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divergence of scaled residuals (steady simulation) |
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November 23, 2010, 08:55 |
divergence of scaled residuals (steady simulation)
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#1 |
New Member
Hana Matheislova
Join Date: Oct 2010
Posts: 7
Rep Power: 16 |
Hi everybody,
do you have any suggestions why scaled residuals (mainly continuity) in steady simulation drop at first, but started to grow after ca. 20th iteration? Smaller mesh cells and second order upwind discretisation quite helped, but the trend is the same. Are these results even relevant? Thanks for any suggestions or ideas |
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November 24, 2010, 03:06 |
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#2 |
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Join Date: Oct 2010
Posts: 45
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what flow are you solving for ? Your problem might be due to
- improper BCs - complicated physics or - inefficient mesh field other causes might exist, but could say the above in general |
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November 30, 2010, 05:09 |
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#3 |
New Member
Hana Matheislova
Join Date: Oct 2010
Posts: 7
Rep Power: 16 |
Hi Siri, thanks.
It is a non-spreading fire in an empty cuboidal room. The fire is defined only by energy source (W/m3), which is generated in a cylinder area. There is a door opening (interior) to the next empty cuboidal room, where the walls are pressure outlet. I am interested especially in temperatures in the room and a flow through the door. The mesh is hexahedral with the same size in the whole domain. If I made the cell size smaller, the residuals got better - increasing of continuity and energy was only very slight (other residuals were nicely decreasing). I am only a beginner in Fluent (I have more experiences only with CFD programmes specialised to compartment fires), so I am sorry, if my questions are quite stupid or irrelevant...But is it possible to use these results or does this behaviour mean that there is a serious problem? Or what do you think about it in general? |
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November 30, 2010, 06:16 |
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#4 |
New Member
Hana Matheislova
Join Date: Oct 2010
Posts: 7
Rep Power: 16 |
I have probably detected the main reason: the energy source of the fire is too high. The residuals do not increase if this value is much lower. The problem is, that this simulation should correspond to a real fire test, so the released heat and the fire size can not be changed.
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November 30, 2010, 07:07 |
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#5 | |
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Quote:
Whatever your real test data be, it does not matter. As long as you successfully fed the computer with the right mathematics & physics governing your problem, your residuals MUST converge in all circumstances. If not, erroreous results will be conveyed. Every CFD user must appreciate the implicit pitfalls such as these while using codes. |
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November 30, 2010, 07:30 |
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#6 |
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Join Date: Oct 2010
Posts: 45
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Hana,
I was writing this reply when I read your other post. I submitted a long response, but it didn't get posted unfortunately. Let me summarize instead brief pointers. I am sure you will appreciate this if you have worked on other CFD codes. As I still suspect the problem lies either in material BCs or equations being solved.
I wonder, why should you bother to model the adjecent room walls at all ? when you are only "interested especially in temperatures in the room and a flow through the door" Why should you not just define the outlet for the flow at the location of the door opening? |
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December 16, 2010, 05:45 |
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#7 |
New Member
Hana Matheislova
Join Date: Oct 2010
Posts: 7
Rep Power: 16 |
siri, thank you very much for your detailed reply. First, I was thinking and studying the influence of the adjacent room, which you mentioned in your reply. This is also the reason why I did not reply sooner. It sounds maybe weird, but it has an influence on temperatures and mainly on the flow through the door. If I consider no adjacent room, the flow through the door is not correct.
Now, I will finally check your suggestions... |
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