How to Define a Free Surface in 2D River Modeling Using Fluent and Gambit?

[farhad_mech]

hello
i want to model and analyze a 2d river which passes a step, in fluent and gambit, but i don't have any idea how should i define free surface between water and air, here is my geometry:

i really appreciate if anybody can help me.

 

[minger]

Well, you don't know where the surface is before hand, so meshing directly to it isn't possible. I believe in most commercial codes, there is a variable called volume ratio or something like that which is defined.

If you have access to CFX tutorials, version 11, tutorial number 7: Free surface flow over a bump is exactly what you're looking for. That tutorial simply defines the free surface using a volume ratio expression, and then let's the mesh adapt using the volume fraction (just looked up, fraction is the correct variable) as the criteria.