The problems of heat flow with convection, discussed in the preceding chapter, pertain to simple systems with laminar flow. Despite the simplicity of laminar flow problems, they should not be underestimated. Many simple solutions have been applied to real systems with approximating assumptions and, besides, the simpler systems provide models for interpretation of complex systems. The more complex nature of turbulent flow and its limited accessibility to mathematical treatment requires, however, an empirical approach to heat transfer. On the other hand, the study of turbulent flow is not entirely empirical; it is possible to establish certain theoretical bases for the analyses of turbulent transfer processes and an introduction to this complex area is given in Chapter 16.