Comment by generuso

If we look at the flow lines, they converge and diverge even when the "nozzle" is a section of a pipe.

In rockets, there is a so called "aerospike" engine [1], which does not have any bell-shaped nozzle at all, but instead uses the ambient air to always shape the flow for an ideal expansion ratio. In theory it is great, but in practice it remained mostly a pie in the sky.

[1] https://en.wikipedia.org/wiki/Aerospike_engine