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

It shouldn't really matter provided you define pressure ICs and BCs consistently with your choice of reference density. If roundoff is a concern, use double precision. I usually use air density.

Thanks Phil, I am also using air density at the present time. What do you mean by consistent BC for the pressure? I am using zero reference pressure, hydrostatic pressure at the outlet boundary and imposing the water surface elevation at the inlet boundary.

Which kind of fluid model do you use for air, continuous or dispersed?

Thanks for your comments,

Luis

The interpretation of pressure depends on the choice of buoyancy reference density. So for a quiescent flow, if you choose the BRD=air density, then the pressure will be constant in the air and have a large gradient in the water, while if you choose BRD=water density, then the pressure will be constant in the water and have a large gradient in the air. This is explained in the documentation. Your pressure BCs and ICs need to be defined to be consistent with your choice of BRD.

Note that the hydrostatic component is added back when calculating 'Absolute Pressure', so that variable is the same independent of your choice for BRD.

Ok, that's what I am doing. I am using BRD=air density, and at the outlet boundary zero static pressure for the air, and an hydrostatic pressure distribution for the water.

Just one more thing regarding the air fluid model variable. Is it better to model air as a continuous fluid or as a dispersed fluid (continuous fluid is better for free surface flow and discontinuous for dispersed bubbles).

Luis

It is a little bit arbitrary, and may not matter much. Try both and see if there is much difference.