Air flows in a cylindrical duct of diameter D = 6 in. At section ①, the turbulent boundary layer is of thickness δ1 = 0.4 in. and the velocity in the inviscid central core is U1 = 80 ft/s. Further downstream, at section ②, the boundary layer is of thickness δ2 = 1.2 in. The velocity profile in the boundary layer is approximated well by the
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