Factors That Influence Flow
The viscosity, density, and compressibility of a fluid are the properties that determine how the liquid or gas will flow. Viscosity measures the internal friction or resistance to flow. Water, for instance, is less viscous than honey and so flows more easily. All gases are compressible whereas liquids are practically incompressible and cannot be squeezed into smaller volumes. Flow patterns in compressible fluids are more complicated and difficult to study than those in incompressible ones. Fortunately for automobile designers, air, at speeds less than 220 MPH (354 km/h) or one-third the speed of sound, can be treated as incompressible for all practical purposes. Also, for incompressible fluids, the effects of temperature changes can be neglected.