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I'm new to CFD, and I'm having a tough time determining which settings to use in FLUENT. I'm modeling a flux chamber, which is basically a box, that has an air inlet and outlet (the flow is forced through the box by a fan at the outlet). Initially, it will be 0.5m x 0.5m x 0.5m (I'm trying to create CFD models to determine how changes in geometry affect the flow pattern in the box). The box has no floor (it is open to the ground), and it is placed in a field of grass (varying height of 15-80 cm), which is exposed to the air in the chamber.

After doing some reading, I think the realizable k-epsilon model should work be an acceptable viscous model. Does that sound reasonable?

I'm having trouble with two things: surface roughness and mass transfer. I'm trying to use surface roughness to model the grass in the bottom of the chamber. Based on some reading, it seems that 30 cm is an appropriate roughness height for tall grass. From what I've been able to find, there isn't really a great way to determine an appropriate roughness constant. The FLUENT User's Guide says that a value of 0.5 represents uniform, tightly-packed sand grains in a pipe, and values of 0.5-1 represent non-uniform roughness. Should I use a number close to 1 for grass, since it's less uniform than sand? Also, the manual says that the surface roughness essentially acts as a blockage effect at about 50% of the roughness height. That doesn't sound like a great way to model grass, but I don't have any better ideas. Does anyone have a suggestion?

Finally, I'd like to apply a flux of NH3 from the bottom of the chamber, and I thought that using a mass-flow inlet may be the easiest way to do that, but I have 2 problems. The first problem is that if I change the BC of the floor from "wall" to "mass-flow inlet," I can no longer apply surface roughness. Secondly, I can't figure out how to set one mass-flow inlet for air and another for NH3. Do you have any suggestions on how to apply the NH3 flux from the floor?

I know that my post is long, but I’m very inexperienced at CFD, and I’ve reached a point where reading the manuals seems to be getting me nowhere. I hope that the answers to some of these questions are obvious to experienced users. Thanks for your help!