Adiabatic compressibility

It is a measure of the compressibility of a body or thermodynamic system when it is subjected to a quasi-static transformation") of pressure under conditions of perfect thermal insulation, it is given by (in Spanish notation it is usually represented as, but not to be confused with the adiabatic compressibility modulus):.

In an adiabatic process of pressure variation, the body will experience some change in temperature.

All fluids are compressible, including liquids. When these volume changes are too large we choose to consider the flow as compressible (showing a significant variation in density as a result of flowing), this happens when the speed of the flow is close to the speed of sound. These changes usually occur mainly in gases since to reach these flow speeds in liquids, pressures of the order of 1000 atmospheres are required, whereas a gas only requires a pressure ratio of 2:1 to reach sonic speeds. The compressibility of a flow is basically a measure of the change in density. Gases are generally very compressible, however, most liquids have a very low compressibility. For example, a pressure of 500 kPa causes a density change in water of only 0.024% (at room temperature). On the other hand, this same pressure applied to air causes a density change of 250%. For this reason, the study of compressible flows is normally known as gas dynamics, this being a new branch of fluid mechanics, which describes these flows.

In a flow there are usually changes in pressure, associated with changes in velocity. In general, these pressure changes will induce density changes, which influence the flow; if these changes are important, the temperature changes presented are appreciable. Although density changes in a flow can be very important, there are a large number of situations of practical importance in which these changes are negligible.

The flow of a compressible fluid is governed by the first law of thermodynamics in energy balances and by the second law of thermodynamics, which relates heat transfer and irreversibility to entropy. The flow is affected by kinetic and dynamic effects, described by Newton's laws, in an inertial reference frame – the one where Newton's laws are applicable. Furthermore, the flow meets mass conservation requirements. It is known that many properties, such as the velocity of the fluid in a tube, are not uniform along the stream.

Isothermal compressibility

It is a measure of the compressibility of a body or thermodynamic system when it is subjected to a thermodynamic process of quasi-static pressure transformation while its temperature remains constant and uniform, it is given by (in Spanish notation it is usually represented as , but not to be confused with the isothermal compressibility modulus):.

In a process of pressure variation at constant temperature, the body will have exchanged a certain amount of heat with the outside, so its total energy, which can be obtained as the sum of the work done on the body and the heat exchanged by it, will not remain constant.

Adiabatic compressibility

It is a measure of the compressibility of a body or thermodynamic system when it is subjected to a quasi-static transformation") of pressure under conditions of perfect thermal insulation, it is given by (in Spanish notation it is usually represented as, but not to be confused with the adiabatic compressibility modulus):.

In an adiabatic process of pressure variation, the body will experience some change in temperature.

All fluids are compressible, including liquids. When these volume changes are too large we choose to consider the flow as compressible (showing a significant variation in density as a result of flowing), this happens when the speed of the flow is close to the speed of sound. These changes usually occur mainly in gases since to reach these flow speeds in liquids, pressures of the order of 1000 atmospheres are required, whereas a gas only requires a pressure ratio of 2:1 to reach sonic speeds. The compressibility of a flow is basically a measure of the change in density. Gases are generally very compressible, however, most liquids have a very low compressibility. For example, a pressure of 500 kPa causes a density change in water of only 0.024% (at room temperature). On the other hand, this same pressure applied to air causes a density change of 250%. For this reason, the study of compressible flows is normally known as gas dynamics, this being a new branch of fluid mechanics, which describes these flows.

In a flow there are usually changes in pressure, associated with changes in velocity. In general, these pressure changes will induce density changes, which influence the flow; if these changes are important, the temperature changes presented are appreciable. Although density changes in a flow can be very important, there are a large number of situations of practical importance in which these changes are negligible.

The flow of a compressible fluid is governed by the first law of thermodynamics in energy balances and by the second law of thermodynamics, which relates heat transfer and irreversibility to entropy. The flow is affected by kinetic and dynamic effects, described by Newton's laws, in an inertial reference frame – the one where Newton's laws are applicable. Furthermore, the flow meets mass conservation requirements. It is known that many properties, such as the velocity of the fluid in a tube, are not uniform along the stream.