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Seminars
Viscoelastic flow simulations in a 3D benchmark flow - Velocity overshoots inside a smooth contraction
Fernando Tavares de Pinho
Mechanical Engineering Department
University of Minho
When: Thursday, February 9, 2006
Time: 2:30 PM to 3:30 PM
Where: 1070 Duncan Hall
Abstract:
Industrial flows tend to be three-dimensional, and often contain separated flow regions to improve mixing, heat and mass transfer. However, when looking back at viscoelastic flows we realize how little has been done under such conditions and, not surprisingly, we often see remarkable differences in comparison with Newtonian fluid behaviour. The computational rheology community is even more focused into solving such challenges as the Weissenberg number problem and dedicate a fair amount of time at investigating geometrically simple flows, such as the sudden contraction or the flow around a cylinder.
Measurements of the laminar flow of various aqueous solutions of polyacrilamide downstream a backward facing step (BFS) by Poole et al [1,2] have shown unexpected differences relative to Newtonian flow features. This motivated a more in-depth experimental investigation of the flow upstream of the BFS, namely inside the preceding square duct and smooth contraction, where large velocity overshoots with very strong gradients appearing near the side walls were observed. Subsequently, an extensive numerical investigation was undertaken in the five parameter space of a fluid described by the Phan-Thien—Tanner constitutive equation with non-zero second normal stress difference and a solvent contribution, aimed at understanding the flow characteristics and its relation with fluid rheology.
The numerical simulations were able to qualitatively capture the velocity overshoots on the XZ-centerline profiles, and observed also overshoots on the XY-mid plane and the so far unreported enhanced velocity peak at the duct corners. The overshoots took place for particular combinations of the independent parameters, in particular they required large Weissenberg numbers, large second-normal stress differences, strain-hardening of the extensional viscosity and intense shear-thinning, together with non-negligible inertia.
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