2. Flow equations for a compressible gas

This paragraph 2 gives a simple reminder of the equations of flow of a compressible gas in a tube of slowly varying cross-section. For more detailed information, please refer to the article Unsteady flows [A 1 920] in the treatise Sciences fondamentales.

Euler variables are used for the equation.

This is a one-dimensional model: in each straight section of the tube, the gas velocity is assumed to be uniform (since flow is turbulent, this assumption is verified) or, at least, the...