Rayleigh-Bénard convection in tall rectangular enclosures

Natural convection in air-filled, 2-D rectangular enclosures heated from below and cooled from above is studied numerically under the assumption of adiabatic sidewalls. A computational model based on the SIMPLE-C algorithm is used for solving the mass, momentum, and energy transfer governing equations. Simulations are performed for different values of the height-to-width aspect ratio of the enclosure in the range A=2-6, by progressively increasing and successively decreasing the Rayleigh number in the range Ra=10^3-2×10^6. After the departure from motionless conduction takes place, the following flow-pattern evolution is detected: one-cell steady; two-cell steady; two-cell periodic; one-to-three-cell periodic; three-cell periodic. At each bifurcation, either abrupt or smooth changes in the Nusselt number are found to occur, according to whether the flow-transition is either sudden or more gradual. Hysteresis phenomena occurrence is documented. The effects of tilting the enclosure upon the stability of the different flow structures are also analysed.