Adiabatic compression and rarefaction

These methods provide an accurate means of investigating translation-vibration and translation-rotation transfer. The passage of a sound wave through a gas involves rapidly alternating adiabatic compression and rarefaction. The adiabatic compressibility of a gas is a function of y, the ratio of the specific heats, and the classical expression for the velocity, V, of sound in a perfect gas is... 

Rapid aerodynamic flow past obstacles involves adiabatic compressions and rarefactions, and is influenced by relaxation of internal degrees of freedom in a way similar to shock phenomena. This effect has been quantitatively treated by Kan-trowitz18, who developed a method for obtaining relaxation times by measuring the pressure developed in a small Pitot tube which forms an obstacle in a rapid gas stream. This impact tube is not a very accurate technique, and requires a very large amount of gas it has been used to obtain a vibrational relaxation time for steam. 

On 27 June 1800, Dalton read Experiments and Observations on the Heat and Cold produced by the Mechanical Condensation and Rarefaction of Air. Joule (a pupil of Dalton) says that Dalton ascertained that about 50 of heat are evolved when air is compressed to one-half its original bulk and that, on the other hand, 50 are absorbed by a corresponding rarefaction and this result can be inferred from what Dalton says, although it is not explicitly stated. The method (of adiabatic expansion) was afterwards adapted by Clement and Desormes to the determination of the ratio of specific heats of a gas. ... 

For most systems the change from high to low pressure in a sound wave occurs in such short times that the system cannot exchange heat with its surroundings, and so the pressure change is adiabatic. However, adiabatic compression causes the molecules to undergo more collisions and so raises the temperature. Adiabatic rarefaction lowers the temperature. So, the sound wave is effectively... 

This sound wave contains regions of rarefactions and compressions. The temperature of the material increases in the compression regions and then cools due to adiabatic expansion. In an explosive composition... 

VELOCITY OF SOUND. The velocity of sound through a continuous material medium, also called the acoustical velocity, is the velocity of a very small compression-rarefaction wave moving adiabatically and frictionlessly through the medium. Thermodynamically, the motion of a sound wave is a constant-entropy, or isentropic, process. The magnitude of the acoustical velocity in any medium is... 

The second law of the thermodynamics, however, shows that only compression shocks are possible (see Prob. 8.44). A compression shock results in an increase in entropy whereas a rarefaction shock, if it existed, would result in a decrease in entropy, which is impossible in an adiabatic, steady-flow system. Thus, we conclude that this type of shock waves always requires a supersonic flow upstream and a subsonic flow downstream. 

This sound wave contains regions of rarefactions and compressions. The temperature of the material increases in the compression regions and then cools due to adiabatic expansion. In an explosive composition the compression part of the wave is sufficiently high to cause the temperature to rise above the decomposition temperature of the explosive crystals. As the explosive crystals decompose just behind the wave front, a large amount of heat and gas is generated. This in turn raises the internal pressure which contributes to the high pressures at the front of the wave. These high pressures at the wave front must be maintained for the wave front to move forward. 

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