# why is there minus concentration of AV

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 January 15, 2009, 06:59 why is there minus concentration of AV #1 Li Guest   Posts: n/a Hi, I am doing a simulation in a model with a emission surface in a wind tunnel. I defined an additional variable. The inlet concentration is 1.83E-06kg/m3, but there is small area where the concentration is higher than the inlet concentration before the emission surface and there is also a small area where the concentration is minus (please see the figures). I checked the simulations results in CFX11.0.It converged well to Max RES in 10E-05 and the monitor point value is quite stable. http://img132.imageshack.us/my.php?image=modelnm4.png http://img132.imageshack.us/my.php?i...trationee6.png Thank you very much!

 January 15, 2009, 18:29 Re: why is there minus concentration of AV #2 Glenn Horrocks Guest   Posts: n/a Hi, What discretisation scheme are you using for volume/mass fractions? Some discretisation schemes can generate over/undershoots. Glenn Horrocks

 January 16, 2009, 04:03 Re: why is there minus concentration of AV #3 Li Guest   Posts: n/a Hi Glenn Horrocks, thank you for your response. I used second order discretisation scheme. Thanks.

 January 18, 2009, 19:26 Re: why is there minus concentration of AV #4 Glenn Horrocks Guest   Posts: n/a Hi, Do you get the over/undershoots when using upwinding for the AV? Glenn Horrocks

 January 19, 2009, 05:33 Re: why is there minus concentration of AV #5 Li Guest   Posts: n/a Hi Glenn, I tried the upwind scheme and then the unphysical area disappeared. I can understand why the second scheme can cause the over/undershoots somehow, but do you think it is ok to keep the results with second scheme even though there are over/undershoots because I still think the second scheme can give better results for the transport equation. Thank you very much! Best Regards, Li

 January 19, 2009, 18:52 Re: why is there minus concentration of AV #6 Glenn Horrocks Guest   Posts: n/a Hi, Whether the under/over shoot is a problem depends on what you want. Is this a significant region? What are the key parameters for the simulation results and does this region affect them? You will want to use second order differencing if at all possible as it is much less diffusive. Glenn Horrocks

 January 20, 2009, 04:15 Re: why is there minus concentration of AV #7 Li Guest   Posts: n/a Hi Glenn, Thank you for your responding very much. I will compare the results with the experiments and have a look at the simulation results in detail. Best Regards, Li

 January 23, 2009, 18:54 Re: why is there minus concentration of AV #8 CycLone Guest   Posts: n/a Hi Li, Use the High Resolution scheme for your scalar. It will give you bounded second order results. -CycLone

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