Entrance and exit effects for a viscoelastic liquid displacing a simple liquid through a contraction

•We studied an original situation, i.e. the flow of a viscoelastic fluid pushing a Newtonian one through a restriction.•Although of practical importance this situation has almost never been considered.•The interface shape between the fluids depends on the Weissenberg number and the ratio of normal to tangential stress.•The interface shape is reminiscent of that between vortices and convected regions for a single viscoelastic fluid. We studied the displacement of the interface between a viscoelastic fluid pushing a simple liquid through a rectangular contraction by following the front interface deformation in time. The progressive deformation of the interface until apparent stabilization is followed, which makes it possible to identify a transient and a stationary regime. For low Weissenberg number the shape of the interface is essentially similar to that between two simple liquids. For sufficiently large Weissenberg number the shape of the interface is different: it is narrower before the entrance and wider just after the exit. The characteristics of this shape are qualitatively analogous to those of the interface between the vortices and the convected regions for the flow of a single viscoelastic fluid through a contraction–expansion. This suggests that the entrance effect is due to extensional effects and the exit effect is due to normal stress effects.