Consider a steady laminar and fully developed flow through a straight pipe of circular cross-section

Consider a steady laminar and fully developed flow through a straight pipe of circular cross-section. The axis of the pipe is considered to be the z-axis about which the flow is azimuthally symmetrical. All fluid particles travel along the z-axis. Vz not equal to zero, but Vr=0, and Vtheta=0.
Hagen-Poiseuille flow
Use the Cylindrical coordinate system to set up the conservation of momentum (Navier-Stokes) and the conservation of mass (continuity equation).
Cylindrical Navier-Stokes
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