When a large amount of data is available, it can be tempting to build a rheological model involving a broad relaxation spectrum, even ranging up to 100 s or beyond. In most cases, this will be impractical and not very useful. Indeed, in a cessation of steady shear flow, measurements reveal that the relaxation mechanism occurs with a time scale on the order of , where is a typical shear rate involved in the experiment.

This observation allows for the identification of a typical time scale for the description of mechanisms occurring in steady flow processes, such as extrusion, fiber spinning, or film casting. An extrusion flow is characterized by a typical wall shear rate , while fiber spinning and film casting are characterized by a typical elongation rate . Consequently, if a single-mode constitutive equation is selected, the corresponding relaxation time should be specified as about or , respectively. For a multi-mode model, the relaxation times should be selected in the vicinity of or , respectively. This is quite important, since it enables the setup of a model that is in agreement with the typical time scales involved in the simulation. Note that the previous comments raise questions about the relevance of Weissenberg numbers as high as 10 or 100.

For most applications, the computational domain is open, with fluid entry and exit. The residence time of fluid particles in the computational domain usually remains moderate, so extremely long relaxation times are not usually effective. Fluid particles trapped in vortices usually do not affect the main flow; they are instead a consequence of it. Finally, in extrusion, the extruded material solidifies long before the effects of these long relaxation times become visible.