Visualizations of viscoelastic flow in a 4:1 square/square contraction

Abstract

Visualizations of the three-dimensional flow in a 4:1 square-square sudden contraction were carried out for a Newtonian and two viscoelastic fluids under conditions of negligible inertia. In the case of the Newtonian fluid the influence of inertia was also studied, and for Reynolds numbers above 0.1 a significant decrease of the recirculation was observed. In the case of viscoelastic fluids increasing the flowrate lead to different flow patterns depending on the Deborah number and the type of viscoelastic fluid: whereas for the Boger fluid a complex flow pattern was observed, with a decrease on the vortex size up to Deborah numbers of 0.2, followed by a moderate increase up to Deborah values of 0.8, for the shear-thinning viscoelastic fluid there was a dramatic increase on the size of the vortex. With both viscoelastic fluids an elastic instability lead to the onset of a periodic flow at high flowrates (under conditions of negligible inertia). For the shear-thinning viscoelastic fluid the periodic flow was characterized by a constant Strouhal number of 0.5. In the steady flow regime the vortices are not closed: fluid particles enter the middle-plane vortex, rotate towards its eye, drift to the diagonal-plane vortex, then rotate to its periphery and exit to the downstream duct, a dynamic process that is opposite to that observed and predicted numerically for Newtonian fluids.