What does the term 'compressibility' refer to in fluid mechanics?

The term 'compressibility' in fluid mechanics specifically refers to the measure of how much a substance can be compressed under pressure. This property is crucial in understanding how fluids behave when subjected to changes in environmental conditions, such as pressure and temperature.

Compressibility quantifies the relative change in volume of a substance in response to a change in pressure. For gases, where molecules are spaced far apart, compressibility is significant, resulting in substantial volume changes when compressed. For liquids, which are typically less compressible, the effect is much smaller but still present.

Understanding compressibility is essential in various engineering applications, including hydraulics and aerodynamics, where accurate predictions of fluid behavior are necessary for effective design and operational efficiency.

Other options presented do not address the specific definition of compressibility. For example, the ability of a substance to expand relates to thermal expansion, while viscosity refers to a fluid’s resistance to flow, and density describes mass per unit volume. Each of these properties plays a role in fluid mechanics, but they do not define compressibility itself.