I'm trying to understand Wikipedia's derivation of the isentropic velocity-area relationship, here under "Flow analysis":
I don't quite understand what happens to the energy equation. I suppose $q$ and $w$ refer to energy added and the work done by the flow? Then later at "The energy equation is:" the sum of the work/energy and the $h$ (enthalpy?) becomes the ratio involving the specific heat ratio, pressure and density. How did this step occur? This is probably quite simple but it puzzles me and I couldn't really find another derivation that explains this step properly. If you could derive it in detail here or point to a derivation I would appreciate it, thank you!