A heaving cylinder
 

Hello;
I want to make a simulation of a heaving cylinder placed at the free surface. I used "rigid body motion" option to make the whole domain move vertically with a sinusoidal pattern. I wrote this field function to specify free surface position: ($$Position[2]<=(free surface level))?1:0, in which "free surface level" is a constant value and the written function defines the volume fraction of the water in a domain that has an unsteady vertical motion (sinusoidal). So the free surface will follow this movements too and that s not what happens in reality. How can I define this function so that the free surface remains fixed relative to laboratory reference frame?
Thank you

the easiest way to do this is to follow the steps in the Boat tutorial, but with these changes:
-use a flat wave
-no flow in or out - eg hydrostatic pressure field function on all external boundaries
-replace DBFI motion with either Translation motion (requires a macro to drive the vertical motion) or User Defined Vertex motion (new feature in v606 which can be driven by a field function)
-maybe a few other minor things too - cant remember it that well.
The other way is to use Morphing motion, then fix the outer walls and they can then be 'walls' since the fluid is going nowhere, and drive the tank up and down via a field function. then the mesh will morph to the required shape - takes a bit of effort in some cases and takes longer to solve.