermittently at {epsilon}{approx}0.55. The number of spirals fluctuated with time, but the average number increased with {epsilon} until, at {epsilon}{approx}0.8, 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 {ital S}({ital k}). The oscillatory instability was observed at {epsilon}{approx}3.0, in agreement with the stability analysis for straight rolls.