Air flows in a horizontal cylindrical duct of diameter D = 100 mm. At a section a few meters from the entrance, the turbulent boundary layer is of thickness δ1 = 5.25 mm, and the velocity in the inviscid central core is U1 = 12.5 m/s. Farther downstream the boundary layer is of thickness δ2 = 24 mm. The velocity profile in the boundary layer is approximated well by the
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