If the shear rate goes out of bounds during convergence, viscosity will be limited to the min and max specified in the Power Law viscosity model settings. At very high shear rates, the minimum viscosity is applied and at at lower shear rates, maximum viscosity is applied. Convergence usually dictates the limits of viscosity, so the settings may need to be determined iteratively. If the extremes are unknown, a reasonable range is usually [0.1, 10000] cp (Approximately the range from air to honey. It will depend strongly on the fluid being modelled, however.) Higher or lower viscosities than the suggested range are generally avoided to allow good convergence, so these are essentially numerical limits. Non-Newtonian behavior of fluids is be caused by several factors, all of them related to structural reorganization of the fluid molecules due to flow. This phenomenon can be unpredictable, so extrapolating the power law viscosity to very high shear rates may not only cause difficult convergence, but may be in error.