The flow of polymeric liquids through an abrupt contraction is known to become unstable at high shear rates. To clarify these phenomena, we visualized the flow and measured fluid velocity with a laser-doppler velocimeter. This instability is characterized by two circulating secondary flows near the contraction becoming larger and smaller periodically and alternately. The shear rate at the onset ofinstability and the periodic time of flow instability depend on the elastic property of polymeric liquids.

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