VELOCITY AND STRESS-FIELDS OF POLYMERIC LIQUIDS FLOWING IN A PERIODICALLY CONSTRICTED CHANNEL .2. OBSERVATIONS OF NON-NEWTONIAN BEHAVIOR

The apparatus described in the preceding Part 1 of this work has been used to investigate the flow behavior of a 20% polystyrene solution in a periodically constricted channel. Local velocity and stress measure ments made over a range of flow rates, corresponding to creeping flow and shear-rate based Weissenberg numbers as large as 15, exhibited cle ar departures from Newtonian behavior. These departures included norma l stress growth delay, local maxima and sign reversal in the shear str ess near the flow cell surfaces, and significant deviation of the axia l velocity profile from Newtonian predictions. Finite element simulati ons for creeping flow of the generalized Newtonian, upper convected Ma xwell and White-Metzner fluids predict all of these features to some e xtent, but in general fail to describe the overall behavior of the flu id. A simplified analysis, using the Maxwell model with a shear-thinni ng velocity profile, indicates that the most striking non-Newtonian ef fect, the shear stress sign reversal, is associated with elastic recoi l as fluid elements near the wall move from a region of high shear rat e into a region of low shear rate. That this can occur only if the she ar rates along the wall exceed the Newtonian predictions is consistent with our observations.