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January 4, 2017, 12:02 
Anisotropic Species Diffusion

#1 
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Hello all.
I am attempting to model multicomponent species transport through a randomized porosity field. My model dictates that I use the following equations for the axial and transverse dispersion coefficients respectively: Where α_os , λ_X, and λ_Y are numerical constants, D_m is effective molecular diffusivity of the acid species,uis the magnitude of Darcy velocity, r_p is the radius of the pore, and ɛ is the porosity. From what I have gathered from the help manual as well as previous posts on this forum, I need to use the anisotropic species diffusion porous cell zone condition in ANSYS Fluent. Unfortunately, the manual did not help me understand how I would apply these equations using this method. Enabling anisotropic species diffusion only allows the input of a matrix as seen by the following image taken from the manual: My question is as follows: how would I apply the equations listed previously as the dispersion coefficients? I am familiar with UDFs and have looked into using the DEFINE_DIFFUSIVITY macro but do not understand how to apply it to anisotropic diffusion or even if that is its proper usage. I have also considered the possibility of using UDS, but I am not sure if that is applicable either and if it is, how to use it. 

January 8, 2017, 03:11 

#2 
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I've been stuck on this problem for about a week with no end in sight.
The only thing I can think of is to rewrite the species transport equation using UDS but that seems rather complicated to simply implement anisotropic diffusivity. Any advice or ideas would be appreciated. 

January 8, 2017, 06:47 

#3 
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AA Azarafza
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Hi Baden,
You need to write your own anisotropic diffusivity using UDS macros and UDF provided for this aim. I've done this before for my PEMFC model a few times. It is pretty straightforward and easy to work. For getting some insights, you can see the examples has been offered in Fluent UDF manual. Be careful that you need to learn how to correctly compile or interpret the written UDF too. I hope it helps. Abouzar
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January 8, 2017, 13:11 

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Quote:


January 8, 2017, 17:22 

#5 
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You right. As there is no anisotropic diffusivity UDF for species transport equations in Fluent, you should employ generic scalar transport equation to use its capability of using userdefined function for your purpose. Please, refer to the related section in UDF manual.
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January 9, 2017, 11:49 

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Quote:
Last edited by Baden; January 10, 2017 at 14:49. 

January 17, 2017, 05:06 

#7  
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Hi, In reference to your questions: 1 Yes. Only two UDS are employed; however, in your UDF you should ensure the conservation of mass for the last species using only a simple algebraic equation. 2 For the UDS inlet velocities, as far as I know, you can again employ a macro to inject the UDSs in your simulations. I hope it helps, and sorry for late response!
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