A flow of air develops in a horizontal cylindrical duct, of diameter \(D=15\) in., following a well-rounded entrance. A turbulent boundary grows on the duct wall, but the flow is not yet fully developed. Assume that the velocity profile in the boundary layer is \(u / U=(y / \delta)^{1 / 7}\). The inlet flow is \(U=50 \mathrm{ft} / \mathrm{s}\) at section (1). At section (2), the boundary-layer thickness is \(\delta_{2}=4 \mathrm{in}\). Evaluate the static gage pressure at section (2), located at \(L=20 \mathrm{ft}\). Find the average wall shear stress.