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Zusammenfassung: Die Stabilit~it der ebenen Couette-Str6mung zweier nichtnewtonscher Fluidmodelle mit Relaxation wird ffir kleine St6rungen in der viskosimetrischen Ebene untersucht. Um die Darstellung der Ergebnisse zu vereinfachen, wird sie auf einFluidmodell mit konstanten Stoffdaten beschrgnkt. Abstract:The stability of plane Couette flow for two models of non-Newtonian liquid having relaxation properties was investigated for small perturbations in the viscometric plane. To simplify the presentation of the results only models with constant material properties are considered.

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Several publications describing the measurement of rheological properties of non-Newtonian liquids have reported a surface-instability phenomenon, which places an upper limit on the useable shear rates. The disturbances grow with time and slowly extend into the inner part of the flow field. A semi-empirical criterion for the onset of the instability is also given in the literature. It involves the first normal-stress coefficient of the liquid. This paper, on the other hand, shows the importance of the second normal-stress coefficient with its negative sign as a possible cause of the instability.

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