Boundary-layer thickness

This page describes some parameters used to measure the properties of boundary layers. Consider a stationary body with a fluid flowing around it, like the semi-infinite flat plate with air flowing over the top of the plate (assume the flow and the plate extends to infinity in the positive/negative direction perpendicular to the ${\displaystyle x-y}$ plane). 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 sharp point at which the boundary layer becomes the free stream, yet this layer has a well-defined characteristic thickness. The parameters below provide a useful definition of this characteristic, measurable thickness. Also included in this boundary layer description are some parameters useful in describing the shape of the boundary layer.

The boundary layer thickness, δ, is the distance across a boundary layer from the wall to a point where the flow velocity has essentially reached the 'free stream' velocity, ${\displaystyle u_{0}}$. This distance is defined normal to the wall. It is customarily defined as the point ${\displaystyle y}$ where:

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