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negation concentration values when calculating convention diffusion of particles
Posted Nov 8, 2015, 7:06 a.m. EST Fluid & Heat, Computational Fluid Dynamics (CFD) Version 5.0 2 Replies
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HI all,
I want to get the concentration values of particles in oil,and i am tyring to use the models of Transport of Dilutes Species（tds） and Turbulent Two-Phase Flow，Level Set(tpf) . The initial concentration is 10^(-12)mol/m^3, but there are negative values in the calculation results，and the negative values is about -10^(-12)mol/m^3.
when the initial concentration is 1mol/m^3, the negative value is about -0.01，which can be accepted.
I want to know whether the COMSOL software is suitable for the calculation of my case，in which the concentration values of particles are small.
Does anyone have a similar problem?
Thank's in advance,
Liangwang Wang
I want to get the concentration values of particles in oil,and i am tyring to use the models of Transport of Dilutes Species（tds） and Turbulent Two-Phase Flow，Level Set(tpf) . The initial concentration is 10^(-12)mol/m^3, but there are negative values in the calculation results，and the negative values is about -10^(-12)mol/m^3.
when the initial concentration is 1mol/m^3, the negative value is about -0.01，which can be accepted.
I want to know whether the COMSOL software is suitable for the calculation of my case，in which the concentration values of particles are small.
Does anyone have a similar problem?
Thank's in advance,
Liangwang Wang
2 Replies Last Post Nov 19, 2015, 4:28 a.m. EST
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Posted:
9 years ago
Hi
The issue of negative concentrations has been discussed in this forum quite frequently. There appears to be no single solution to the problem. I had a similar problem with the convective diffusion equation. The solution was better meshing in the narrow areas, but I could not get rid off it completely. Convective diffusion is a notoriously difficult problem because the convective term easily is orders of magnitude larger than the diffusion term. It is thus analogous to a problem where a fast chemical reaction takes place along diffusion.
You could try a higher relative accuracy than the default 0.01.
BR
Lasse
The issue of negative concentrations has been discussed in this forum quite frequently. There appears to be no single solution to the problem. I had a similar problem with the convective diffusion equation. The solution was better meshing in the narrow areas, but I could not get rid off it completely. Convective diffusion is a notoriously difficult problem because the convective term easily is orders of magnitude larger than the diffusion term. It is thus analogous to a problem where a fast chemical reaction takes place along diffusion.
You could try a higher relative accuracy than the default 0.01.
BR
Lasse
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Posted:
9 years ago
HI, Lasse,
Think you for your reply, i had tried your suggestion, but the negative concentration still existed. And i finally solved my problem using the measures on cn.comsol.com/community/forums/general/thread/29200
I am a new user on comsol. I should learn from you more in the future. Thank you very much for your help！
Think you for your reply, i had tried your suggestion, but the negative concentration still existed. And i finally solved my problem using the measures on cn.comsol.com/community/forums/general/thread/29200
I am a new user on comsol. I should learn from you more in the future. Thank you very much for your help！