The passive scalar spectrum and the Obukhov-Corrsin constant

It is pointed out that, for microscale Reynolds numbers less than about 1000, the passive scalar spectrum in turbulent shear flows is less steep than anticipated and that the Obukhov-Corrsin constant can be defined only if the microscale Reynolds number exceeds this value. In flows where the large-scale velocity field is essentially isotropic (as in grid turbulence), the expected 5/3 scaling is observed even at modest Reynolds numbers. All known data on the Obukhov-Corrsin constant are collected. The support for the notion of a "universal" constant is shown to be reasonable. Its value is about 0.4.