Limiting relation for the eddy diffusivity close to a wall

Experiments on mass transfer for fully developed flow in a pipe show that the eddy diffusivity for mass close to the wall is described by the functionality The best interpretation of presently available data in a pipe and in an annulus is given by the following, more explicit relation The evidence for this latter relation is not as conclusive as one would like. Accurate measurements of concentration profiles close to a wall are needed.