A fluid which has shear stress, is proportional to the rate of shear strain is called:
Try it first: pick an option, then read the explanation.
Correct answer: D. Real fluid
Asked in SSC JE 23-09-2019 (Evening)
Detailed Explanation
A fluid that exhibits shear stress proportional to the rate of shear strain possesses viscosity. Because ideal fluids are defined to have zero viscosity, any fluid that experiences viscous shear stress must be a real fluid.
Why “Real fluid” Is Correct
Newtonian fluids obey τ = μ(du/dy) with μ > 0. Since ideal fluids have zero viscosity (τ = 0), Newtonian fluids are classified as real fluids in the absence of a distinct Newtonian option.
| Fluid type | Viscosity changes with rate of shear? | Examples |
|---|---|---|
| Ideal fluid | No (μ = 0) | Theoretical only |
| Newtonian fluid | No (μ = const) | Water, air, oil, mercury |
| Non-Newtonian | Yes | Blood, toothpaste, ketchup, paint |
| Ideal plastic (Bingham) | No, but needs yield stress τ₀ first | Fresh concrete, drilling mud |
Why the Other Options Are Wrong
- A. Non-Newtonian fluid: Non-Newtonian fluids do not exhibit proportionality between shear stress and rate of shear strain.
- B. Ideal fluid: Ideal fluids are non-viscous (μ = 0) and cannot have shear stress proportional to strain rate.
- C. Ideal Plastic fluid: Ideal plastic fluids require an initial yield stress before deforming.
Key Point
A fluid exhibiting viscous shear stress is a real fluid; ideal fluids have zero viscosity.
Key Concepts for Students
- All real fluids possess viscosity, surface tension, and compressibility.
- Ideal fluid is an idealized concept used in potential flow theory.
- Newtonian fluids are a major subclass of real fluids.
Practice more: this is question 33 in Properties of Fluid MCQs – Part 3. Try the whole set and check every answer instantly.