“…36. Wang 23 reports that for Stokes flow through a straight, twisting tube with wavy walls, the axial pressure gradient is independent of ⑀ to first order when the helical pitch is infinite, that is, the cross section does not twist, as is the case here. Note that other researchers, including Dean himself, use the maximum pipe velocity in determining C, which produces C = 4, see for example Refs.…”