Rahul Mule
Ansys Employee

Hi,

This error may occur when unreasonable displacements were received by fluent or unreasonable forces were sent to Mechanical APDL. 

Using under-relaxation on the data transfers can help here. This will make the solution more stable 
at the expense of more coupling iterations to reach convergence. Note that some cases can 
remain unstable even when under-relaxation is used, to stabilize these types of cases Solution 
Stabilization is available in Fluent in the Dynamic Mesh settings on the Solver Options panel.

It is very important to build up the FSI simulation in stages as opposed to setting up the 2-way FSI right at the start. This document "Best Practices for Coupled Fluid-Structure Interaction (FSI)" describes this process and is available here: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v221/en/sysc_ug/sysc_bestpractices_fsi.html

Hope this helps!