Which one of the following is true about ideal fluid?
Try it first: pick an option, then read the explanation.
Correct answer: B. It is incompressible
Asked in SSC JE 23.01.2018 (Morning Shift)
Detailed Explanation
The concept of an ideal fluid simplifies hydrodynamic analyses by omitting viscous dissipation and density variations.
Why “It is incompressible” Is Correct
By definition, an ideal fluid is both frictionless (non-viscous) and incompressible (mass density remains constant regardless of applied pressure).
| 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. It is compressible: Ideal fluids are assumed incompressible, not compressible.
- C. It has high shear force: Ideal fluids have zero viscosity, meaning shear forces are identically zero.
- D. It has high value of viscosity: Ideal fluids possess zero viscosity, not high viscosity.
Key Point
Ideal fluid properties: non-viscous (frictionless) and incompressible.
Key Concepts for Students
- Ideal fluid flow is governed by Euler's equation and potential theory.
- In an ideal fluid, d'Alembert's paradox states that net drag on a submerged body is zero.
- Boundary layers do not exist in ideal fluids because of the absence of viscosity.
Practice more: this is question 47 in Properties of Fluid MCQs – Part 4. Try the whole set and check every answer instantly.