consult for the multiphase flow

poiuliu

My job is to simulate a bubble column with water,granule and air bubbles in it.The column has no cover and air flow out from the surface of the water.But no water and granules flow out of the column.So the water surface is a free surface. At the beginning,water and granules are resting in the column.Then air bubbles flow into the column from the bottom and they make the other two phase begin to flow in the column. My question is how to set the initial volume? I use E-E three-phase model and i patch the up half part of the colume with air volume fraction of 1,water and granules total of 1 in the down half part of the column.But after computing 0.01s,the volume fraction of water in the top half of the colume is nearly 0.4.I don't know why water can't get down for the function of the gravity. And the velocity of air bubble is not very large,only 0.01 m/s. I need your help! Thank you,every one!