Explain phenomenon of bounday layer separation. Also explain methods to prevent it.

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. Then, the boundary layer will be separated fro....

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. Then, 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.

Disadvantage of boundary layer seperation

• Separation of boundary layer greatly affect the flow as a whole. In particular, the formation of a weak zone of disturbed fluid on the downstream, in which the pressure is approximately constant and much less than that on that on the upstream, gives rise to boundary forces. Thus, the separation of boundary layer gives to additional resistance to flow.

• Separation of boundary layer from the surface of a body is a accompanied by reversed  flow in the vicinity of the body. Reversal of flow and consequent eddy formation and generally undesirable because considerable amount of energy is lost in the process of eddying.

It is therefore necessary to control the separation of boundary layer as far as possible.

Method  of controlling separation of boundary layer

• Acceleration of the fluid in the boundary layer

• Suction of the fluid from the boundary layer:

• Motion of solid boundary 

• Guidance of flow by guide vanes.

Acceleration of the fluid in the boundary layer:

This method consist of supplying additional energy to the particle of fluid which are being retarded in the boundary layer. This may be achieved by injecting fluid into the the region of boundary layer from the interior of the body with the help of some suitable device  shown in fig.

Suction of the fluid from the boundary layer:

In this method, the slow moving fluid in the boundary layer is removed by suctions through slots, so that on the downstream of the point of suction, a new boundary layer starts developing which is able to withstand an adverse  pressure gradient and hence separation is prevented.

Motion of solid boundary : 

The formation of the boundary layer is due to the difference between the velocity of the flowing fluid and that of the solid boundary.

As it is possible to eliminate the formation of boundary layer by causing the solid boundary to move with the flowing fluid.

Using guide vanes in case of bend:

When the flow takes place round a bend, a pressure gradient is generated and there is a tendency of separation at the inner radius of the bend. In such case, guide vanes can be used to prevent boundary layer separation.