Peter,
If pj goes up, say, ps/pj goes down. Also if pj goes up the primary jet will expand some more and the 2ndary annulus area would decrease.
But 2ndary annulus area decreasing arrow is pointing up. I'd have expected down.
I'm thinking of what's happening at a particular nozzle plane. Perhaps the "2ndary annulus area decreasing" arrow means in the direction of flow?
Ah! I see your problem. My fault - when I edited the original for general consumption I removed some of the labeling. The three lines actually represent three
different nozzle designs with varying
physical annulus area for a constant primary jet area. The top line has a minimum secondary area 1.66Aj, the middle line 1.83Aj and the lowest line 2.0Aj. The 'aerodynamic' annulus area will of course depend on this and the jet expansion
So when the arrow shows a lower annulus area (pointing up) that is more restrictive of the secondary flow and is consistent with your point - if the secondary flow is squeezed it will require a bigger Ps/Pj to pass a given secondary flow. So when, for a given nozzle geometry, Pj changes the flow characteristic moves
along whichever of those three lines is relevant to that nozzle, not
across them.
Sorry about that, hope it is clear now
Clive