Control of purely-elastic instabilities in cross-slot geometries

Abstract

<h2>Abstract</h2> The cross-slot stagnation point flow is one of the benchmark problems in fluid mechanics as it allows large strains to develop and can therefore be used in extensional rheometry. In such a flow, for purely-elastic cases in creeping flow regimes, elasticity can break symmetry which is perhaps an unwanted phenomenon if used as a rheometer and will limit the maximum deformation rate in which these tests can be performed, or beneficial once used as mixing device. Here, this instability will be investigated in more detail using a combination of numerical, experimental and analytical analysis and a series of methods will be proposed that can potentially be used to delay and control the start point of the instability. The first part of this presentation focuses on the effect of elongational dominated flow on the onset criteria of symmetry-breaking purely-elastic instability in the cross-slot geometry by applying a fundamental change on the kinematics of the flowfield in this region. Here, the standard geometry is modified by adding a cylinder at the geometric centre and replacing the free stagnation point by pinned stagnation points at the cylinder walls. Next, two-phase flows of Newtonian and/or viscoelastic fluids in a "cross-slot" geometry will be investigated in the creeping-flow limit. In this part, the effect of injecting two fluids with different elastic properties from each inlet arm, and effects of interfacial tension and the viscosity ratio of these flow streams will be investigated.