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 ?1 = 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 ?2 = 24 mm. The velocity profile in the boundary layer is approximated well by the 1/7-power expression. Find the velocity, U2, in the inviscid central core at the second section, and the pressure drop between the two sections.