We don't tend to worry about the Darcy Number, the resistance coefficients are what Fluent needs, which come from the equations. From your last comments the authors have used the Darcy Number to correlate something, that doesn't mean they set it in the model to get the data. I can do an experiment get velocity v pressure in a system and then report Reynolds Number v Pressure in a paper; if I don't define diameter in the paper it's impossible to work out what I did.