The basic concept of a three-dimensional boundary layer is reviewed from both physical and mathematical viewpoints. Emphasis is placed on the various causes of secondary flow, particularly geodesic curvature of the surface streamlines of inviscid flow. Various exact solutions for steady, incompressible laminar flow are reexamined, a proposal for a finite-difference scheme for arbitrary inviscid flows and surface conditions is sketched, and the momentum-integral method and other approximation schemes are briefly discussed. Also considered are compressibility effects, laminar-flow stability, transition to turbulence, displacement thickness of a three-dimensional boundary layer, and the incompressible turbulent boundary layer. It is concluded that very successful three-parameter models of mean velocity profiles exist, but methods for predicting the variation of the profile parameters are essentially deficient. Suggestions for future work include further study of the method of characteristics for solving momentum-integral equations, mapping of the skin friction field in laminar flow, and synoptic exploration of three-dimensional turbulent boundary layers. 108 pp. Bibliog.
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