Hagen–Poiseuille flow from the Navier–Stokes equations

Derivation

The laminar flow through a pipe of uniform (circular) cross-section is known as Hagen–Poiseuille flow. The equations governing the Hagen–Poiseuille flow can be derived directly from the Navier–Stokes momentum equations in 3D cylindrical coordinates by making the following set of assumptions:

1. The flow is steady ( ∂ ( . . . ) / ∂ t = 0 ).
2. The radial and swirl components of the fluid velocity are zero ( u r = u θ = 0 ).
3. The flow is axisymmetric ( ∂ ( . . . ) / ∂ θ = 0 ) and fully developed ( ∂ u z / ∂ z = 0 ).

Then the angular equation in the momentum equations and the continuity equation are identically satisfied. The first momentum equation reduces to ∂ p / ∂ r = 0 , i.e., the pressure p is a function of the axial coordinate z only. The third momentum equation reduces to:

Since u z needs to be finite at r = 0 , c 1 = 0 . The no slip boundary condition at the pipe wall requires that u z = 0 at r = R (radius of the pipe), which yields

Thus we have finally the following parabolic velocity profile:

The maximum velocity occurs at the pipe centerline ( r = 0 ):

The average velocity can be obtained by integrating over the pipe cross section:

The Hagen–Poiseuille equation relates the pressure drop Δ p across a circular pipe of length L to the average flow velocity in the pipe u z a v g and other parameters. Assuming that the pressure decreases linearly across the length of the pipe, we have − ∂ p ∂ z = Δ p L (constant). Substituting this and the expression for u z m a x into the expression for u z a v g , and noting that the pipe diameter D = 2 R , we get:

Rearrangement of this gives the Hagen–Poiseuille equation: