Non-Newtonian Fluids

Definition

Non-Newtonian fluids are a class of fluids whose flow properties differ in fundamental ways from those of Newtonian fluids. Unlike Newtonian fluids, the viscosity of non-Newtonian fluids can change depending on the applied shear stress or shear rate, leading to complex flow behaviors that are important in the context of fluid dynamics and rheology.

5 Must Know Facts For Your Next Test

1. Non-Newtonian fluids do not obey the linear relationship between shear stress and shear rate that is characteristic of Newtonian fluids.
2. The viscosity of non-Newtonian fluids can change with the rate of shear, leading to behaviors such as shear-thinning (viscosity decreasing with increasing shear rate) or shear-thickening (viscosity increasing with increasing shear rate).
3. Examples of non-Newtonian fluids include ketchup, toothpaste, blood, and certain types of paints and polymers.
4. The flow of non-Newtonian fluids is often described using the Herschel-Bulkley model, which includes parameters for yield stress, consistency, and flow behavior index.
5. The complex flow behavior of non-Newtonian fluids can have important implications in various engineering applications, such as the design of pumps, valves, and processing equipment.

Review Questions

• Explain how the viscosity of non-Newtonian fluids differs from that of Newtonian fluids and how this affects their flow behavior.
  • Unlike Newtonian fluids, the viscosity of non-Newtonian fluids is not constant but can change depending on the applied shear stress or shear rate. This leads to complex flow behaviors, such as shear-thinning (where viscosity decreases with increasing shear rate) or shear-thickening (where viscosity increases with increasing shear rate). These changes in viscosity result in flow patterns that deviate from the simple linear relationship between shear stress and shear rate observed in Newtonian fluids, making the flow of non-Newtonian fluids more challenging to predict and model.
• Describe how the flow of non-Newtonian fluids through a pipe, as described by Poiseuille's Law, would differ from the flow of Newtonian fluids.
  • Poiseuille's Law, which describes the volumetric flow rate of a Newtonian fluid through a cylindrical pipe, does not directly apply to non-Newtonian fluids. Due to the complex, non-linear relationship between shear stress and shear rate in non-Newtonian fluids, their flow through a pipe will not follow the same parabolic velocity profile as Newtonian fluids. Instead, the flow behavior of non-Newtonian fluids in pipes can be influenced by factors such as yield stress, shear-thinning or shear-thickening effects, and the specific rheological model used to describe the fluid's properties. As a result, the volumetric flow rate and pressure drop for non-Newtonian fluids in pipes will differ from the predictions of Poiseuille's Law for Newtonian fluids.
• Analyze the importance of understanding the flow behavior of non-Newtonian fluids in the context of viscosity and laminar flow, and explain how this knowledge can be applied to practical engineering problems.
  • Understanding the flow behavior of non-Newtonian fluids is crucial in various engineering applications where these types of fluids are encountered. Unlike Newtonian fluids, the viscosity of non-Newtonian fluids can change dramatically with shear rate, leading to complex flow patterns that deviate from the simple laminar flow described by Poiseuille's Law. This knowledge is essential for the design and optimization of equipment and processes involving non-Newtonian fluids, such as the flow through pipes, valves, and pumps, as well as in the processing and handling of materials like paints, polymers, and biological fluids. By accurately modeling the rheological behavior of non-Newtonian fluids, engineers can develop more efficient and reliable systems, optimize process parameters, and ensure the proper functioning of critical components in a wide range of industries.

Related terms

Viscosity:

The measure of a fluid's resistance to flow, which determines how easily the fluid can move or deform under an applied force.

Laminar Flow:

A smooth, orderly flow of a fluid in parallel layers, with no disruption between the layers.

Poiseuille's Law: An equation that describes the volumetric flow rate of a Newtonian fluid through a cylindrical pipe, based on the pressure difference, pipe radius, and fluid viscosity.