Boundary Layer Thickness
The boundary layer thickness, , is the distance across a boundary layer from the wall to a point where the flow velocity has essentially reached the ‘free stream’ velocity, , or . This distance is defined normal to the wall, and the point where the flow velocity is essentially that of the free stream is customarily defined as the point where
Figure: A simple sketch of boundary layer thickness definition. The sketch also shows the boundary layer development. When the free-stream inlet velocity is uniform and laminar (or low intensity of turbulence). When approaching the wall, the boundary layer is first laminar and then becomes transitional and finally turns to turbulence.