Swirling flow of viscoelastic fluids. Part 1. Interaction between inertia and elasticity

A torsionally driven cavity, consisting of a fully enclosed cylinder with r otating bottom lid, is used to examine the confined swirling flow of low-vi scosity Boger fluids for situations where inertia dominates the flow field. Flow visualization and the optical technique of particle image velocimetry (PIV) are used to examine the effect of small amounts of fluid elasticity on the phenomenon of vortex breakdown. Low-viscosity Boger fluids are used which consist of dilute concentrations of high molecular weight polyacrylam ide or semi-dilute concentrations of xanthan gum in a Newtonian solvent. Th e introduction of elasticity results in a 20% and 40% increase in the minim um critical aspect ratio required for vortex breakdown to occur using polya crylamide and xanthan gum, respectively, at concentrations of 45 p.p.m. Whe n the concentrations of either polyacrylamide or xanthan gum are raised to 75 p.p.m., vortex breakdown is entirely suppressed for the cylinder aspect ratios examined. Radial and axial velocity measurements along the axial cen treline show that the alteration in existence domain is linked to a decreas e in the magnitude of the peak in axial velocity along the central axis. Th e minimum peak axial velocities along the central axis for the 75 p.p.m. po lyacrylamide and 75 p.p.m. xanthan gum Boger fluids are 67% and 86% lower i n magnitude, respectively, than for the Newtonian fluid at Reynolds number of Re approximate to 1500-1600. This decrease in axial velocity is associat ed with the interaction of elasticity in the governing boundary on the rota ting base lid and/or the interaction of extensional viscosity in areas with high velocity gradients. The low-viscosity Boger fluids used in this study are theologically characterized and the steady complex flow held has well- defined boundary conditions. Therefore, the results will allow validation o f non-Newtonian constitutive models in a numerical model of a torsionally d riven cavity flow.