Real (practical) fluids: which intrinsic properties do they possess?

Introduction / Context:
Unlike the idealized “ideal fluid,” real or practical fluids exhibit a suite of physical properties that affect flow behaviour, measurement techniques, and design outcomes in pipelines, open channels, and equipment such as pumps and turbines.

Given Data / Assumptions:

• Continuum assumption holds.
• Ambient temperature and pressure ranges common to engineering applications.
• Real fluids include liquids and gases with molecular interactions.

Concept / Approach:

Viscosity quantifies resistance to shear and is central in laminar and turbulent flow predictions (Reynolds number). Surface tension governs capillarity, droplet formation, and free-surface curvature at small scales. Compressibility, though small for liquids, is finite and important in water hammer, acoustics, and high-pressure systems; for gases, it is significant and modeled via equations of state.

Step-by-Step Solution:

Verification / Alternative check:

Empirical data: dynamic viscosity of water ~1 mPa·s at 20°C; surface tension ~0.072 N/m; bulk modulus ~2.2 GPa—none are zero, proving the point for a common fluid.

Why Other Options Are Wrong:

(a), (b), or (c) alone are incomplete. Only (d) captures the full set of inherent properties of practical fluids.

Common Pitfalls:

Assuming incompressibility equates to infinite bulk modulus; in reality, “incompressible” is an approximation where density change is negligible for the process in question.

Final Answer:

All the above.