Question

A real fluid, in which the shear stress is directly proportional to the rate of shear strain (or, velocity gradient), is known as

• A. Ideal fluid
• B. Real fluid
• C. Newtonian fluid
• D. Non-Newtonian fluid

Hint:

Examples of Newtonian fluids: Water, Kerosene, Air, Gasoline.
Non-Newtonian fluids: Paints, Starch slurry, Toothpaste, Polymer solutions.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Fluids are classified rheologically based on their shear stress-shear strain rate behavior.
Fluids obeying Newton's law of viscosity exhibit a constant dynamic viscosity independent of shear rate.
Key Formula or Approach:
\[ \tau = \mu \frac{du}{dy} \]
where \(\tau\) is shear stress, \(\mu\) is dynamic viscosity, and \(\frac{du}{dy}\) is velocity gradient.

Step 2: Detailed Explanation:
According to Newton's law of viscosity, the shear stress between adjacent fluid layers is directly proportional to the velocity gradient (rate of shear deformation).
- Newtonian fluids: Follow this direct linear relationship (e.g., water, air, light oils).
- Non-Newtonian fluids: Do not exhibit a constant linear proportionality between shear stress and strain rate (e.g., slurries, pastes, blood).
- Ideal fluids: Incompressible and have zero viscosity (theoretical).

Step 3: Final Answer:
Therefore, a fluid with shear stress directly proportional to shear strain rate is a Newtonian fluid, matching option (C).