Contraction and expansion flows of non-Newtonian fluids

Abstract

An experimental study is reported concerning the flow of non-Newtonian fluids through contractions and expansions, with emphasis on the latter. Flow visualization is used to illustrate how the flow fields are affected by changing the non-Newtonian character of the fluid. In particular, it will be shown that for the range of conditions covered by the study, elastic fluids exhibit vortex enhancement in contraction flow but vortices are inhibited in expansion flows. The effect of inertia is opposite: vortex motion in the contraction is reduced but in an expansion inertia causes vortex enhancement. A different scenario is obtained by adding a very small concentration of inextensible long fibres to a Newtonian fluid. In this case both contraction and expansion flows are characterized by the occurrence of large vortices (although vortex enhancement is not significant). Thus, dilute fibre suspensions behave similarly to polymeric solutions in contractions but not in expansions. The study also indicates that the fibres fail to flow affinely with the bulk flow. This is particularly apparent in expansion flow where the fibres tend to an orientation which is transverse to the flow direction and also close to boundary walls where substantial fibre-free regions develop.