Textbook

Boundary Layers

Because of friction , when a fluid flows over a surface, an interesting pattern develops. The fluid actually stops ; there is no velocity or movement at the surface. A new layer develops on top of the stopped flow. There is less friction on this new surface and thus a little more flow. New layers develop, each with less friction, until some distance away from the original surface the remaining layers of the fluid travel at the original velocity. The distance from the original surface to the layer of the flow traveling at the original velocity is called the boundary layer thickness.

In general, the boundary layer gets thicker as the flow moves along the surface. How fast and how big the boundary layer grows depends on the smoothness and shape of the surface and the velocity of the liquid.

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