STRESSING THE 'FREE' IN FREE CONVECTION RESEARCH: THE BASIC SCALES OF HEAT AND MASS TRANSFER IN FLUIDS AND FLUID-SATURATED POROUS MEDIA.
The parallel structure of the fields of natural convection in fluids and natural convection in porous media is reviewed in terms of the most basic scales of flow, heat and mass transfer. In each field, the review includes single vertical boundary layers, enclosures, heat and mass transfer in the presence of temperature and concentration buoyancy effects, Benard flow and new or still unexplored directions of research. In particular, an original scale analysis of the nonDarcy regime in a porous layer heated from below shows that the Darcy-nonDarcy transition occurs when the Darcy-Rayleigh number Ra//K exceeds O(Pr//p), where Pr//p is an important new dimensionless group called 'the porous medium Prandtl number. ' In the nonDarcy regime the Nusselt number scales as Ra//K** one-half Pr** one-half . The importance of choosing the problem-solving method freely, and that of maximizing Return On Investment in the research domain is emphasized throughout the paper.
