Non-Newtonian fluids are fluids that do not obey the linear relationship between shear stress and shear rate observed in Newtonian fluids. In other words, their viscosity (resistance to flow) is not constant and can vary with the rate of deformation, shear rate, temperature, or other factors. Non-Newtonian fluids exhibit a wide range of behaviors, including shear-thinning, shear-thickening, and viscoelastic behavior. One commonly used model for non-Newtonian fluids is the power-law model, which relates the shear stress ($\tau$) to the shear rate ($du/dy$) as follows:

Where:

• $\tau$ is the shear stress,
• $du/dy$ is the shear rate, 
• $K$ is the consistency index,
• $n$ is the flow behavior index.