Buoyancy reference density

Luis

Hello,

I am modelling a free surface mixing tank with water and dispersed air bubbles. I am not sure which is the most adequate buoyancy density reference for such a kind of flow. For free surface flow it is better to use air density (because then the water goes down), but on the other hand for dispersed air bubbles in water is better to use the water density (because then the air bubbles go up). So, if you have a free surface and dispersed air bubbles, which is the best value for the buoyancy density reference?

Thanks,

Luis