I am modelling a tank where water flows in at the base and then flows through a porous region before leaving through an outlet close to the top. The top of the tank is a free surface which i am modelling as a slip wall. Air is also added which i am trying to model as a series of discrete phase inputs. The model converges fine in 2d but not in 3d, although it will converge without the discrete phase. Can anyone suggest something that i can try?

I've just realised that i've missed a bit out. I have tried setting the slip wall (free surface) as an escape boundary for the discrete phase. It works and converges in 2d but not in 3d. Any help will be appreciated.

Do You want to get a stationary solution? I think the plume in a gas-stirred tank won't be stationary and You should try to get a transient solution.