 Boundary layer thickness

The conventional boundary layer thickness ${\delta }_{99}$ is the normal distance at which the streamwise velocity first reaches $0.99U$ in a monotone velocity profile. A Blasius boundary layer approaches $U$ smoothly, so its apparent edge depends on the chosen percentage; there is no discontinuity. Solving $f^{\prime }({\eta }_{99})=0.99$ in the Blasius equation gives ${\eta }_{99}\simeq 4.910$, henceThe factor $4.910$ belongs to this definition, whereas the boundary-layer scaling specifies only order of magnitude. Near this edge the velocity changes slowly, making a threshold position sensitive to measurement error; displacement thickness and momentum thickness summarize the whole velocity profile.^[ref]