Modelling of a Hydrocyclone

Hello,

I'm modelling a hydrocyclone for solid-liquid seperation.

I have two inlet pipes placed opposite to each other at the bottom of a cylinder, water is pumped with a constant velocity, it swirls in the cylinder to reach an outlet placed horizontally at the top of the cylinder. The pressure at the outlet is assigned to zero as well.
I modeled the liquid flow by using Laminar flow and the study is stationary and I have added the particle trajectory interface to model the solid particles with a time dependent study.
According to my supervisor the flow of liquid shouldn't occupy all the horizontal outlet and should fill only half of it, I have assigned everything and every possible boundary condition, the problem is that I can't assign the gravity force (F=-g_const*rho), when I add this term the solution is not converging.
I have tried almost everything but still no solution.

Any ideas how can I solve these problems?

Thank you in advance