INFLUENCE OF PRANDTL NUMBER ON INSTABILITY MECHANISMS AND TRANSITION IN A DIFFERENTIALLY HEATED SQUARE CAVITY

The transition from laminar to turbulent of the natural-convection how inside a square, differentially heated cavity with adiabatic horizont al wails is calculated, using the finite-volume method. The purpose of this study is firstly to determine the dependence of the laminar-turb ulent transition on the Prandtl number and secondly to investigate the physical mechanisms responsible for the bifurcations observed. It is found that in the square cavity, for Prandtl numbers between 0.25 and 2.0, the transition occurs through periodic and quasi-periodic flow re gimes. One of the bifurcations is related to an instability occurring in a jet-like fluid layer exiting from those corners of the cavity whe re the vertical boundary layers are turned horizontal. This instabilit y is mainly shear-driven and the visualization of the perturbations sh ows the occurrence of vorticity concentrations which are very similar to Kelvin-Helmholtz vortices in a plane jet, suggesting that the insta bility is a Kelvin-Helmholtz-type instability. The other bifurcation f or Prandtl numbers between 0.25 and 2.0 occurs in the boundary layers along the vertical walls. It differs however from the related instabil ity in the natural-convection boundary layer along an isolated vertica l plate: the instability in the cavity is shear-driven whereas the ins tability along the vertical plate is mainly buoyancy-driven. For Prand tl numbers between 2.5 and 7.0, it is found that there occurs an immed iate transition from the steady to the chaotic flow regime without int ermediate regimes. This transition is also caused by instabilities ori ginating and concentrated in the vertical boundary layers.