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convergence problem in a long pipe

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Old   February 4, 2010, 18:04
Question convergence problem in a long pipe
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hi freinds
i'm trying to simulate a long pipe (180 m length) with 1.5 m diameter that has a 90 degree curve in one third of length from begining.inlet boundry conditions are velocity of .8 m/s and static pressure. outlet boundry condition is velocity of .8 m/s normal to the boundry. after about 2000, residules don't change and they approximately became a smooth line parallel to each other. i use hexa mesh that each cell along the pipe has 1 meter length. i use multiphase option. does anyone know what can i do to obtain convergence.
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Old   February 4, 2010, 18:57
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do you need to model the problem using cfd? you didnt say what you want to model anyway.
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Old   February 4, 2010, 19:37
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i want to model the water and air flow.
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Old   February 5, 2010, 11:49
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i work with CFX 11.0.
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Old   February 5, 2010, 12:22
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I will reiterate; you did not say what you want to model, no information on the physical or how you modeled it. if you want an answer please post an apropriate question.
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Old   February 7, 2010, 17:20
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For general comments on convergence see here:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   February 8, 2010, 18:45
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1m mesh length is large if you are modeling multiphase Air/water, you haven't told the other mesh dimensions but relatively they should be large as well. you need smaller mesh size for better resolved water surface, in most of the Air/Water models, this is the reason for flat line convergence.
you can also try smaller physical timescale while reducing the mesh size.
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Old   February 16, 2010, 09:05
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i solved it by changing the time scale. tanks a lot everyone
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