Change the time step setting under 'Analysis Settings' in system coupling and not in Fluent.
Using under relaxation of data transfer will make the solution more stable at the expense of more COUPLING ITERATIONS to reach convergence. So, if you reduce the URF, consider increasing the max coupling iterationsÂ
according.
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.
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