Rayleigh-Benard convection in limited domains: Part 1 - Oscillatory flow

Transition from the steady state to an oscillatory regime in three-dimensional limited aspect ratio boxes, filled with an incompressible fluid and heated from below, has been examined by direct numerical simulation. Two different physical problems have been considered: the first is related to a domain 3.5 X 1 X 2.1 filled with water at 70 degrees C (Prandtl number 2.5); Be second considers a domain 2.4 X 1 x 1.2 filled with water at 33 degrees C (Prandtl number 5). The Rayleigh number has been varied from 20,000 to 80,000. A new procedure based on a statistical approach for evaluation of the critical Rayleigh number for transition from steady state to oscillatory flow (Ra-II) has been introduced in order to reduce numerical errors and estimate the error bars. A systematic study for the determination of Ram has been conducted as a function of the geometries considered and the different flow structures observed.