ABSTRACT
The compressibility of a fluid is basically a measure of the change in density that will be produced in the fluid by a specified change in pressure. Gases are, in general, highly compressible whereas most liquids have a very low compressibility. Now, in a fluid flow there are usually changes in pressure associated, for example, with changes in the velocity in the flow. These pressure changes will, in general, induce density changes, which will have an influence on the flow, i.e., the compressibility of the fluid involved will have an influence on the flow. If these density changes are important, the temperature changes in the flow that arise due to the kinetic energy changes associated with the velocity changes also usually influence the flow, i.e., when compressibility is important, the temperature changes in the flow are usually important. Although the density changes in a flow field can be very important, there exist many situations of great practical importance in which the effects of these density and temperature changes are negligible. Classical incompressible fluid mechanics deals with such flows in which the pressure and kinetic energy changes are so small that the effects of the consequent density and temperature changes on the fluid flow are negligible, i.e., the flow can be assumed to be incompressible. There are, however, a number of flows that are of great practical importance in which this assumption is not adequate, the density and temperature changes being so large that they have a very significant influence on the flow. In such cases, it is necessary to study the thermodynamics of the flow simultaneously with its dynamics. The study of these flows in which the changes in density and temperature are important is basically what is known as compressible fluid flow or gas dynamics, it usually only being in gas flows that compressibility effects are important.
