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August 28, 2013, 09:36 |
Convergence problem
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#1 |
Member
Harry
Join Date: May 2013
Posts: 68
Rep Power: 12 |
Hi friends. I am simulating flow over 2d airfoil at Reynold number of 1000. Flow is laminar and steady state. I run with two different pressure schemes. First I used PRESTO Pressure scheme and 2nd order upwind and the residual became stable (very small oscillations) at 10e-6 and were not decreasing any more. I was monitoring CL and was constantly decreasing with a very minor change.
Then I restarted the whole process with 2nd order Pressure scheme and the solution converged but the result is over predicted from experimental. I know this is steady state case and not transient and my mesh quality, skewness, and Angle is also fine. Anyone know where is the problem? Thanks in advance |
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August 28, 2013, 10:34 |
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#2 |
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
Posts: 3,399
Rep Power: 46 |
Why do you thing that this is a problem with convergence? You say that both simulations converge
How high is the error between experiment and simulation? |
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August 28, 2013, 10:51 |
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#3 |
Member
Harry
Join Date: May 2013
Posts: 68
Rep Power: 12 |
Thanks Alex for your response. The correct CL is about 0.1634 (at 3 degree angle of attack) but i got 0.167 with 2nd order pressure and momentum schemes. My value was lower when i used coarser mesh but with refining it become over predected. I think it should be equal or less than correct value. When I changed pressure scheme to PRESTO, i got about 0.1620 but not totally stable, it still decreasing (very minor change 0.1% decrease in 1000 iterations) but residuals not decreasing. I run through about 32000 iterations. What do you say about this?
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August 28, 2013, 13:41 |
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#4 |
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
Posts: 3,399
Rep Power: 46 |
I say that a mismatch of 2.2% between experiment and simulation is very good.
Unless you know for certain that the error of the measurement is lower than those 2.2%, there is no reason in pushing the simulation towards the "exact" measurement value. |
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August 28, 2013, 23:20 |
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#5 |
Member
Harry
Join Date: May 2013
Posts: 68
Rep Power: 12 |
Actually I am doing grid validation at angle of attack 3 degree. one of my friend did the same case and he got almost the same result to the actual (He is not here). I wonder why with two different pressure schemes result is different. In my opinion, using PRESTO pressure scheme the result looks ok but the residuals become stable as i told before so i don't think this is correct.
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August 29, 2013, 04:06 |
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#6 | |
Member
Harry
Join Date: May 2013
Posts: 68
Rep Power: 12 |
Quote:
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