The performance of a typical numerical simulation for complex flows of viscoplastic materials was examined. The inertialess flow of viscoplastic materials through an axisymmetric channel formed by an abrupt expansion followed by a contraction was employed with this purpose. Flow visualization experiments were performed with a well characterized Carbopol aqueous solution. Numerical solutions of the mass and momentum balance equations were obtained, using the Generalized Newtonian Liquid model with a biviscosity function. The flow visualization results showed that the flow pattern is essentially Newtonian for large expansion lengths. For smaller expansion lengths, however, flow is observed only in an inner axisymmetric region whose diameter is approximately the same as the one of the inlet and outlet tubes. Outside this region the flow is stagnant, and a slip interface between these two regions seems to occur. The corresponding numerical solution was not capable of predicting the observed flow pattern.
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