Rahul Mule
Ansys Employee

Hi,

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

Try with better mesh and increased number of substeps with loads applied at very small incremental value.

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