Contraction-ratio variation and prediction of large experimental pressure-drops in sharp-corner circular contraction-expansions–Boger fluids

• Predictive matching of Boger-fluid experiments in circular contraction-expansions.• Flow-transitions predicted for all aspect-ratios (α) and with rise in flow-rate.• Precise capture of enhanced pressure drops, epdmax∼600% (α=10).• α≥6: salient-c/lip-vortex co-existence observed, then dominated by elastic c-vortex.• Tight correspondence in flow-structure between streamline patterns and N1-fields. This study is concerned with the continuum modelling of sharp-corner contraction-expansion axisymmetric flows, under contraction-ratio variation, and more particularly, in the precise capture of the large-levels of experimental excess pressure-drops (epd) for Boger fluids. The particular contraction-ratios (α) considered are those studied experimentally by M. Pérez-Camacho, J.E. López-Aguilar, F. Calderas, O. Manero, M.F. Webster, J. Non-Newton. Fluid Mech. 222 (2015) 260–271; of α={2, 4, 6, 8, 10}. Their experimental PAA/corn-syrup Boger fluids have been characterized and modelled with the so-called swanINNFM model through dissipative continuum-scale modelling. This facilitates the precise capture of experimental-levels of epd-data (largest epd=O(6) under α = 10 contraction-ratio and sharp corners). The swanINNFM model has already proven capable of reproducing the large excess pressure-drops reported by J.P. Rothstein, G.H. McKinley, J. Non-Newton. Fluid Mech. 98 (2001) 33–63, in their experiments (epd=O(3) for α=4 contraction-ratio and PS/PS Boger fluids); it is also capable of reproducing the Boger-fluid pressure-drop rise, relative to Newtonian-instance, in axisymmetric α=4 contraction-flow, as opposed to the null rise observed in the planar counterpart reported by S. Nigen, K. Walters, J. Non-Newton. Fluid Mech., 102 (2002) 343–359. In the present study, at each contraction-ratio and under De-rise (flow-rate-increase), one may identify two main phases: i) an epd plateauing-region at low deformation-rates, and ii) a sudden epd-rise above the Newtonian unity reference- line. With elevation in contraction-ratio, the first plateaued-epd phase is elongated and the maximum epd-levels rise significantly. Such epd-elevation is captured theoretically and numerically, with counterpart rise in extensional-viscosity. In addition, this position in epd-response correlates well against trends in vortex-dynamics - correctly capturing lip-vortex appearance, lip-vortex and salient-corner vortex co-existence and coalescence, and ultimate elastic corner-vortex domination. In th