  We present experimental evidence of global viscoelastic flow transitions in
2:1, 8:1 and 32:1 planar contractions under inertia-less conditions. Light
sheet visualization and laser Doppler velocimetry techniques are used to probe
spatial structure and time scales associated with the onset of these
instabilities. The results are reported in terms of critical Weissenberg
numbers characterizing the fluid flow rates. For a given contraction ratio and
polymer fluid, a two-dimensional, steady flow with converging streamlines
transitions to a two-dimensional pattern with diverging streamlines beyond a
critical Weissenberg number. At even higher Weissenberg numbers, spatial
transition to three-dimensional flow is observed. A relationship between the
upstream Weissenberg number for the onset of this spatial instability and the
contraction ratio is derived. For contraction ratios substantially greater than
unity, we observe that further increase in the flow rate results in a second
temporal transition to a three-dimensional, time-dependent flow. This complex
flow arises due to a combination of the effects of stress-curvature interaction
and three dimensional perturbations induced by the walls bounding the neutral
direction. Comparison with studies on other geometries indicates that boundary
shapes deeply influence the sequence and nature of flow transitions.
