Ideal Fluid: Definition and Characteristics

The question asks to identify a fluid that is both incompressible and has no viscosity. Let's break down the properties mentioned:

• Incompressible Fluid: A fluid whose density remains constant, regardless of changes in pressure. The volume of the fluid does not change significantly when pressure is applied.
• No Viscosity (Non-viscous Fluid): Viscosity is a measure of a fluid's resistance to flow (internal friction). A fluid with no viscosity offers no resistance to shear stress and flows without friction.

A fluid possessing both these properties – being incompressible and having zero viscosity – is defined as an Ideal fluid. This is a theoretical concept used in fluid mechanics to simplify analysis, as all real fluids exhibit some degree of viscosity and compressibility.

Comparing Fluid Types

To understand why "Ideal fluid" is the correct answer, let's look at the other options:

• Real fluid: This term refers to any fluid that actually exists and flows in reality. All real fluids possess viscosity (even gases like air have viscosity, albeit low) and are generally compressible to some extent, although some may be approximated as incompressible under certain conditions (like liquids).
• Newtonian fluid: This type of fluid follows Newton's law of viscosity, meaning the shear stress is directly proportional to the rate of shear strain. Examples include water, air, and oil. Newtonian fluids can have viscosity and may be compressible or incompressible.
• Ideal plastic fluid: This is a fluid that behaves like a solid until a certain critical shear stress (yield stress) is exceeded, after which it flows like a viscous fluid. It possesses viscosity and a yield stress, distinguishing it from an ideal fluid.

Based on these definitions, only the concept of an Ideal fluid matches the description of being both incompressible and having no viscosity.