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Flux definition in Transport of Dilute Species Module

Posted 2 sept. 2020, 23:22 UTC−4 Battery Design, Chemical Reaction Engineering, Electrochemistry Version 5.5 2 Replies

Donovan Wang
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Hi COMSOL users,

In the Equation View of Transport Properties under Transport of Dilute Species Module, there are two kinds of flux definitions. One is defined on all the boudaries, called Normal total/diffusive/electrophoretic flux. The other is defined on domains, called Total flux whose magnitude is calculated from the total flux components (defined in domains). See images attached.

Assuming that flux tangent to the boundary is 0, the normal total flux should equal to the total flux magnitude. However, what I found is that they are not the same.

I understand that boundary nodes are treated differently from those internal nodes. This should ensure the flux calculated from the domain values and boundary treatments are the same.

I would appreciate it if there is any helpful explanation or related comments! Thanks!

2 Replies Last Post 10 sept. 2020, 15:56 UTC−4
Lasse Murtomäki
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Posted: 4 years ago 4 sept. 2020, 01:55 UTC−4

sqrt(a²+b²+c²) is not equal to a+b+c (domain flux picture). But that you know already. Flux magnitude is always positive but normal total flux can be also negative.

sqrt(a²+b²+c²) is not equal to a+b+c (domain flux picture). But that you know already. Flux magnitude is always positive but normal total flux can be also negative.
Donovan Wang
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Posted: 4 years ago 10 sept. 2020, 15:56 UTC−4

sqrt(a²+b²+c²) is not equal to a+b+c (domain flux picture). But that you know already. Flux magnitude is always positive but normal total flux can be also negative.

Thank you Lasse for the reply. In my case of a 2D problem (0 flux in z axis), I set concentration boundary condition so the flux along the boundary is 0. Then the magnitude of sqrt(a^2+b^2+c^2) should be the same as that of a. In my simulation it's not. I guess the flux from the domain nodes are extrapolated values and may not be as accurate as boundary flux.

>sqrt(a²+b²+c²) is not equal to a+b+c (domain flux picture). But that you know already. Flux magnitude is always positive but normal total flux can be also negative. Thank you Lasse for the reply. In my case of a 2D problem (0 flux in z axis), I set concentration boundary condition so the flux along the boundary is 0. Then the magnitude of sqrt(a^2+b^2+c^2) should be the same as that of a. In my simulation it's not. I guess the flux from the domain nodes are extrapolated values and may not be as accurate as boundary flux.

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