Upstream vortex and elastic wave in the viscoelastic flow around a confined cylinder

The flow of a viscoelastic fluid past a cylinder is a classic benchmark problem that is not completely understood. Using novel holographic particle velocimetry to measure 3D flow fields, we report two main discoveries of the elastic instability upstream of a single cylinder in confined microchannel flow. After the onset of corner vortices upstream of the cylinder, we find that the vortex becomes unsteady and switches between two distinct flow states, leading to symmetry breaking perpendicular to the cylinder axis that is highly three-dimensional in nature. Second, we show that the disturbance of the elastic instability propagates far upstream via an elastic wave and is weakly correlated with that in the cylinder wake. The wave speed and the extent of the instability increase with Weissenberg number, indicating an absolute instability in viscoelastic fluids.