PATTERN SELECTION FOR RAYLEIGH-BENARD CONVECTION IN INTERMEDIATE ASPECT RATIO BOXES

The present numerical study examines the pattern selection process in Rayleigh-Benard convection for boxes with intermediate aspect ratio. T he three-dimensional rectangular enclosure has insulated side walls. T he top wad is cooled and the bottom wad is heated both isothermally. T he Boussinesq approximation is invoked. Results of numerical simulatio ns are presented and discussed for a Prandtl number of 3.5 and aspect ratios of 10.61 and 5.32. The computations show the exact mechanism of the transition process by a detailed examination of the flow and temp erature field for three different cases. The first documents a decreas e in the number of rolls from 10 to 8 as the Rayleigh number is increa sed from 10,000 to 17,000. In the second, the change was from an eight -roll planform to a flow structure with a nonroll polygonal planform a s the Rayleigh number was increased from 17,000 to 24,000. A transitio n from 8 rolls to a T-shaped 4-roll pattern was also documented when t he Rayleigh number was stepped up from 17,000 to 20,000.