Abstract
Experiments were conducted using a particle image velocimetry (PIV) technique and a polarization imaging technique, in a channel flow of viscoelastic fluids into a sudden contraction at viscoelastic Mach numbers of 6.5 and 0.65 based on the maximum velocity of the upstream plane Poiseuille flow and the shear wave speed of the fluid. A viscoelastic wormlike micelle solution of CTAB/NaSal was used for the test fluid. The velocity and vorticity fields were measured in a plane normal to the slit. At a Mach number of 6.5, the vorticity field in the experiments on a 5:1 contraction exhibits a unique structure of symmetric two streaky layers, each of which is composed of a pair of counter-rotating vorticity sheets starting from the slit. While no streaky layers are observed at a Mach number of 0.65. The observed vorticity layers are consistent with the geometry of the characteristics for the plane Poiseuille flows of the upper-convected Maxwell model. Polarization imaging results show that the micelles orient parallel to the flow direction in the streaky layers. The orientation angle maps also show that the crossing of the streaky layers is allowed in the flow into a double slit, which supports the consistency of the description of the flow phenomena with characteristic lines. For the streaky flows, the results suggest that the onset can be predicted by a critical viscoelastic Mach number and an elasticity number.
