Concept : Boundary Layer Separation
When a solid body is immersed in a flowing fluid, a thin layer of fluid called the boundary layer is formed adjacent to the solid body.
In this thin layer of fluid, the velocity varies from zero to free-stream velocity in the direction normal to the solid body. Along the length of the solid body, the thickness of the boundary layer increases.
The fluid layer adjacent to the solid surface has to do work against surface friction at the expense of its kinetic energy.
This loss of the kinetic energy is recovered from the immediate fluid layer in contact with the layer adjacent to solid surface through momentum exchange process. Thus the velocity of the layer goes on decreasing. Along the length of the solid body, at a certain point a stage may come when the boundary layer may not be able to keep sticking to the solid body if it cannot provide kinetic energy to overcome the resistance offered by the solid body.
In other words, the boundary layer will be separated from the surface. This phenomenon is called the boundary layer separation. The point on the body at which the boundary layer is on the verge of separation from the surface is called point of separation.
Methods of Preventing the Separation of Boundary Layer
When the boundary layer separates from the surface, a certain portion adjacent to the surface has a back flow and eddies are continuously formed in this region and hence continuous loss of energy takes place. Thus separation of boundary layer is undesirable and attempts should be made to avoid separation by various methods.
The following are the methods for preventing the separation of boundary layer:
Suction of the slow moving fluid by a suction slot.
Supplying additional energy from a blower.
Providing a bypass in the slotted wing.
Rotating boundary in the direction of flow.
Providing small divergence in a diffuser.
Providing guide-blades in a bend.
Providing a trip-wire ring in the laminar region for the flow over a sphere.