How to add gas velocity in turbulent bubbly phase?

Hi,

I am trying to model a gas-liquid system. I selected turbulent bubbly flow model as it is based on Euler-Euler equation. But I don't know how to add gas velocity field at inlet. Liquid phase in my situation is just agitated by inlet gas and there is no liquid in or out the tank. I've already checked the BCs but it always says 'Segregated group X1. Matrix has zero on diagonal.' Could anyone give me some advises? Many thanks.

Best wishes,

Dd. Zhang