Viscosity Model Panel

AFT Impulse can model systems with Newtonian or non-Newtonian fluids. A Newtonian fluid is one where the fluid viscosity is fixed and independent of fluid velocity, while a non-Newtonian fluid is one where the fluid viscosity is variable and dependent on fluid velocity. Most industrial fluids are Newtonian fluids, although there are important applications for non-Newtonian fluids.

AFT Impulse offers seven viscosity models:

Modeling non-Newtonian fluids is a complex task. A myriad of test equipment and theoretical models exist, and each have various names unique to an industry or application. This discussion along with other discussion related to non-Newtonian flow in Impulse seeks to help the reader understand what the viscosity model is meant for, how to use the model, and how the model calculates hydraulic behavior in a system.

Frictional Losses

Each non-Newtonian fluid model adjusts the frictional losses in the system from the Newtonian fluid model. The equations and correlations used to determine pipe frictional losses are detailed on each model’s page. These methods to determine frictional losses replace the pipe friction model specified in the Pipe Properties window. For pipe Fittings and junction-specific losses, all non-Newtonian models use the same method, as detailed on the Laminar and Non-Newtonian Corrections panel topic.

Applicability in Transient Flow

AFT Impulse uses the same non-Newtonian models and correlations for both steady-state and transient flow. The applicability of these viscosity models to transient flow is uncertain. Studies suggest that the viscosity models available in Impulse remain valid in transient systems, but no definitive answer exists. Users should treat all transient non-Newtonian hydraulic results with caution.

Impulse models these non-Newtonian fluids in a transient system by recalculating friction factor or pressure loss information each time step as dictated by the settings on the Transient Friction panel. It is strongly recommended that transient systems with non-Newtonian fluids are modeled with the Time Varying – Simple model (selected by default for all models created in Impulse 8 or later). The Constant model will not capture the non-Newtonian behavior of the fluid as velocity changes. The Time Varying – Advanced (Unsteady Brunone) model has not been studied for use with non-Newtonian fluids.