 If the momentum and thermal eddy diffusivities are zero (no apparent turbulent shear stress and heat flux), then the turbulent flow equations reduce to the laminar equations.We can define the eddy diffusivities for momentum transfer formula_1 and heat transfer formula_2 asformula_3 and formula_4where formula_5 is the apparent turbulent shear stress and formula_6 is the apparent turbulent heat flux.The turbulent Prandtl number is then defined asformula_7

The turbulent Prandtl number has been shown to not generally equal unity (e.g.Malhotra and Kang, 1984; Kays, 1994; McEligot and Taylor, 1996; and Churchill, 2002).