= Displacement thickness
{title2=$\delta^*$}
For constant-<mass density> <incompressible flow>, the <displacement thickness> measures the lost <volume flux> per unit span divided by the outer <velocity>:
$$
\delta^*=\int_0^\infty\left(1-\frac{u}{U}\right)\,dy,\qquad U\delta^*=\int_0^\infty(U-u)\,dy.
$$
An outer <inviscid flow> encountering the same <volume flux> deficit behaves, at leading order, as though the wall were displaced outward by $\delta^*$. In a <Blasius boundary layer>, numerical <integration> gives $\delta^*\simeq1.720788\sqrt{\nu x/U}$. This equivalent displacement is smaller than the <boundary layer thickness> and is not the position of a physical surface.<Modules on High Reynolds Number Flows/Boundary-layer Thickness, Skin friction, and Energy dissipation>{ref}
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