Which of the following is listed as a characteristic of the ideal fluid in hydraulic theory?

• A. Thermal stability
• B. Hydrolytic stability
• C. Low chemical corrosiveness
• D. Constant viscosity, regardless of temperature

Answer: (D)

In hydraulic theory, the ideal fluid is treated with simplifying assumptions that remove viscous effects so the flow is governed mainly by pressure and velocity. Treating viscosity as a constant that does not change with temperature keeps the mathematical model straightforward and predictable, since real fluids have viscosity that varies with temperature and produces frictional losses. By assuming constant viscosity, the analysis stays clean and aligns with the concept of an ideal, frictionless (or near-frictionless) flow. The other properties describe how a real fluid interacts with materials and temperature, which are important in practice but do not define the ideal-fluid model itself.