| Summary: | Elastic instabilities of entangled polymer melts are common in industrial processes but the physics responsible is not well understood. We present a numerical linear stability study of a molecular-based constitutive model which grants us physical insight into the underlying mechanics involved. Two constriction flows are considered - one shear dominated, the other extension dominated - and two distinct instabilities are found. The influence of the molecular structure and the behaviour of the polymer dynamics are investigated and in both cases chain relaxation and orientation play a crucial role. Additionally, convective-constraint release is also shown to be critical, but with a contrasting effect in the two flows: when the molecular weight is high, slow release is found to be stabilising in the extension dominated constriction but destabilising in the shear dominated flow. This suggests a molecular-based physical interpretation of the underlying mechanisms responsible for flow instabilities.
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