Fluid Behavior – Viscosity Independent of Shear Rate A fluid whose viscosity remains constant regardless of the rate of shear is termed a Newtonian fluid. Water, air, and light oils are common examples.

Introduction:
Classifying fluids by how shear stress relates to rate of deformation is crucial for selecting correct constitutive equations. In many engineering calculations, assuming Newtonian behavior simplifies momentum and energy analysis.

Given Data / Assumptions:

• Steady, simple shear in a continuum fluid.
• Newtonian law of viscosity: tau = mu * (du/dy).
• Viscosity mu is constant for given temperature and pressure.

Concept / Approach:

In Newtonian fluids, shear stress is directly proportional to shear rate with a proportionality constant mu that does not depend on the shear rate itself. In contrast, non-Newtonian fluids have mu_eff that varies with shear rate, time, or both (shear thinning, shear thickening, viscoelasticity).

Step-by-Step Solution:

Verification / Alternative check:

Rheometer data for simple liquids show linear tau versus shear-rate plots passing through the origin with slope mu independent of shear rate, confirming Newtonian behavior.

Why Other Options Are Wrong:

Real fluid: Generic term not tied to shear-rate independence. Ideal fluid: Zero viscosity, not a physical fluid. Non-Newtonian: Viscosity changes with shear rate. Plastic fluid: Requires yield stress before flow.

Common Pitfalls:

Equating temperature dependence of viscosity with non-Newtonian behavior; confusing apparent viscosity changes due to turbulence with constitutive properties.

Final Answer:

newtonian fluid