[c_underwood]

I am want to produce a field file which is the number density for my mesh area.
I am simulating a gas jet nozzle using sonicFoam.
I don't think there is a premade function to do this, although I may be wrong as I am still very new to openFoam. I am assuming that the gas I have is an ideal gas, and therefore I can get the number density by

n = P / (k_b T)

Therefore I want to produce this as I go along. I was thinking of making a function in system/controlDict but I cannot find a way of entering this. Please could someone advise me on how to do this, or another way of getting the number density out.

Thanks