The flow visualization experiments of shear-thinning aqueous polyacrylamide solutions through a rectangular channel with an abrupt contraction were carried out, using not only the tracer method but also dye solution injection technique in order to study the transition from a steady flow regime to an unstable flow regime and the structure of the unstable flows. It is found that two basic transition patterns can be identified accoding to the flow properties of polymer solutions and channel

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... y. The one follows a rapid enhancement of the salient corner vortex in flows of rather concentrated solutions through a channel with a large contraction ratio, and the unstable flow is characterized by the fact that two salient corner vortices grow and decay alternately. The other is observed in rather dilute solution flows through a channel with a small contraction ratio. In this case the salient corner vortices are almost completely diminished. In the tracer-visualization experiments, it is clear that diverging flow occurs first and the radius of curvature of the main flow around the salient corner becomes reduced with an increase in flow rate. The so-called bundle-like streams are then generated near the salient corners in unstable flows as the flow rate is further increased. The dye-solution injection technique reveals that a stream bundle consists of two counterrotatingvortex tubes, i.e. Goertler-like vortices, whose cross-section is approximately rectangular.