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If you look through the manual section there are a few ways to find the coefficients. You'll also note the fluid density and viscosity feature in the equation: if you do the maths what does that tell you? 

Another option is to build a sub model of part of the whatever it is you want to model as a porous block. Run the CFD model in exactly the same way as you would the experiment and take that data to get the coefficients. We've done that for grates, chemical plant and the like.