As stated in Chapter 1, the unaveraged unsteady form of the Navier-Stokes equations remains valid for the entire flow regime from laminar to turbu­ lent, as long as appropriate values are specified for the transport coefficients v and a. However, the extremely small length and time scales associated with turbulent motion require unpractically dense computational grids and small time steps when attempting to solve the unaveraged unsteady govern­ ing equations for engineering applications at realistic Reynolds numbers. Turbulence modeling is the method of incorporating the critical effects of turbulent motion on the mean unsteady flow and heat transfer without having to resolve the actual small scales of turbulent motion.