Possible issue on using aircraft flight velocity for EPM initial concentrations

[marcoslogrono]

In this line we are calculating the initial water vapor concentration for the EPM box model. From my understanding the numerator here is the number emission per second for the input fuel flow, and we are dividing that by the exit volumetric flow aircraft_.VFlight()*Ab0 to get an average concentration at the engine exit plane. If that is so, why are we using the aircraft flight velocity, instead of the absolute velocity of the exhaust (i.e exit engine velocity - aircraft velocity)? What is the appropriate frame of reference?

APCEMM/Code.v05-00/src/EPM/Models/Original/Integrate.cpp

Line 175 in 4ae88d5

VAR_[ind_H2O] += EI_.getH2O() / ( MW_H2O * 1.0E+03 ) * aircraft_.FuelFlow() / double(aircraft_.EngNumber()) / aircraft_.VFlight() * Na / Ab0 * 1.00E-06 ;

[sdeastham]

I think the current formulation is correct, but not useful. I would suggest we go to a more straightforward formulation which asks the user for quantities they are more likely to actually have access to - did you have something in mind for this?