# Two norm diff = 1 at first timestep of FSI simulation **URL:** \ **Category:** Using preCICE\ **Tags:** adapters, fsi\ **Created:** [January 25, 2021, 4:08pm UTC](https://precice.discourse.group/t/two-norm-diff-1-at-first-timestep-of-fsi-simulation/438 "2021-01-25T16:08:12Z")\ **Posts on this page:** 1\ **Showing post:** 2
**Author:** ![DavidSCN](https://avatars.discourse-cdn.com/v4/letter/d/c37758/32.png) [@DavidSCN](https://precice.discourse.group/u/DavidSCN)\ **Post date:** [January 25, 2021, 9:39pm UTC](https://precice.discourse.group/t/two-norm-diff-1-at-first-timestep-of-fsi-simulation/438/2 "2021-01-25T21:39:58Z")
Hi @Claudio, the logical sequence of your time loop looks fine. You can also find a quite minimal FSI cpp example [in the current develop version of our tutorials](https://github.com/precice/tutorials/blob/develop/FSI/rigid_flap/OpenFOAM-RigidBody/Solid/rigid_body_solver.cpp) in order to compare your setup. > [@Claudio](#): > > The fact that the two-norm diff is always 1 makes me think that one set of displacements is zero at the numerator, but I don’t see where. That’s indeed bad, the two norm diff should only be one in your first iteration. Here some ideas and common issues: Does your coupling work with an `explicit` coupling scheme? Have you tested your solver in a standalone mode without coupling? Do you store/reload your coupling data correctly in the checkpoints (this part is missing in your snippet)? Do you have the interface boundary accidentally constrained in your setup? Could you provide some additional information on this? Regards, David --- _[View the full topic](https://precice.discourse.group/t/two-norm-diff-1-at-first-timestep-of-fsi-simulation/438)._