help for gravitational flow model
 

I've just started to use the program (so unfortunately I'm a beginner! 8(, anyway) and I need to simulate a laboratory experiment which examines gravitational flow in a flume. Should I define two different liquids in a surface body? If that's the case could to help me assigning those fluids in the body of my 2D model?

Thank you so much for taking your time and reading my problem!