How to maintain the total gravity?
 

I want to simulate granulars dropping from hopper,and I choose the Eulerian Granular Model. The gravity of granulars in the hopper is settled to 4Kg. Then I calculate the volume fraction using the formulation: cumulate desity/real density. when I mark the region in fluent, I choose the Hex and the height of Z axis is calculated by the formulation:4Kg/(cumulate density)=the volume of the hopper; the right of the equation includes unknown variable Z(the height of the granular in the hopper). when I finish the simulation, I find that the total gravity of the dropped granulars is not 4 Kg , less than 4Kg. why is this? Is the way of the calculation right?

Thanks!