(234y) Stability of Stagnation Point Flows of Newtonian and Complex Fluids

We present a summary of our recent results examining flow instabilities arising in stagnation point flows generated in cross-slot-type geometries. Our studies span purely inertial flows of simple Newtonian fluids, through inertio-elastic flows of highly dilute polymer solutions, to purely-elastic instabilities in more concentrated polymeric systems. In Newtonian fluids, axially-aligned spiral vortices are formed along the outlet channels of the cross-slot for flow above a modest Reynolds number, Re_c ~ 40. The addition of small quantities of polymer has a striking effect on the inertial instability, causing reductions in both the value of Re_c and the strength of the spiral vortex that forms. For higher polymer concentrations, the vorticity is completely suppressed and a purely-elastic instability mode becomes dominant above a critical Weissenberg number, Wi_c. Potential mechanisms for the various flow instabilities are also discussed.