What are the definitions of viscous and incompressible fluids? What should be used for differentiation?

The definitions of viscous and incompressible fluids are usually based on the viscosity of the fluid. Viscosity is a manifestation of the intermolecular forces within a liquid; it determines the degree of viscosity of the fluid, that is, the amount of resistance exerted by the molecules within the fluid during flow. Fluids with high viscosity are known as viscous fluids, which exhibit flow resistance and stickiness ； Fluids with lower viscosity are known as non-viscous fluids, and they exert less resistance to flow. In the design of separation equipment, it is important to distinguish between viscous and non-viscous fluids, as their separation behaviors differ significantly. The classification of specific parameters is usually based on the viscosity value of the fluid; a common standard is to use the values of dynamic viscosity or kinematic viscosity for such classification. For example, some literature classifies fluids with a dynamic viscosity of less than 100 mPa·s as non-viscous fluids, while those with a dynamic viscosity greater than 1000 mPa·s are classified as high-viscosity or extremely viscous fluids. Regarding design specifications, different industries and application areas may have their own standards and guidelines. In the chemical industry, organizations such as the American Petroleum Institute (API) and the American Institute of Chemical Engineers (AIChE) have issued various standards and manuals for the design of liquid separators, which can be used as references. .