**Boundary-layer Thickness**

This page describes some parameters used to measure the properties of boundary layers. Consider a stationary body with a turbulent flow moving around it, like the semi-infinite flat plate with air flowing over the top of the plate. At the solid walls of the body the fluid satisfies a no-slip boundary condition and has zero velocity, but as you move away from the wall, the velocity of the flow asymptotically approaches the free stream mean velocity. Therefore it is impossible to define a clear point at which the boundary layer becomes the free stream. The parameters below overcome this and allow the boundary layer to be measured.

Read more about Boundary-layer Thickness: 99% Velocity Thickness, Displacement Thickness, Momentum Thickness, Shape Factor, Further Reading

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“For his teeth seem for laughing round an apple.

There lurk no claws behind his fingers supple;

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Nor antlers through the *thickness* of his curls.”

—Wilfred Owen (1893–1918)