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#1 |
New Member
Greg Hwang
Join Date: Mar 2019
Posts: 3
Rep Power: 7 ![]() |
Hey guys! I am a newcomer to CFX.
Recently I was working on the transient simulation of a rotating seal model with tip leakage(the tip clearance is set to 0.5 mm, 2% height of the blade). I found that it is unable for the transient simulation to reach the prospectively residual (1e-4 or 1e-5, which is required by many papers I read ) since the steady simulation reached well. I tried to change the number of elements from 6.8 million to 70 million, however all the RMS Mom remained between 1e-3 and 1e-4. Then I got rid of the tip clearance in that model, the transient residual could reach below 1e-5 well. Hence, I infer that the small tip clearance might be the very cause of bad residual in transient simulation. Nevertheless the small tip clearance is the focus point of my research, I cannot just get rid of the clearance. I would appreciate it if anyone could help ![]() 1. Is my inference rational? 2. How can I improve the residual for the transient simulation of rotating seal model with small tip clearance? For your information: [Time duration]:total time 0.5s [Time step]:1e-4\5e-5\1e-5 were tried [Coeff loops]:3 to 5 [Turbulence]:SST |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,870
Rep Power: 144 ![]() ![]() ![]() ![]() |
Your question is a FAQ: https://www.cfd-online.com/Wiki/Ansy...gence_criteria
It is normal for a finer mesh to be less stable than a coarser mesh. It as less numerical dissipation, so is less able to damp the motions.
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Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
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#3 |
Senior Member
M
Join Date: Dec 2017
Posts: 703
Rep Power: 13 ![]() |
To be completely sure about the residuals you can let cfx write out the residual magnitudes in the domain. Then in CFD-post you can define a point location or whatever and plot the residuals.
(Point location allows you to find the maximum value instantly) This will give you certainty about the location. |
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#4 |
New Member
Greg Hwang
Join Date: Mar 2019
Posts: 3
Rep Power: 7 ![]() |
Thanks a lot for your reply and I will check FAQ later.
Actually what you talked about that finer mesh shall be less stable than coarser mesh is really contrary to my common sense. However when I use same case by only changing turbulence to k-epsilon or RNG k-epsilong, the residual could reach even 1e-6. What I am confused about is why just SST turbulence work so bad. Thanks again, and wish you a good day. |
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#5 |
New Member
Greg Hwang
Join Date: Mar 2019
Posts: 3
Rep Power: 7 ![]() |
Hey, thanks for your reply.
Actually I am not clear what you suggested me to do. The residual problem I asked happened in the process of solving, and the residual was updated with each timestep. Hence I am confused that how finding the maximum value in the CFD-post could help me improve the residual reaching 1e-4 or better? Please enlighten me with your instructions if I missed anything. Thanks again. |
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#6 | ||
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,870
Rep Power: 144 ![]() ![]() ![]() ![]() |
Quote:
Finer meshes have less numerical dissipation due to reduced truncation error, and reduced dissipation means reduced numerical stability which means harder to converge. A bit of knowledge of how CFD works trumps somebody's "common sense" guesswork. Quote:
And to answer your original question - please read the FAQ I linked to. I have answered this question thousands of times and I got sick of writing it over and over so wrote an FAQ on it. Please read the FAQ, it really does discuss the important issues.
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Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
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