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Author(s): Manuel António Alves
Fernando Tavares de Pinho
Paulo Jorge Oliveira
Title: Viscoelastic flow of boger fluids in a 4:1 square/square contraction
Issue Date: 2004
Abstract: Sudden contraction flows are classical benchmark problems in computational rheology (Hassager, 1988), and alarge number of visualization studies in planar and axisymmetric contractions have been published in the literature. Inthese simple configurations, the flow behaviour of non-Newtonian fluids can be very surprising, and different flowpatterns have been observed even for fluids with apparently similar rheological properties.The first visualizations in circular contractions for viscoelastic fluids were carried out by Cable and Boger (1978a,1978b, 1979) and Nguyen and Boger (1979), who reported a dramatic growth of the salient corner vortex for shear rateshigher than a critical value above which the normal stress grew quadratically. These experiments were performed forcontraction ratios in the range 7.67:1 to 14.83:1 and at very high Weissenberg numbers the flow became asymmetricand eventually time-dependent. In 1986 Boger et al investigated the behaviour of Boger fluids with similar steady anddynamic shear properties and found different vortex dynamics, thus concluding that a different fluid property had to betaken into account. In his 1987 review paper Boger suggested extensional viscosity as that property, and described insome detail the sequence of flow dynamics in the sudden contraction. For some fluids, only a corner vortex exists,which grows in size as elasticity increases whereas for other fluids the corner vortex extends to the re-entrant cornernear which a lip vortex is formed. For high contraction ratios the two vortices are initially separate, as also seen byMcKinley et al (1991) in his 4:1 contraction experiments. As the elasticity increased the lip vortex grew at the expenseof the corner vortex, while the length of the recirculation remained fairly constant. Eventually, the lip vortex occupiedthe whole contraction plane region and a further increase in the Weissenberg number lead to an increase of the nowconcave shaped vortex. At higher Weissenberg numbers a small pulsating lip vortex appeared and lead to unsteadybehaviour.The relevance of extensional viscosity was also emphasized in the experimental investigations of White and Baird(1986) in a planar contraction with polystyrene (PS) and low density polyethylene (LDPE): whereas a vortex was foundfor the LDPE, it was absent from PS and the difference was attributed to their different extensional viscosities. This wasfurther emphasised when they used later (White and Baird, 1988) a constitutive equation that represented correctly themeasured extensional viscosity of both fluids and were able to numerically predict the different vortex patternsobserved in 1986. In the mid 1980s the experimental work on the 4:1 contraction flow concentrated on assessing thevarious flow transitions and instabilities and used several experimental techniques, as in McKinley et al (1991).
Subject: Ciências da engenharia e tecnologias
Engineering and technology
Scientific areas: Ciências da engenharia e tecnologias
Engineering and technology
Source: 10th Brazilian Congress of Thermal Sciences and Engineering - EMCIT 2004
Document Type: Artigo em Livro de Atas de Conferência Internacional
Rights: restrictedAccess
Appears in Collections:FEUP - Artigo em Livro de Atas de Conferência Internacional

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