Hi,

Have you tried using monitors to identify the instability? You can monitor pressure/force convergence for FSI surfaces within each time step. Look at the graph of monitor point, poorly initialised case shows the sudden jump at the start of the run (improve initalization if this causes failure) and an unstable case starts with small oscillation that grows.

As it is unsable case, defults inputs of stabilization may not be sufficient. Look for appropriate inputs. If there are strong non-linearities within the coupled solution, then convergence within approximately 5 – 10 coupling iterations may not be possible. If this is suspected (and the previous recommendations are not sufficient to allow convergence), then increase maximum number of coupling iterations per step.

Try using more max. iterations/time step in Fluent.