# DEFINE_DIFFUSIVITY udf in fluent

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 August 10, 2013, 11:57 DEFINE_DIFFUSIVITY udf in fluent #1 New Member   Mariusz Join Date: Aug 2013 Posts: 2 Rep Power: 0 Sponsored Links Hello, I am new user of fluent and I need udf for traser diffusivity in water. The traser is flowing from small pipe to bigger, filled with water. In manual I found only example for mean age of air, so please help me. I use k-epsilon model. The equation of traser diffusion is: D=(Dm+Dt)*rho where: Dm and rho are constant, Dt=0.1*(k^2/epsilon) k and epsilon are values calculated by fluent in every cell, The udf function I need is DEFINE_DIFFUSIVITY(name,c,t,i) in manual there was an information that c,t,i are variables, but I don't know how they are working. Can anyone help me how to create udf, that fluent can "understand" that k and epsilon are that values calculated?

August 11, 2013, 13:12
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A-A Azarafza
Join Date: Jan 2013
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Quote:
 Originally Posted by maniek940 Hello, I am new user of fluent and I need udf for traser diffusivity in water. The traser is flowing from small pipe to bigger, filled with water. In manual I found only example for mean age of air, so please help me. I use k-epsilon model. The equation of traser diffusion is: D=(Dm+Dt)*rho where: Dm and rho are constant, Dt=0.1*(k^2/epsilon) k and epsilon are values calculated by fluent in every cell, The udf function I need is DEFINE_DIFFUSIVITY(name,c,t,i) in manual there was an information that c,t,i are variables, but I don't know how they are working. Can anyone help me how to create udf, that fluent can "understand" that k and epsilon are that values calculated?
Hey Dude
You can do it either by defining a generic user scalar function (UDS) or applying species transport equation. The most important thing is the diffusivity, of course which you can use DIFFUSIVITY macro. The macro has four arguments: "name" is the name you consider for the function, "c" stands for the cells (cells in the zone), "t" designates thread (actually it represents zone ) and finally "i" is an index which imply to the interest species (the species you want to define a diffusivity). It is defined according to the order you select the species. For instance, if you have three specie, "i" is 0,1 and 2 for the specie, respectively. Let me tell you that if you use UDS then you can find the "i" for the appropriate scalar in user define/ scalars/ UDS index.
I hope it helps somehow
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Regard yours

 August 13, 2013, 17:13 #3 New Member   Mariusz Join Date: Aug 2013 Posts: 2 Rep Power: 0 Hey Azarafza, It helped, thanks so much

 Tags define_diffusivity, diffusion, diffusivity, udf

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