If the fluid is referred to as a gas, the density will change greatly depending on the amount of pressure applied due to the equation of state for gases (p=ρRT). In the study of the flow of liquids, the term used while referring to the little change in density is called incompressible flow. In the study of the flow of gases, the rapid increase due to an increase of pressure is called compressible flow.
If the fluid is described as a gas, its density will change significantly in response to varying pressure due to the equation of state for gases (p=ρRT). In contrast, when studying liquid flow, a small variation in density is referred to as incompressible flow. Conversely, when examining gas flow, a rapid increase in density resulting from increased pressure is known as compressible flow.