text: The dynamic behavior of compressible flows is governed by the dimensionless thermodynamic quantity , which is known as the fundamental derivative of gasdynamics and is defined as
  where
  *  is the speed of sound [m/s];
  *  is the specific entropy per unit mass [J/(kg K)].
  From a mathematical point of view, the fundamental derivative is a non-dimensional measure of the curvature of isentropes in the pressure-volume thermodynamic plane. From a physical point of view, the definition of  tells that the speed of sound increases with pressure in isentropic transformations for values of , while, by contrast, it decreases with pressure for .
  Based on the value of , three gasdynamic regimes can be defined:
  * ideal gasdynamic regime for ;
  * non-ideal classical gasdynamic regime for ;
  * non-classical gasdynamic regime for .