Boundary layer thickness
A quantity, defined for velocity, thermal and diffusion boundary layers as
the distance along the normal to the wall over which the fundamental variable quantities (the axial velocity component, the temperature
or the concentration, respectively) reaches, with some prescribed accuracy, its limiting value away from the wall (i.e., in the external flow).
Comment. The approximate methods of boundary layer theory postulate that the boundary layer has a finite thickness, the outer edge of which is determined at the maximum value of the fundamental variable quantity and the derivative of this variable vanishes along the normal.