Overview

In general, the liquids and gases are the states of a matter that comes under the same category as “fluids”. The incompressible flows are mainly deals with the cases of constant density. Also, when the variation of density in the flow domain is negligible, then the flow can be treated as incompressible. Invariably, it is true for liquids because the density of liquid decreases slightly with temperature and moderately with pressure over a broad range of operating conditions. Hence, the liquids are considered as incompressible. On the contrary, the compressible flows are routinely defined as “variable density flows”. Thus, it is applicable only for gases where they may be considered as incompressible/compressible, depending on the conditions of operation. During the flow of gases under certain conditions, the density changes are so small that the assumption of constant density can be made with reasonable accuracy and in few other cases the density changes of the gases are very much significant (e.g. high speed flows). Due to the dual nature of gases, they need special attention and the broad area of in the study of motion of compressible flows is dealt separately in the subject of “gas dynamics”. Many engineering tasks require the compressible flow applications typically in the design of a building/tower to withstand winds, high speed flow of air over cars/trains/airplanes etc. Thus, gas dynamics is the study of fluid flows where the compressibility and the temperature changes become important. Here, the entire flow field is dominated by Mach waves and shock waves when the flow speed becomes supersonic. Most of the flow properties change across these waves from one state to other. In addition to the basic fluid dynamics, the knowledge of thermodynamics and chemical kinetics is also essential to the study of gas dynamics.

Thermodynamic Aspects of Gases

In high speed flows, the kinetic energy per unit mass is very large which is substantial enough to strongly interact with the other properties of the flow. Since the science of energy and entropy is the thermodynamics, it is essential to study the thermodynamic aspects of gases under the conditions compressible high speed flows.

Perfect gas : A gas is considered as a collection of particles (molecules, atoms, ions, electrons etc.) that are in random motion under certain intermolecular forces. These forces vary with distances and thus influence the microscopic behavior of the gases. However, the thermodynamic aspect mainly deals with the global nature of the gases. Over wide ranges of pressures and temperatures in the compressible flow fields, it is seen that the average distance between the molecules is more than the molecular diameters (about 10-times). So, all the flow properties may be treated as macroscopic in nature. A perfect gas follows the relation of pressure, density and temperature in the form of the fundamental equation.

 

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