Free surface for OffLatticeExternalFlow

Hallo Community,
I’m trying (without success…) to implement a free surface for an external flow with 3D Multigrid. I have a classical Inlet/Outlet Domain with a constant inlet-velocity / outlet-pressure, without any external force.

Has anybody experience with the topic? Is it generally possible to implement such a model with Palabos?

Thanks in advance for your help!