= Boundary layer thickness
{title2=$\delta_{99}$}
{wiki}
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 $f'(\eta_{99})=0.99$ in the <Blasius equation> gives $\eta_{99}\simeq4.910$, hence
$$
\delta_{99}\simeq4.910\sqrt{\frac{\nu x}{U}}.
$$
The 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>.<Modules on High Reynolds Number Flows/Boundary-layer Thickness, Skin friction, and Energy dissipation>{ref}
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