It is called compressible flow to that fluid whose density varies significantly to a pressure change. Both gases, liquids, and solids all decrease in volume when pressure is applied to them. The relationship between the volume variation and the pressure variation is a constant K, characteristic of each material, which depends on its elasticity.
The value of K is very large for solids and liquids, since their volume (therefore their density) varies little when faced with a change in pressure. It is for this reason that most solids and liquids are said to be incompressible . On the other hand, the value of K is very small for gases, given that under high pressure, its volume decreases greatly. So we say that most gases are compressible fluids.
The compressibility of a fluid can also be assessed by the speed at which small disturbances are transmitted within the fluid itself. This speed is called sonic speed, or the speed of sound in the fluid. Compressible fluids have low sonic velocities; in incompressible flows, the sonic speed is high.
In this way, at an atmosphere and 20 degrees Celsius, the speed of sound in water is 1483.2 meters per second, in air, the sonic speed is 331.3 meters per second.
The relationship between the flow velocity (V) and the speed of sound (c) in the fluid medium is called the Mach number.
For Mach values less than 0.3, the density changes are only of the 2% environment under pressure variations, therefore, fluids with Mach less than 0.3 are considered incompressible. In liquids, the speed of sound is high, therefore the Mach is small, being incompressible fluids.
According to their Mach number, compressible fluids can be classified into:
- Subsonicfluids : with Mach less than 1.
- Fluid sonic: Mach equal to 1.
- Supersonicfluids : with Mach greater than 1.
In a more complete classification, we have:
- Incompressibleflow : Mach less than 0.3.
- Subsonicflow : Mach between 0.3 and 0.8.
- Transonicflux : Mach between 0.8 and 1.2
- Supersonicflow : Mach enre 1.2 and 3.0
- Hypersonicflow . Mach greater than 3.0
The compressible flows have applications in various areas. A very common example is compressed air systems that are used in dental equipment and workshop tools, high pressure pipes to transport gases, among others. The effects of fluid compressibility are very important in the design of high-speed aircraft.