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How to make my simulation converge?

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Old   November 21, 2023, 05:23
Question How to make my simulation converge?
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Hello everyone,

I'm a master's student and relatively new to Ansys Fluent. Since September, I have been learning how to use it from scratch by myself.

I'm writing this post because I'm struggling to let my simulation for an abrupt expansion pipe converge.

The geometry of my abrupt expansion pipe is like this:
abrupt expansion pipe.png

Mesh qualities:
- Minimum Orthogonal Quality: 0.16
- Maximum skewness: 0.39
(According to the manual, these qualities seem to be okay, though it's not that good.)

The setup in Fluent that I did is the following:

Solver setting in General:
- Type: Pressure-Based
- Velocity Formulation: Absolute
- Time: Steady

Since the flow is laminar(Re for the thin pipe = 2000, Re for the thick pipe = 666), I choose the Laminar model.

Boundary conditions:
- Inlet velocity = 0.172 m/s
- Outlet pressure: Gauge pressure = 0 Pa

When I set smaller inlet velocity (0.001 m/s, 0.003 m/s, 0.005 m/s, 0.011 m/s, 0.021 m/s, 0.043 m/s), the simulation successfully converged. However, with a larger inlet velocity like this time (0.172 m/s), the simulation no longer converges...

I tried the following solutions, but none worked well:
- Increase the number of iterations
- Change fluid time scale
- Change relaxation factors

I have been trying my best to think about possible solutions by myself, but unfortunately, I have not succeeded. I would appreciate it if some of you could give me possible suggestions.
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Old   November 21, 2023, 15:11
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Lorenzo Galieti
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Do you have flow separation when the pipe area increases?
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Old   November 21, 2023, 21:41
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Alexander
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get converged solution for 0.043 case
don't reinitialize, increase inlet speed and run simulation again
increase inlet speed gradually
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Old   November 27, 2023, 11:18
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Hi LoGal,

Some eddies were observed.

I did meshing again and then my simulation successfully converged.

Anyway, I really appreciate that you kindly gave me a prompt reply. Thank you for your help.

Best regards,
Hanako
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Old   November 27, 2023, 11:23
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Hi AlexanderZ,

Thank you very much for your suggestion.

After re-meshing, my simulation successfully converged.

But I'll try your suggestion later as well for my learning.

Best regards,
Hanako
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converge case setup, converge error, laminar, laminar flow in pipe

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