CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)
-   FLUENT (https://www.cfd-online.com/Forums/fluent/)
-   -   1st order vs 2nd order (https://www.cfd-online.com/Forums/fluent/36237-1st-order-vs-2nd-order.html)

 ken April 1, 2005 21:25

1st order vs 2nd order

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?

 ap April 2, 2005 14:54

Re: 1st order vs 2nd order

Try to increase the pressure under-relaxation factor to 0.6 or so.

Also consider a grid refinement.

Regards, ap

 ken April 3, 2005 15:46

Re: 1st order vs 2nd order

Unfortunately that didn't work. In fact the integral of the pressure in the system is also fluctuating and can't converge.

 Luca April 4, 2005 03:51

Re: 1st order vs 2nd order

Check your grid elements. If they are high skewed that could be the possible reason. Luca

 ken April 4, 2005 20:55

Re: 1st order vs 2nd order

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?

 ken April 4, 2005 21:34

Re: 1st order vs 2nd order

Just found of that PISO dosen't work. I tried it with and without skewness correctiona and used the QUICK scheme with it.

 Luca April 5, 2005 03:04

Re: 1st order vs 2nd order

You should try to correct your grid...I had your same problem and when I remeshed the problem was solved. Luca

 ken April 6, 2005 01:23

Re: 1st order vs 2nd order

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.

 k_tafazoli March 14, 2013 04:43

Van leer second order and AUSM+

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?