At a viscous solid wall satisfying no slip, the fluid velocity matches the wall velocity. Away from the wall, the external flow may move much faster. The thin region across which velocity changes from the wall value to the outer-flow value is the boundary layer.

Boundary-layer growth

As fluid travels along a surface, viscous effects act for longer and the boundary layer generally grows thicker.

Its behavior depends strongly on Reynolds number and can be laminar, transitional, or turbulent.

Why the boundary layer matters

Velocity gradients are strongest near the wall, so viscous shear and momentum diffusion are concentrated there even when most of the outer flow behaves approximately as if viscosity were weak.

This is why a thin viscous region can control wall drag and strongly influence the pressure field around a body.

Adverse pressure gradients

If pressure increases in the direction of flow, fluid in the boundary layer is forced toward higher pressure. The low-momentum fluid nearest the wall can slow strongly or even reverse direction.

When the near-wall flow can no longer remain attached to the surface, the boundary layer separates.

Wakes

Separated flow creates a wake with a different pressure and velocity field behind the body. This can increase pressure drag dramatically and can also cause an airfoil to lose lift.

Boundary layers therefore connect wall boundary conditions, viscosity, Reynolds-number flow regimes, and the aerodynamic forces experienced by real bodies.