Hello, I need to know how you estimate the solution error in simulation using Fluent or have an idea about it. Regards, Chouki

There is no simple method as there are so many potential sources of error. Amongst other things you need to:

Do a grid independance study.
Do a timestep independance study.
Test sensitivity of solution results to inputs e.g. viscosity.
Validate your results against experimental data.
...
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