Characteristic properties of Pressure Gradient Elastic Waves

We single out the main differences between Pressure Gradient Elastic Waves from conventional “sound” waves (here the term “sound” combines the elastic oscillations regardless of their frequency).

To compare the properties, two types of waves are presented in the table.

The normal  “sound” waves

Pressure Gradient Elastic Waves

 The sound waves always arise in compressible fluids when there is a sound source, creating density fluctuations.
 Three conditions are necessary for Pressure Gradient Elastic Waves arising

  1. The substance must be a compressible fluid.
  2. A pressure gradient must exist inside a space or a volume filled with a compressible fluid.
  3.  Density fluctuations must be present. These fluctuations may be a result of sound or turbulence.
 The source of the sound determines the characteristics of the sound wave (frequency and amplitude). All the energy of the sound wave is received from the source of the sound.
 The external forces, which create a pressure gradient in a gas, create the Pressure Gradient Elastic Waves.
 The sound waves propagate out from the sound source. In the case of a point sound source placed in homogeneous infinite space, the surface of the front of the sound wave is an expanding sphere (full solid angle).
 Pressure Gradient Elastic Waves propagate along the vector of the pressure gradient.
 If the sound source pulsates or carries out oscillatory motion, the pressure in a point of space changes periodically and the molecules of gas carry out oscillatory motion 
 During the PGEWs propagation, the oscillatory motion of the gas molecules is absent. The pressure in a point of space can changes periodically (when sound create the PGEWs) or non periodically (when turbulence create the PGEWs) 
 In sound wave, the compression and rarefaction zones alternate and move together in the same direction, moving away from the sound sours.
 In the Pressure Gradient Elastic Waves, the waves of compression are directed toward the pressure increasing, while the waves of rarefaction are directed in the opposite direction toward the pressure decreasing.
 The process of sound waves propagation in a gas is an isentropic process. 
 The process of Pressure Gradient Elastic Waves propagation is adiabatic (no heat supply or removal), but is not an isentropic process. The field of forces did the work. (This field creates the pressure gradient.) It compresses or expands the density fluctuation area.
 The energy of sound waves in gases consists of two components: the potential energy, which is due to the magnitude of the relative elastic strain, and the component of the kinetic energy of the oscillatory movement of gas molecules. Adiabatic compression and rarefaction have to change the gas temperature in the wave disturbance areas. However, since in the sound waves these zones alternate, the net effect is zero.
 The component, which is due to the kinetic energy of the oscillatory movement of gas molecules, is absent in the energy of PGEWs.
The energy of the PGEW consists of two components: the energy of starting sound disturbances, including the component associated with the change in temperature and pressure of the disturbance area, and the energy equivalent of the work done by pressure forces creating pressure gradient, which compresses and rarefies the wave fronts. 
 Inside a bonded space, the sound waves are reflected from the walls.
 Inside a bonded space, the compressed and rarefied fronts of PGEWs are reflected from the walls but immediately are extinguished by next fronts due to interference. The effect is equivalent to absorption, and the entire energy of the wave is transferred to the walls in the form of heat or cold.
The PGEW cannot pass through an extremum point, thus if the pressure gradient has an extremum (the centre of rotation), PGEW is absorbed in this place
 The sound wave transfers the energy obtained from the sound transmitter. Their absorption usually has very small changes the thermodynamic characteristics of the system. 
 Pressure Gradient Elastic Waves take energy over all space, and carry it to the direction of increasing pressure.
The heat transfer increases the temperature in the high-pressure zone and reduces it in the low-pressure area.