Flow through porous medium

Hi CFDEM users,

I just started to use cfdem-coupling and try to simulate a pressure driven fluid flow through randomly packed spheres with certain distribution. The fluid is flowing from top to bottom with particle settling along with the fluid direction. The flow rate profile I've got seems a bit counterintuitive to me. I've attached the screen shot to this post. The flow rate appears to increase when it reaches the particle domain where I would expect volumetric flow rate would decrease when the fluid flow through the porous medium. I'm not sure where went wrong or if I interpret the result incorrectly.

The known variables would be inlet pressure and outlet pressure.

I modified the tutorial in tutorial/cfdemSolverPiso/voidfractionTest with the following boundary condition
0/p:
inlet {type fixedValue; value uniform 200;}
outlet {type total pressure; p0 uniform 0;U U;phi phi;rho rho;psi none; gamma 1; value uniform 0;}
0/U:
inlet {type pressureInletOutletVelocity; value uniform (0 0 0);}
outlet {type zeroGradient;value uniform(0 0 0);}

U is in the unit of m/s, I couldn't figure out how to output a reasonable volumetric flow rate which I expect would decrease as fluid encounter with more particles. I tried to specify inlet type as flowRateInletVelocity but the code won't even run, I also tried to add 0/phi with similar setup like 0/U but when I plot the results in paraView, it didn't have correct format.

I'm looking forward to your comments and suggestions,

Thanks and regards,