[sern]

Hello everyone, I'm a chemical engineering master degree student and I'm very new to CFD. I need to use CFD to simulate the fluidized bed for further use in separation process. I've studied and tried fluidized bed workshop from Fluent. Then I'm trying to draw my own mesh and do it myself.
I draw the very simple geometry (just a 3D column with air inlet at the bottom) and generate mesh setting solver preference as Fluent. Then I import it into Fluent and try to do the same as the workshop, calculate it using Euler-Granular model and patch a volume fraction of particles at the bottom of column.
But I've got a problem, the simulation shows the particles flow with air at first as picture attached, some of them just flow out of column with air and then some of them fall down and start acting like fluidzed bed. How can I fix this?

Thank you in advance.