I have the following question regarding nozzles; It is known that in a CD nozzle there can be only one of the following outcomes;
- a flow fully subsonic in the convergent, subsonic at the throat and then subsonic in the divergent area; That's in the case that the pressure ratio is below the critical pressure ratio.
- a flow fully subsonic in the convergent, sonic at the throat and then supersonic in the divergent (no shock) or supersonic and then subsonic (with a shock).
So far so good; my question now about the second case. From a physical point of view, why can't the sonic transition take place before the nozzle throat? For the sake of simplicity in exercises we know that the nozzle has a CD shape, so we assume a priori that if there is a sonic transition it will be right at the throat. But physically speaking, the flow is being accelerated and "it does not know" what's happening ahead and if the cross section is going to be even smaller later. So what prevents it from going sonic before the nozzle throat? Can someone help with this thought? Am I missing something fundamental or approaching it in a completely wrong way?
Thank you and cheers, ndjojo