How to simulate Free Surface ?

Using Fluid scenario creation, how to simulate open channel flow, since closed domain is required. If we use the closed channel to simulate open channel flow, How do we specify the interaction between air and water, 

Edu ​​​​​​​3DEXPERIENCE ​​​​​​​SIMULIA ​​​​​​​Fluid Scenario Creation ​​​​​​​