Axial Couette–Poiseuille flow of power-law viscoplastic fluids in concentric annuli

The laminar axial flow of non-Newtonian fluids obeying Robertson–Stiff model in concentric annuli is analysed. Fluid flow is produced by the inner cylinder moving along its axis and by the pressure gradient imposed in the axial direction. Both cases (where pressure gradient either assists the moving cylinder or opposes the moving cylinder) are considered. All possible cases with respect to the positions of the plug flow regions are uniquely diversified by the derived semi-analytical criteria using the entry (geometric, kinematic, and rheologic) parameters. The explicit semi-analytical expression is derived for each possible case of the volumetric flow rate.