Boundary Layer Definition
- Boundary layer thickness (d): defined as the distance away from the surface where the local velocity reaches to 99% of the free-stream velocity, that is
u(y=d)=0.99U?. Somewhat an easy to understand but arbitrary definition.
- Displacement thickness (d*): Since the viscous force slows down the boundary layer flow, as a result, certain amount of the mass has been displaced (ejected) by the presence of the boundary layer (to satisfy the mass conservation requirement). Image that if we displace the uniform flow away from the solid surface by an amount d*, such that the flow rate with the uniform velocity will be the same as the flow rate being displaced by the presence of the boundary layer.