Hi folks!

I am trying to model an open channel flow, I read recommandations to see the free surface, I should use the VOF method, eventhough, I have read in the documentation that the fluid "Solid element" shall be defined using VOF section,as you can see in the attached picture, I have modelled the channel as a solid element, and the volume of fluid as solid element in light transparent blue.

In the documentations here, it states that I shall define a reference material as "water" fluid, and the secondary materal as ambient fluid "Air" and shall assign the water in the inner side of the channel and air outside.

I cannot see where this shall take place, shall I define two separate solids?

Any hint is appreciated. Thanks in advance!