Convection in a Vertical Slot Near the Onset of Multi-cellular States

Experimental observations of the motion in a laterally-heated, vertical slot will be presented. Particular attention is given to conditions for which instabilities occur in the flowfield leading either, to stationary transverse rolls or, travelling wave states. For large Prandtl number fluids, the evolution to multi-cellular transverse rolls is found to occur smoothly as the Rayleigh number increases at fixed aspect ratio. Evidence of a similar transition in broader enclosures, will be described for an air-filled cavity (Prandtl number = ~ 0.7). The observations suggest that the secondary motions arise from conditions near the mid-cavity where the dissipative effects associated with the slot end regions are weakest. Comparisons will be made between the current results and a recent asymptotic analysis for the onset of the unstable region.