In view of the many dimensionless groups thatgovern the natural convection phenomenon, the orderof-magnitude analysis of the flow promises to be ascomplicated as the task of selecting the ‘best’ cases fordirect numerical simulation. For this reason it is helpfulto structure the scale analysis : one way to do this is torecognize from the very start two important extremes inwhich the phenomenon can exist, namely :(1) Heat-transfer-driven flows, which are dominatedby the buoyancy effect due to side heating,