Rob
Ansys Employee

Mesh does make a significant difference if the mesh is rubbish compared to good: ie a poor mesh may work in single phase but not with multiphase models. Similarly, Fluent Meshing uses poly cells and is often able to give a better mesh as the built in algorithms tend to override poor sizing choices! Check the cell count, you want to be running 250k-1M cells per core (or node, I've lost track of the multi-cpu terminology) and also check you've got sufficient RAM.