BOUNDARY LAYER AND SKIN FRICTION RELATIONS

This chapter discusses the boundary layer and skin friction relations. One of the usual difficulties encountered when presenting boundary layer theory to engineers lies in the provision of design equations suitable for engineering use. In a turbulent boundary layer, the turbulent stresses that provide the turbulent mixing are much greater than the viscous stresses. On account of the inability of a laminar layer to persist far into a region of increasing pressure without separating, it is usually sufficient, for engineering purposes, to assume that separation occurs just downstream of the point where the pressure begins to rise. There are at least four main simple configurations for which estimates of skin friction are required : (1) two-dimensional flow, (2) fully developed flow in a pipe, (3) initial flow in a pipe prior to fully developed conditions, and (4) flow over an axisymmetric body. As in the case of laminar flow, no distinction will be made between flow along a flat plate in a uniform stream and the fully developed flow in a pipe. In the case of laminar flow, the properties of the layer can, in the presence of a velocity gradient, be expressed in terms of local flow parameters, such as Λ and λ.