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Oscillating Residuals in Finer Mesh Case

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Old   June 15, 2005, 17:56
Default Oscillating Residuals in Finer Mesh Case
  #1
Jake
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I am able to get convergence for a coarse (non-conformal) mesh case but only after I drop the under-relaxation factor for momentum to 0.3. Thinking that this works, I made the mesh finer. But with this fine mesh case, the continuity and momentum equations residuals are oscillating and never quite converge. Interestingly, while the continuity and the momentum equations residuals are out of phase while oscillating.

Any advice will be appreciated.

Thank you.
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Old   June 15, 2005, 22:47
Default Re: Oscillating Residuals in Finer Mesh Case
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HK
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A) It may so happens that when you refine the domain further, it captures more physics which may be unsteady by nature.

B) When you refine it, be careful about the quality (aspect ration for example)

C) Try to locate the maximum residuals and animate it for 200 iterations and check the origin of the maximum residuals and how it propagates?

D) To get a converged solution on a refined mesh, you may want to interpolate the results from a coarse mesh.

HK
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Old   June 16, 2005, 10:25
Default Re: Oscillating Residuals in Finer Mesh Case
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KP
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Hi, I have a similar experience with my solution. I am trying to model bubbly flow in an upwardly pipeflow. When I have 10 nodes in radial direction I observe lesser number of oscillations in the solution where as when I increase nodes to 15 and 20 it becomes worse. Well what I infered with little reading is the pressure velocity coupling in FLUENT needs to undergo revision. One can refer paper by Kunz et al., (1997)where some similar behaviour is discussed. However for an end user it might be a bit hard to implement his reccommendation, as one has to tweak and modify the source code which might not be possible with UDF. CITATION: Kunz R. F., B. W. Siebert. W. K. Cope, N. F. Foster, S. P. Antal and S. M. Ettore, " A Coupled Phasic Exchange Algorithm for Three Dimensional Mutli-field Analysis of Heated Flows with Mass Transfer", Comp. and Fluids 27, 741-768 (1998). LINK TO THE PAPER http://www.sciencedirect.com/science...&_coverDate=09%2F30%2F1998&_cdi=5694&_orig=search&_st=13&_sort=d &view=c&_acct=C000051270&_version=1&_urlVersion=0& _userid=1069263&md5=34214d96d9ea71a0f7ab620b036441 ab KP
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Old   June 16, 2005, 10:34
Default Re: Oscillating Residuals in Finer Mesh Case
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Jake
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HK and KP, Thanks for the advice, I really appreciate it. Jake
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