When is a fluid compressible

In the next column, the difference between a compressible fluid and an incompressible fluid will be described in specific. Want to know more Compressible? Compressibility effect in a fluid flow can be obtained with Mach number. Mach number is a dimensionless number representing the ratio of flow velocity to sound velocity and expressing the level of density change caused by a flow. Therefore, Mach number 0. The threshold of density change can be set arbitrarily.

Note that because sound velocity changes depending on temperature and pressure, Mach number changes under different conditions even if flow velocity is the same. On the other hand, density change of a liquid like water is quite small and the sound velocity is faster than that in a gas.

Thus, a liquid is generally treated as an incompressible fluid. It expresses how many times faster the velocity is than sound velocity. As is the case with Mach number, the definition of the velocity differs depending on the conditions of a surrounding fluid. His doctoral research focused on numerical method for fluid-solid interaction problem. He is a consulting engineer at Software Cradle and provides technical support to Cradle customers.

Remember that a fluid can be a liquid or a gas. Consider a simple example of a gas in a cylinder as shown in Figure 3. The cylinder is sealed so that gas cannot enter or escape. The fluid volume changes as the piston moves. However, the mass of the system does not change because the gas is not allowed to enter or leave the cylinder. Therefore, the fluid density must change because of the change in volume. Figure 3. On the other hand, when compression and expansion do not significantly affect the fluid density, the fluid is called an incompressible fluid.

The volume of an incompressible fluid does not change and its density is treated as a constant. Consider a liquid in a cylinder. If the cylinder is sealed, the piston will stop moving once it contacts the liquid. As the piston retracts, an empty space is created above the liquid surface. The amount of space volume the liquid occupies does not change actually the volume does change but the change is very tiny.

Liquids are always considered to be incompressible fluids, as density changes caused by pressure and temperature are small. While intuitively gases may always seem to be incompressible fluids if the gas is permitted to move, a gas can be treated as being incompressible if its change in density is small.

Consider the cylinder filled with a gas as shown in Figure 3.