Isothermal flow is described as compressible flow with constant temperature. Which statement best reflects this property?

• A. Temperature remains constant
• B. Pressure remains constant
• C. Density remains constant
• D. Velocity remains constant

Answer: (A)

Isothermal flow describes a compressible flow where the temperature does not change along the flow path. Because the temperature is fixed, the pressure and density can still vary as the fluid is compressed or expanded, with their relationship given by p = ρ R T and T constant. The velocity is not required to stay the same; it can change as the flow accelerates or decelerates in response to geometry and forces. So the defining property is that the temperature remains constant, while pressure and density may vary and the flow remains compressible.