“…It is used to unify the models (1.2), (1.3), (1.4), and as a framework for interpretation of different experimental or field data. It is analyzed numerically in [8,21,27], theoretically in [2,[12][13][14][15]18] for single-phase flows, and also in [16,17] for two-phase flows. For compressible fluids, especially gases, the dependence of coefficients a i 's on the density ρ is essential.…”