Hi, I've got a strange doubt.I model an elastic diaghfram that move water in piping system (a simple pump) so i've got used the 3D FSI multiphicis. Note that in Comsol's examples is the fluid that move the solid structure: i use the same procedure but for the opposite case. I create a simple geometry with a thin elastic rectangular solid (the diaghpram) directly attached on a cube(the fluid subdomain). I set on solid fixed constrains on four laterals buondaries and a surface load on upper wall. So i've found an absurd: if i leave the default setting on the boundary condition on the fluid-structure wall (that corrisponding at the "structural velocity" time derivates of solid displacements (ut,vt,wt) ) the fluid doesn't see the perturbation. BUT if in the same field i set DIRECTLY the SOLID DISPLACEMENTS (u,v,w) the model works perfectly and it rapidly converge. BUT Dimensionally is an absurd, why it works? Please help me to understand.