Engine mass-flow rate coupling for zero-thrust simulation

[m-minervino]

Added flag to allow mass-flow rate coupling between all exhaust boundaries towards the engine inlet.

Proposed Changes

When simulating a turbofan engine in idle regime (no thrust), total free-stream conditions can be imposed at fan and core exhaust boundaries (i.e. no energy variation by the engine), but no information is usually available at the fan inlet boundary (for example, the target mass-flow rate to be used).
Assuming that both engine bleed and fuel addition are negligible, the sum of mass-flow rates computed at all exhaust boundaries can be specified as a target for the inlet (ENGINE_INFLOW_TYPE= FAN_FACE_MDOT).
The proposed modification, then, changes the target value at run-time (instead of using a fixed pre-defined value specified in the configuration file) to match the overall outlet mass-flow rates.

by: Angelo Passariello and Mauro Minervino (Italian Aerospace Research Centre).

Related Work

None.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

[pcarruscag]

Can you add a test? The engine features are not covered very well.