The thickness up to which the velocity approaches 99% of the free stream velocity. The distance measured perpendicular to the boundary by which the free stream is displaced on account of the formation of the boundary layer.
The displacement thickness for the boundary layer is defined as the distance the surface would have to move in the y-direction to reduce the flow passing by a volume equivalent to the real effect of the boundary layer.
The thickness of the boundary layer represented by δ is arbitrarily defined as that distance from the boundary surface in which the velocity reaches 99% of the velocity of the mainstream.
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