Local Topology of Three-Dimensional Separation and Attachment Lines
In contrast to the situation in two-dimensional flow the separation of three-dimensional boundary-layers is characterized by the fact that, in general, the wall shear stress does not vanish along the separation line, except in singular points. Usually a distinctive pattern of limiting wall streamlines, i.e. skin friction lines, can be realized near separation lines, namely that the skin friction lines converge towards the separation line from both sides. Here the separation line itself is considered to be a skin friction line, which, however, cannot be proved in the frame of local topology. It is noted that the convergence of the skin friction lines near the separation line may be a necessary but not sufficient condition for the occurence of separation , see for example the attached flow near the leeward symmetry line of a blunt body at small angle of attack. The separation line in three-dimensional flow is the location along which the boundary-layer detaches from the wall, forming a dividing surface which separates the converging boundary-layers on each side of the separation line.
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