A note on the flow regimes in the unsteady, rectilinear flow of a perfect gas

Homentropic, unsteady, rectilinear flows can be classified in (i) constant flows, (ii) simple waves, (iii) general flows. After a short discussion of this standard theory, an attempt is discussed to extend this scheme to non-homentropic flows. The principal result is that a flow with the constraint du+(1/a) dp=0 everywhere is physically impossible. In two final sections two suggestions are discussed to remedy the situation, and the linearized case is discussed where analogous features are present.