Boundary Layers

Abstract

A boundary layer forms when a fluid flows over a solid surface. The fluid velocity goes to zero at the surface because of the roughness of a real surface. A general definition for a boundary layer is “a region in which the velocity gradient and related shear stresses become large enough that they cannot be neglected.” (Prandtl and Tietjens 1934). Thus the consideration of the effects of a boundary layer is left to the user. Even very highly polished surfaces are rough on the scale of gas molecule separation distances. From Sect. 2.2 on the discussion of sound propagation we showed that the intermolecular distance was approximately 2.e-7 cm for sea level air. The surface would need to be smooth to a few times this distance for the surface to not form a boundary layer.