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strange residuals - converging -then diverging- then oscillating - then getting weird |
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March 26, 2015, 09:44 |
strange residuals - converging -then diverging- then oscillating - then getting weird
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#1 |
New Member
Juste
Join Date: Mar 2015
Posts: 14
Rep Power: 11 |
hi guys
i am trying to simulate an open channel flow (like simulating a river in a lab flume) in fluent..but without using vof...instead i am defining the top surface as a moving wall....so that the air-water interface is preserved to some extent...i am not interested in the interface and hence not using vof as of now... the problem i am facing is that the residuals are not converging properly....initially they start converging, then start diverging, then flat, and then all goes weird.....please have a look at the attached pic... however, the velocity profile and turbulence KE obtained from this simulation is very close to the experimental data i obtained from the lab flume.... i am just simulating this to verify and validate my model first... do you have any idea why this is happening...any suggestions/solutions... mesh details: i guess i am already having good enough mesh 1. it is structured hexahedral mesh 2. aspect ratio (AR) = 1 - 178 (higher AR in boundary layer only, used inflation) 3. orthogonality = 0.16 -1.0 (so no skewed cells) 4. total cells = 1.4 million 5. i have used inflation for the boundary layer fluent details: in fluent also, i guess i am using pretty simple stuff 1. pressure based, steady state, gravity enabled solver 2. turbulence model - kepsilon with scalable wall function 3. material = liquid-water 4. inlet BC = mass-inflow, specified turbulence intensity and hydraulic dia 5. otlet BC = outflow 6. bottom and side = wall no slip 7. top = moving wall as mentioned before 8. urf - default values 9. SIMPLEC / least square / all second order discretization methods used i am not sure why the residuals are behaving like this, although the output velocity and turbulence are matching...the geometry as well as the model is pretty simple (just a rectangular flume with a cavity) so i guess there shouldn't be much of a problem.. please let me know if you all have any ideas...thanks a lot.. |
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March 30, 2015, 15:41 |
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#2 |
Member
Ethan Doan
Join Date: Oct 2012
Location: Canada
Posts: 90
Rep Power: 14 |
try starting with first order discretization then switching up to second order after 500 or so iterations
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March 31, 2015, 08:52 |
found solution
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#3 |
New Member
Juste
Join Date: Mar 2015
Posts: 14
Rep Power: 11 |
hi ethan and desmond, thanks for both of your replies
i just changed the mesh and now it is converging very nicely, 1e-6 residuals in 2700 iterations...i guess that's okay with me.. i didn't change any other thing in fluent...but i wonder why was it behaving so weird previously... earlier too i had a hex mesh (using cutcell and inflation layers) and now I am having hex mesh too but using edge sizing and biased divisions, no inflation... anyway, it has converged now.... thanks so much for both of your replies. |
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December 3, 2016, 18:26 |
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#4 |
New Member
Mohsin
Join Date: Dec 2016
Posts: 1
Rep Power: 0 |
Hi gentlemen,
I am new to this subject and this forum so please take it easy on my ignorance. I am trying to run a few supersonic cases on a jet model. Due to the time constraint I didnt want to create a very fine mesh. These are the current residuals graph. [IMG][/IMG] I know it doesnt show converging results. Is the only solution for me is refining mesh. If i do it causes the iterations to become really slow and I dont have a week to run this. Any other options for a quick conversion. I mean i can run the simulation for a day at most. Any help is highly appreciated. Please feel free to give any comments. |
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