Boundary layer concept and related terms

Boundary layer concept

When a real fluid flows past a solid body, fluid at the surface sticks to it (no-slip). Velocity increases from zero at the wall to the free-stream velocity $U$ over a thin layer. Viscous shear is large only within this layer, called the boundary layer. Outside it the flow is treated as ideal (potential) flow. This idea (Prandtl, 1904) lets the flow be split into a thin viscous layer and an outer inviscid region.

Boundary layer thickness ($\delta$)

It is the distance from the wall at which the velocity equals 99% of the free-stream velocity: $u = 0.99U$ at $y = \delta$.

Laminar sub-layer

Under a turbulent boundary layer there is a very thin layer next to the wall in which flow stays laminar and the shear is purely viscous: $\tau_0 = \mu\,du/dy$. Its thickness is $\delta' = 11.6\nu/u_*$.

Point of separation

It is the point on the surface where flow moving against an adverse pressure gradient ($dp/dx > 0$) is brought to rest at the wall, so $(du/dy)_{y=0} = 0$. Downstream the flow near the wall reverses and the boundary layer leaves the surface, forming a wake.

 U -> ===========================
            turbulent BL         separation
                      ~~~~~~~~~~~~  S
 solid ========================\===\====
      laminar sub-layer        <-- back flow