•    Boundary layer is that thin layer of fluid the immediate vicinity of bounding surface (solid body), over which velocity gradients and shear stresses are large.

•    If the boundary surface is stationary and the velocity of fluid at boundary will be zero i.e. as the fluid moves past the solid object, the molecules right next to the surface stick to the surface.

•    The velocity is higher away from the boundary layer due to retardation of each layer of fluid from the boundary. This create a thin layer of fluid near surface in which velocity changes from zero at boundary surface to free stream velocity away from the surface. This is called as Boundary layer.


•    It is very important in many problem of aerodynamics, because it is directly responsible for the drag experienced by a body immersed in a fluid.

•    In design of an aircraft, more concentration is given on controlling the behaviour of the boundary layer, to minimized drag.

Effect of boundary layer:

1.    Boundary layer add the effective thickness of the body though the displacement thickness hence increase in the pressure drag.

2.    The shear force at the boundary surface create the skin friction drag.

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