Boundary layer concept

Ludwig Prandlt, a German Fluid dynamics expert; introduced the boundary layer concept in the year 1904.

What is fluid mechanics?
It is a science that deals with movement of fluids (Liquids/gas) and its interaction with solid surface. 99% of the practical flow is wall bounded.

What is meant by continuum?
 All matters are made up of atoms, fluids too. Atoms have some gap in between them.  Now just consider the flow over a circular cylinder of diameter d. The mean distance that an atom travels between two successive collisions is called as Mean free path (λ).  When comparing λ and d, if λ is too small than d, we can assume that the flow over the cylinder as continuum.

What is no- slip condition?
Consider a fluid flow over a solid surface. The layer of fluid which is attached to the solid surface sticks to the solid surface and so it has zero relative velocity with the solid surface. This is what called as no-slip condition. Due to no slip phenomenon the layer of fluid attached with the surface has zero relative velocity and the next layers are moving with some velocity. So shear stress comes into picture. Also there is a velocity change or velocity gradient between any two layers of fluid within boundary layer region.

What is boundary layer?
Due to frictional effect no slip condition occurs in a fluid flow, thus a velocity gradient forms. If you keep your eye on the upper layers you can grasp that these layers aren't disturbed by frictional effect. The thin viscous region adjacent to the body is called as boundary layer. Within the boundary layer viscous flow analysis is used. Apart from boundary layer region invisid flow analysis will be used.

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