From Acheson: Elementary Fluid Dynamics:

Consider a steady viscous flow under constant pressure gradient dp/dz = -P through a pipe that runs along the z-axis and has triangular cross-section. The sides are given by the planes x = 1, x+2-y*sqrt(3) = 4, and x+2+y*sqrt(3) = 0.

Show that the axial flow speed w is a (Constant)*(x-1)*(x+2-y*sqrt(3))*(x+2+y*sqrt(3)) and find the Constant.

Find the tangential force per unit length on the pipe due to the viscous stress.