Degassing boundary condition depending on mesh
 

Hi everyone,

I am simulating a dissolved air flotation unit (water + air bubbles), using multiphase Eulerian. The boundary condition on the top surface of the unit is set as degassing, so the air bubbles come out.

The results show a decrease in air fraction on the top rear portion of the tank (right image). I know that air bubbles escape through the top layer, but I don't understand why the air fraction is not higher on the top portion of the tank.

I have also found that modifying the mesh (making a boundary layer on top) gives different results (left image). It seems that if the mesh is thinner on top, the air fraction on the top is higher, but the escaped air mass flow is lower, so the air accumulates on the tank.

Has anybody faced something like this?

Any advice will be appreciated.

Thanks!