5. Interest of similarity in compressible fluids

Note that compressibility somewhat reduces the possibilities offered by similarity in incompressible fluids.

• It follows from the equality of the γ coefficients that the laws of similitude can only be applied to geometrically similar turbomachinery operating with the same fluid.

  There are, however, methods that allow us to predict to some extent the operation of a turbomachine with a given gas, based on its characteristics with another gas of different γ; but this does not involve rigorous similarity. We can, for example, try to find an operation that respects the similarity of the velocity triangles at certain points, but the Mach numbers will no longer be equal.

  These methods, which are not developed here, are all the more representative when the gases have little difference in γ....