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Explain what is meant by separation of boundary layer.

Describe with sketch the methods to control separation.

(12 Marks) Dec-2018

Subject Fluid Mechanics 2

Topic Boundary Layer Flow

Difficulty Medium

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separation of boundary layer

• 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 0 to free stream velocity in the direction normal to the solid body.

• Along the length of the body thickness of 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. Thus the velocity of layer goes on decreasing.

• Along the length of the solid body at a certain point, a stage will come when the boundary layer may not able to keep sticking to the solid body if it cannot provide kinetic energy to overcome the resistance offered by the solid body.

• 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.

Control Of Boundary Layer Separation :

• The total drag on a body is attributed to form drag and skin friction drag. In some flow configurations, the contribution of form drag becomes significant.

• In order to reduce the form drag, the boundary layer separation should be prevented or delayed so that better pressure recovery takes place and the form drag is reduced considerably. There are some popular methods for this purpose which are stated as follows.

• By giving the profile of the body a streamlined shape( as shown in Fig.).

1. This has an elongated shape in the rear part to reduce the magnitude of the pressure gradient.
2. The optimum contour for a streamlined body is the one for which the wake zone is very narrow and the form drag is minimum.
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