ON THE HIGH-RAYLEIGH-NUMBER STRUCTURE OF STEADY LAMINAR NATURAL-CONVECTION FLOW IN A SQUARE ENCLOSURE

Natural-convection flow in an enclosure with adiabatic horizontal wall s and isothermal vertical walls maintained at a fixed temperature diff erence has been investigated. At high values of the natural-convection parameter, the Rayleigh number, a recirculating pocket appears near t he corners downstream of the vertical walls, and the flow separates an d reattaches at the horizontal walls in the vicinity of this recircula tion. There is also a considerable thickening of the horizontal layer. In some previous studies by different authors, this corner flow was c onsidered to be caused by an internal hydraulic jump, and the jump the ory was used to predict bifurcation of the steady flow into periodic f low. The present work examines the corner phenomenon closely to decide if it is indeed caused by a hydraulic jump. The results of the analys is reveal the oversimplification of the problem made in the previous s tudies: there is no connection of the corner phenomenon with a hydraul ic jump. The separation of flow at the ceiling is not a feature of hyd raulic jumps, and the essential energy loss associated with hydraulic jumps is not observed in the corner flow. It is shown that the corner structure is caused by thermal effects. Owing to the temperature under shoots in the vertical boundary layer, which are known to be caused by the stable thermal stratification of the core, relatively cold fluid reaches the upper corner. This cold fluid detaches from the ceiling li ke a plume at high Rayleigh numbers, and causes the separation and rec irculation.