Rheological classification: what type of fluid is water under ordinary conditions?

Introduction / Context:
Classifying fluids by their stress–strain-rate behavior guides the selection of models and correlations. Water is the most common working fluid in civil and environmental engineering.

Given Data / Assumptions:

• Moderate pressures and temperatures near ambient.
• Shear rates typical of pipe/channel flows.

Concept / Approach:
A Newtonian fluid has a linear relationship between shear stress τ and shear rate du/dy: τ = μ (du/dy), where μ is the dynamic viscosity, constant for given temperature and pressure. Water obeys this linear law across a wide range of practical conditions, making it Newtonian. Non-Newtonian fluids exhibit shear-thinning, shear-thickening, yield stress, or viscoelastic effects.

Step-by-Step Reasoning:

Verification / Alternative check:
Viscometry data for water show linear τ–du/dy over typical engineering ranges; contrast with paints (Bingham), polymer solutions (shear-thinning), and slurries (non-Newtonian).

Why Other Options Are Wrong:
Non-Newtonian and Bingham plastic behaviors do not apply to pure water. Water is effectively incompressible at low Mach numbers; labeling it “compressible fluid” is misleading for standard hydraulics.

Common Pitfalls:
Assuming incompressible means Newtonian—these are independent properties; many incompressible fluids are non-Newtonian.

Final Answer:
Newtonian fluids