Inertioelastic flow instability at a stagnation point

High molecular weight polymer additives can suppress inertial flow instabilities and reduce turbulent drag. Yet exactly how the polymer does this is difficult to study. For Newtonian fluids, beyond a critical Reynolds number (Re_c), an inertial instability in the cross-slot geometry results in the formation of a single steady streamwise vortex, providing a good model system to study vortex flow. A novel configuration of cross-slot allows flow velocimetry in the cross-section of the vortex as Re is increased. Using polymeric fluids of incrementally increasing elasticity (El), we are able to precisely quantify the critical conditions for instability and the subsequent intensification of streamwise vorticity. Increasing El at low levels causes dramatic reductions in both Re_c and the vorticity growth. For higher El, vorticity is completely suppressed and a purely-elastic instability emerges beyond a critical Weissenberg number (Wi_c). Dimensionless phase diagrams in Re-Wi and Re-El phase space delineate regions of stable flow from those of inertia and elasticity-dominated instabilities. Our findings shed valuable insight into the action of polymer on vortices and bridge the gap between inertial and elastic instabilities in intersecting flows.