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\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{\varepsilon} {\nu} = g({y^+}) $$\end{document}The best interpretation of presently available data in a pipe and in an annulus is given by the following, more explicit relation\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{\varepsilon} {\nu} = 0.00032y^{+ 4} $$\end{document}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.