Approximate solutions of fluid mechanics and their physical interpretation

Approximate relations for the determination of frictional drag in the case of a planar plate with a turbulent boundary layer are examined. The momentum equation of fluid mechanics provides the basis for the theoretical study of the problem. Other approximate solutions considered are related to the problems of sudden pipe enlargement, unsteady tank efflux, and lift on an airfoil. The discussed examples illustrate that an analysis of the basic considerations involved in the derivation of an approximate solution can provide important information regarding the quality of the selected mathematical model concept.