In Chapter 6, we discussed the fundamentals of turbulence and reviewed the mixing length and eddy diffusivity models. As mentioned there, these classical models do not treat turbulence as a transported property, and as a result, they are best applicable to equilibrium turbulent fields. In an equilibrium turbulent field at any particular location, there is a balance among the generation, dissipation, and transported turbulent energy for the entire eddy size spectrum, and as a result, turbulence characteristics at each point depend only on the local parameters at that point.