Considering a Non-Constant Anisotropicity Parameter in the Giesekus Model

The Giesekus model has proven to be one of the most successful constitutive rheological models. Although Giesekus introduced the anisotropicity parameter as a constant, recent evidence suggests that it should not be. We elaborate herein on the implications of having a variable anisotropicity coefficient; to our knowledge, this is only the second such model. We find that the modification leads to important differences in the predictions of the second normal stress coefficient in simple shear flow, of which the most significant is the shift of the linear viscoelastic envelope of the second normal stress coefficient to higher values in the case of start-up simple shear flow, which is more in line with experimental data in the literature.