5.2 Three Thicknesses of a Boundary Layer bL Thickness d(名义厚度,厚度) The locus of points where the velocity u parallel to the plate reaches 99 percent of the external velocity U 2 Displacement Thickness 8*(排移厚度,移厚度) U5.2 Three Thicknesses of a Boundary Layer 2. Displacement Thickness  * (排移厚度，位移厚度） d U y u x 1. BL Thickness d (名义厚度，厚度） The locus of points where the velocity u parallel to the plate reaches 99 percent of the external velocity U