flow

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Etienne-Jules Marey,Three associated concave surfaces, 30-degree angle, fourth and last version of the smoke machine equipped with 57 channels,© Cinémathèque française
FYFD
Any time there is relative motion between a solid and a fluid, a small region near the surface will see a large change in velocity. This region, shown with smoke in the image above, is called the boundary layer. Here air flows from right to left over a spinning spheroid. At first, the boundary layer is laminar, its flow smooth and orderly. But tiny disturbances get into the boundary layer and one of them begins to grow. This disturbance ultimately causes the evenly spaced vortices we see wrappin
NASA Technical Reports Server (NTRS)
Schlieren photograph of transonic flow over an airfoil. The nearly vertical shock wave is followed by boundary layer separation that adversely affects lift, drag, and other flight parameters.