When a solid body is immersed in a flowing fluid, there is a narrow region of the fluid in the neighbourhood of the solid, where the velocity of fluid varies from zero to free stream velocity is known as:

Explanation:

Boundary layer:

When a real fluid flows past a solid body or a solid wall, the fluid particles adhere to the boundary and the condition of no-slip occurs i.e velocity of fluid will be the same as that of the boundary.

Farther away from the boundary, the velocity will be higher and as a result of this variation, the velocity gradient will exist.

Boundary-Layer Thickness:

It is defined as the distance from the boundary of the solid body measured in the perpendicular direction to the point where the velocity of the fluid is approximately equal to 0.99 times the free stream velocity (U). It s denoted by the symbol (δ).

Displacement thickness (δ* or δ+): 

It is defined as the distance by which the body surface should be shifted in order to compensate for the reduction in mass flow rate on account of boundary layer formation.

The mass flow rate of ideal fluid flow = 

The mass flow rate of real fluid flow = 

The loss is compensated by displacement layer thickness,

Momentum thickness (θ): 

It is defined as the distance from the actual boundary surface such that the momentum flux (momentum transferred per sec) corresponding to mainstream velocity (u∞) through this distance θ is equal to the deficiency or loss in momentum due to the boundary layer formation.

Given as

Energy thickness (δE): 

It is defined as the distance from the actual boundary surface such that the energy flux corresponding to the mainstream velocity u∞ through the distance δE is equal to the deficiency or loss of energy due to the boundary layer formation.  

Given as