3D-Flow: Tank with 2 fluids with no separation
 

Hi! I´d like to simulate:

3D-Tank with 2 fluids. At the begining should one fluid (density ρ1) on the left side of the tank, the other one (density ρ2, with ρ1>ρ2) should on the right side, between them there´s not a separation at all. The fluids are the same, but their density.

How can I define at time t=0s. the location of the fluids? What about the mass fraction? The domain is the entire tank, and there´s 2 Composite Regions: Left Side, Right Side. |Fluid1 + Fluid2|

Thanks, Juan G.