[pauls]

Hello Martin and Julien,
I had similar problems. Searching this forum I found that all OpenFOAM solvers are unstable for this setup. The source of instability seems to be the density contrast between air and water. Any momentum transfer across the interface generates a huge velocity jump.
Discussions show that no solution is available in OpenFOAM, and no attempts are made to find one. We are still using commercial codes for FSI including a water-air interface. I don't know what they a doing differently, but obviously there is a solution.

Paul