Boundary-Layer Thickness Definitions
The fluid velocity at a stationary wall is zero, and the velocity at the outer edge of the boundary layer is equal to the mainstream velocity. Moreover, the velocity gradient at the outer edge of the boundary layer should be zero. Therefore, the boundary layer satisfies the following boundary conditions
- At the wall: \(u=0,\quad \partial u/\partial y=\tau_w / \mu\)
- At the outer edge of the boundary layer: \(u=U,\quad \partial u/\partial y=0\)
Nominal thickness of the boundary layer. The nominal thickness of the boundary layer does not have much practical significance, and some other types of boundary layer thickness are more meaningful in engineering.
Derivation of displacement thickness.
1Hongwei Wang (2023). A Guide to Fluid Mechanics. National Defense Industry Press.