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.

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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.

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**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.

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**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.

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**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.

Do you have any suggestions? Please write in comment
box.

### Reference:

Fluid mechanics, By R. K. Bansal

Image courtesy: Google

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