Flow characteristics of a "multiconfigurational", shear thinning viscoelastic fluid with particular reference to the orthogonal rheometer

This paper deals with the flow characteristics of a class of nonsimple visc oelastic fluid models developed by Rajagopal and Srinivasa (1999). The cent ral feature of these models is that the stress response is elastic from a c hanging natural configuration with the viscous dissipation occurring due to changes in the natural state. The class of models considered are character ized by three independent parameters that represent respectively the elasti city, the viscosity and the shear thinning index. The stress relaxation response of the material is compared with experimenta l data reported by Bower er al. (1987) for polyisobutelene in cetane, and p arameters that fit the data are calculated. The flow of such a fluid betwee n parallel disks rotating about noncoincident axes (the orthogonal rheomete r) is then studied. It is shown that the assumed velocity field leads to a system of second-order nonlinear ordinary differential equations (Rajagopal , 1982). A parametric study is then undertaken to see the effect of the various mate rial, geometrical, and flow parameters on the flow characteristics. It is o bserved that inertial effects and shear thinning effects are roughly comple mentary in the range of parameters considered. While it is well known that boundary layers occur in these flows due to inertial effects, it is demonst rated that these boundary effects are insensitive to the Reynolds number bu t rather are determined by the absorption number. Finally, in the range of parameters that are commonly observed in such rheometers, it is shown that neglect of inertia causes significant discrepancies in the calculation of t he boundary shear rates.