We were discussing the basics of Boundary layer theory, laminar boundary layer and turbulent boundary layer, in the subject of fluid mechanics, in our recent posts.

Today we will be interested here to understand the basics of boundary layer thickness, displacement thickness and momentum thickness with the help of this post.

### Boundary layer thickness

Boundary layer thickness is basically defined as the distance from the surface of the solid body, measured in the y-direction, up to a point where the velocity of flow is 0.99 times of the free stream velocity of the fluid.

Boundary layer thickness will be displayed by the symbol δ.

We can also define the boundary layer thickness as the distance from the surface of the body up to a point where the local velocity reaches to 99% of the free stream velocity of fluid.

### Displacement thickness

Displacement thickness is basically defined as the distance, measured perpendicular to the boundary of the solid body, by which the boundary should be displaced to compensate for the reduction in flow rate on account of boundary layer formation.

Displacement thickness will be displayed by the symbol δ*.

We can also define the displacement thickness as the distance, measured perpendicular to the boundary of the solid body, by which the free stream will be displaced due to the formation of boundary layer.

### Momentum thickness

Momentum thickness is basically defined as the distance, measured perpendicular to the boundary of the solid body, by which the boundary should be displaced to compensate for the reduction in momentum of the flowing fluid on account of boundary layer formation.

Momentum thickness will be displayed by the symbol θ.

Further we will go ahead to start a new topic i.e. Energy thickness in boundary layer, in the subject of fluid mechanics, with the help of our next post.

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### Reference:

Fluid mechanics, By R. K. Bansal