Yes. Von: Dumux [mailto:[email protected]] Im Auftrag von Bernd Flemisch Gesendet: Mittwoch, 2. Dezember 2015 10:52 An: DuMuX User Forum Betreff: Re: [DuMuX] A fundamental question concerning diffusion in Dumux
Hi Georg, do you get a correct result if you include the density in the gradient? Kind regards Bernd On 12/02/2015 10:45 AM, [email protected]<mailto:[email protected]> wrote: I forgot the attached file... Von: Dumux [mailto:[email protected]] Im Auftrag von [email protected]<mailto:[email protected]> Gesendet: Mittwoch, 2. Dezember 2015 10:44 An: [email protected]<mailto:[email protected]> Betreff: Re: [DuMuX] A fundamental question concerning diffusion in Dumux Hello Dumux, I am back with the same question and some more infos. The modeling approach for diffusion in Dumux is (in my opinion) wrong and gives unphysical results. This is most pronounced for diffusion in a gas phase and when the intrinsic permeability is low. Consider the model setup depicted in Figure1.jpg. In this setup the only transport mechanism for H2O from the right boundary is diffusion because the sink of N2 is high and the advective flow is from left to right. In this setup, the pressure will drop to the right while the gradient of x_g^H2O is vice versa. However, the concentration of H2O will be lower at the right boundary and diffusion will occur from a lower to a higher concentration! This is completely unphysical. There is no reason why the component should flow against its concentration gradient. The reason for this lies in the formulation of the diffusive fluxes: [cid:[email protected]] Where [cid:[email protected]] In the Dumux formulation, the second (pressure dependent) term on the very right is neglected resulting in the possibility that species diffuse against their concentration gradient. The density gradient is proportional to the pressure gradient from left to right while the molfraction gradient is vice versa. I hope this made things more clear. I would recommend to use grad(rho*x) instead of grad(x) for the calculation of the diffusive fluxes. I am always open for discussion. Kind regards Georg Von: Dumux [mailto:[email protected]] Im Auftrag von [email protected]<mailto:[email protected]> Gesendet: Mittwoch, 25. November 2015 14:30 An: [email protected]<mailto:[email protected]> Betreff: [DuMuX] A fundamental question concerning diffusion in Dumux Hello Dumuxers, I was wondering why the diffusive fluxes in Dumux are defined as D*rho*grad(mol-or-massfraction). Typically one would use D*grad(c) (e.g. Ficks law) where c=rho*mol-or-massfraction. Using the Dumux equation means that local differences in the density are neglected for diffusive fluxes. Is there any reason/justification for this? Or is my thinking just wrong? Best regards Georg Futter -------------------------- German Aerospace Center (DLR) Institute of Engineering Thermodynamics | Computational Electrochemistry | Pfaffenwaldring 38-40 | 70569 Stuttgart Dipl.-Ing. Georg Futter | Ph.D. student Telefon 0711/6862-8135 | [email protected]<mailto:[email protected]> www.DLR.de<http://www.DLR.de> _______________________________________________ Dumux mailing list [email protected]<mailto:[email protected]> https://listserv.uni-stuttgart.de/mailman/listinfo/dumux -- _______________________________________________________________ Bernd Flemisch phone: +49 711 685 69162 IWS, Universität Stuttgart fax: +49 711 685 60430 Pfaffenwaldring 61 email: [email protected]<mailto:[email protected]> D-70569 Stuttgart url: www.hydrosys.uni-stuttgart.de<http://www.hydrosys.uni-stuttgart.de> _______________________________________________________________
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