Ryan O'Connor
Ansys Employee

Hi Florian,

If the fluid is applying a force on the solid the forces will most likely not sum to zero. From your output, it looks like Mechanical is sending small (but non-zero) displacements. I would expect that the motion of the corresponding walls in Fluent to match that in Mechanical. So it's not that the walls will not deform, but instead that the deformations are not building up to produce observable deformations. I recommend to review your mesh and timestep to ensure that they're fine enough to adequately resolve the oscillations. From one of your images, it looks like your fluid mesh is very coarse.