Hi Yasser and Steve,nThanks for your input and suggestions! Sorry for my delayed replay, my model needed some time to run with tighter convergence criteria. Unfortunately, this didn't solve the problem. There aren't any (major) setup differences between the successful and unsuccessful runs. However, the mesh is different and therefore, the Windkessel parameters are different. The parameters are calculated in a same way and the mesh is created in a similar way as well. In my study, I run 2 FSI models for each patient. In one model, the pre-stress that's present in the geometry is estimated. In the other model, the pre-stress estimation is omitted. For the same patient, the model with pre-stress estimation runs fine (pressure and flow plots shown below). However, the model without pre-stress shows the sudden jump in flow, as shown above. In this model, the systolic pressure in the first cardiac cycle is fairly high and the diastolic pressure in the first cardiac cycle is fairly low, since a periodic solution hasn't been reached yet, due to high displacements at the start of the FSI simulation. I tried lowering the stiffness of the wall, but this hasn't solved my problem either. I'm now thinking about trying the following:nRun the simulation with a stiffer wallnRun the simulation with a higher Windkessel compliance, such that the pressure is decreasednHopefully, one of these simulations works well, such that I at least know what is causing this problem. Ofcourse, I'm also going to have another look at the convergence and results near the anomaly. I didn't find anything weird so far, but maybe a fresh look will make things clear nIf you have any further suggestions, I'm more than happy to hear!nBest regards,nJudithnModel with pre-stress estimation - Pressure nModel with pre-stress estimation - Flownn