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1st order vs 2nd order

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Old   April 1, 2005, 21:25
Default 1st order vs 2nd order
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ken
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Hi I'm simulating flow throught 2 parallel plates with inclined inlet and outlets. I tried the second order upwind momentum discretization (and also QUICK) but it couldn't converge very well. For example the average velocity in a plane was fluctuaing at around 1% of its maximum value. But when I switched to the first order upwind, this fluctuation is reduced to 0.2%.

I know first order can converge more easily and 2nd order is more accurate. But will a 2nd order solution that is partially converged be more accurate than a 1st order one?

Are there any ways to help the 2nd order solution converge? I have reduced the momemtum under relaxation to 0.3 but to no avail. I was wonder if I should make some changes to the multigrid entries. Any suggestions?
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Old   April 2, 2005, 14:54
Default Re: 1st order vs 2nd order
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ap
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Try to increase the pressure under-relaxation factor to 0.6 or so.

Also consider a grid refinement.

Regards, ap
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Old   April 3, 2005, 15:46
Default Re: 1st order vs 2nd order
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ken
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Unfortunately that didn't work. In fact the integral of the pressure in the system is also fluctuating and can't converge.
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Old   April 4, 2005, 03:51
Default Re: 1st order vs 2nd order
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Luca
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Check your grid elements. If they are high skewed that could be the possible reason. Luca
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Old   April 4, 2005, 20:55
Default Re: 1st order vs 2nd order
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ken
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Some of the elements are indeed skewed but I can't do much about it because of the geometry of the domain. Can I work around this by using skewness correction in SIMPLEC or PISO? If so, what is a common value to try?
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Old   April 4, 2005, 21:34
Default Re: 1st order vs 2nd order
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ken
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Just found of that PISO dosen't work. I tried it with and without skewness correctiona and used the QUICK scheme with it.
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Old   April 5, 2005, 03:04
Default Re: 1st order vs 2nd order
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Luca
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You should try to correct your grid...I had your same problem and when I remeshed the problem was solved. Luca
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Old   April 6, 2005, 01:23
Default Re: 1st order vs 2nd order
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ken
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I remeshed taking care in reducing the number of skewed elements. It dosen't seem to work. As before, the residuals can't get below 1e-3 when I used 2nd order upwind. Pressure is also fluctuating.
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Old   March 14, 2013, 04:43
Default Van leer second order and AUSM+
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kian
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Hi, I've got a fortran code for solving 2D compressible flow over an airfoil with the method of first order van leer flux vector splitting . I have to change it to second order van leer flux vector splitting method . Can you please help me on how to do it? It's also possible for me to solve this project with the method of AUSM+ . Are you familiar with this method? can you help me with it please?
Thanks a lot in advance
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