Ground effect calculation
 

Firstly: I'm pretty new to CFD and Fluent but after couple of tutorials I can run the simulations without a problem but there is one thing i didn't quite get

From my ground effect simulations I've noticed that while there is a force on the multi (3) wing assembly (-550N, 1m wide 1.2m long, 45 degree angle, 5cm ground clearence at 250 km/h) but there is a much bigger force on the walls (9000N) (right, left, top and bottom (floor) atr all in this "category").
So this got me wondering if that 9000N is also part of the downforce along the 500N as the ground doesn't obviously move up or down.
And if I'm correct I assume that part of the force is from the stream hitting the "roof" and that force wouldn't exist in real life so how could i either make the "windtunnel" fixed (stationary) so all the force is directed to the wing and eliminate the force on the roof?

Thanks in advance