Pressure drop in fluidized bed

Hi,

I try to simulate the pressure drop in a gas-fluidized bed with small particles (dp = 100um). The pressure drop over the packed bed is in good agreement with the Ergun equation but somehow the pressure drop over the bubbling fluidized bed is always about 20% below the analytic solution of the pressure drop due to the particle weight (Delta_p = particle_weight*gravity/A_bed).

Does anybody has an idea why?

Thanks!

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I attached two plots:
pressure.png: Shows the pressure drop over the bed and the pressure on the lower wall due to the particle weight. The inlet velocity is increased steadily.

-> At about t = 0.8s the packed bed starts to fluidize and the pressure drop over the packed bed should be equal (m_particle*g/A).

force.png: Shows the force on the lower wall due to the particles and the drag force on the particles in z-direction.

Is it possible that the momentum exchange from CFD to DEM works but in the other direction a problem exists?