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Old   January 6, 2013, 15:10
Default Low Flow Convergence Help
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Ryan Persaud
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Hey guys, need some help with convergence. The application involves a 3" inlet pipe with 52 m/s velocity discharged into a 26"X50" duct. The duct has a U-bend with a 20" outlet.

This is a very slow flow problem due to the large size of the duct. We are expecting the velocity at the outlet to be approximately 1 m/s.

Used a surface monitor plot at the outlet to look at the velocity, and scaled residuals criteria of 1e-5. The velocity of the outlet appears to have approached a value, however the residuals are quite high. I ran 5000 iterations to exclude the effect of under relaxation factors. I have posted the plots below.

Any suggestions, or direction on how to improve the model will be great. Thanks!

residuals 5000.jpg

velocity 5000.jpg
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Old   January 7, 2013, 01:13
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Lucky Tran
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Seems like everything is okay except for residuals.

Have you tried a finer grid? What about discretization schemes, 2nd order upwind on all?
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Old   January 7, 2013, 12:23
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Ryan Persaud
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I am using a student version of Fluent so I'm limited to 500000 cells and I'm using 2nd order for everything. I'm also using the Simple solver....would using another one help? any other ideas?
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Old   January 8, 2013, 00:59
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It is better to use the simplest form (First Order).
It is obvious that the problem is with your mesh. Refine it and try to make a structured mesh.
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