Mmh, ok.
Good steps are typically to test uni-directional coupling
- OpenFOAM -> MBDyn, so deformation is not handed back
- MBDyn -> OpenFOAM, using some other external force (e.g. a constant force in flow direction) on the structure
For that you have to modify the cplscheme in your config, remove one exchange tag and change to a explicit scheme. See also How to configure one-way coupling