eng URI http://hdl.handle.net/10795/3411 physical sciences applied research university research During extrusion of viscoelastic fluids various flow instabilities may arise resulting in a distorted free surface. In order to investigate the factors generating these instabilities we performed a linear stability analysis at zero Reynolds number around the steady solution of the cylindrical or planar stick-slip flowfor a viscoelastic fluid following the affine exponential Phan-Thien Tanner (PTT) model. Stick-slip flow is an important special case of the extrudate swell problem, since the latter reduces to it in the limit of infinite surface tension but avoids the complications of a free-boundary flow. The linear stability analysis is performed for various values of the rheological parameters of the PTT model in order to determine the effects of all material properties. It is found that the flow becomes unstable as the Weissenberg number increases above a critical value, due to a Hopf bifurcation suggesting that the flow will become periodic in time. Both the critical value of theWeissenberg number and the frequency of the instability depend strongly on the rheological parameters of the viscoelastic model. The corresponding eigenvectors indicate that the perturbed flow field has a spatially periodic structure, initiated at the rim of the die, extending for up to 5–7 die gaps downstream, but confined close to the surface of the extrudate, in qualitative agreement with existing experiments. This suggests that instability is generated by the combination of the singularity in the velocity and stress fields at the die lip and the strong extension that the extruded polymer undergoes near its surface. The elasticity alone can be responsible for the appearance of instabilities in the extrusion process of viscoelastic fluids and the often used assumptions of wall slip or compressibility, although they might be present, are not required. Finally, the mechanisms that produce these instabilities are examined through energy analysis of the disturbance flow 29 pp. LOMv1.0 creator LOMv1.0 publisher LOMv1.0 Project Executing Organisation 2013 Digital Library of the Operational Programme "Education and Lifelong Learning" abstract types Text Viscoelastic fluids Linear stability analysis PTT Rheological parameters Extruded polymer 3142518 application/pdf http://repository.edulll.gr/edulll/bitstream/10795/3411/2/3411_POF_paper1.pdf Πανεπιστήμιο Θεσσαλίας. Τμήμα Μηχανολόγων Μηχανικών. - Δημοσιεύσεις. URI http://hdl.handle.net/10795/3411 LOMv1.0creator 2016-05-25T11:15:19Z LOMv1.0validator 2016-05-25T11:15:19Z LOMv1.0 gre no no Copyright EYD-EPEDBM (Operational Programme "Education and Lifelong Learning")