The Stability of Helical Flow of Pseudoplastic Fluids

The transition in rotational flow from laminar Couette flow to unstable turbulent flow is associated with the development of toroidal vortices (Taylor vortices) regularly spaced along the axis of the cylinders. These vortices can greatly increase the value of the measured torque of a rotating cylinder, above that which would normally be expected. Consequently the presence of these vortices will affect rheological measurements in concentric cylinder geometries. A good knowledge of the onset of these vortices is critical in determining the operating limits of any rotating rheological instruments. It has been shown that the addition of axial flow to Couette flow, resulting in helical flow, can have a stabilising influence on the flow by delaying the onset of Taylor vortices. The onset of Taylor vortices in Newtonian fluids under extremely slow acceleration conditions is well defined. Many practical applications including rheological measurements however involve non-Newtonian fluids in wide gap geometries where the rotational acceleration rates are significantly higher. The development of Taylor vortices under these conditions is examined to develop a stability criterion for Couette and helical flows.