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Old   July 16, 2002, 09:56
Default UDS-diffusivity
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Dear All,

I am asked to use UDF and UDS in my project.To start off with the very basics, I am asked only (for the time being)to set Diffusivity coefficient of air=1

I could not remember exactly what I did, but roughly it goes like this:

Under Define/User-Defined/Scalars, I put number of user defined scalar as =1 and flux function none

Then under the materials panel of air, i have this additional scalar UDS diffusivity and when I click on edit the UDS diffusivity coeff panel pops up and there is a whole list of uds-0, uds-1 etc which also appears in the manuals.

I guess the number under the coefficient will be the number of diffusivity coeff I want air to be.

However I am confused by the procedures. WHat if I am asked to introduce a scalar tat is not diffusivity but something else, should i under Define/User-Defined/Scalars, I put number of user defined scalar as =2? since if i put 1, diffusivity is shown on the materials panel.

And what is the meaning of the uds-0, uds-1, uds-2 etc shown on the UDS diffusivity coeff panel means? Why are there a whole list?and not just a panel that contains just

"Diffusivity Coefficient= 1"

I am really confused and are doing things that I don't even sure what am I doing. Hope can hear some opinions from anyone.
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Old   July 17, 2002, 03:46
Default Re: UDS-diffusivity
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Hi, so I'll try to explain what i understood about this.

the list uds-0,uds-1,uds-2 ... represents the number of scalars you want to use. If you have defined one scalar in Define/User-Defined/Scalarsn, then you have to fix the value of diffusivity for uds-0. You have to watch the unit of the coefficient : (m2/s). And look at the equation for the scalar in Fluent Documentation and take into account if you are in turbulent flow or not. Turbulent diffusion may be much higher than molecular diffusion.

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Old   July 17, 2002, 10:52
Default Re: UDS-diffusivity
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Hi, thanks for your reply!

So does that mean that all the scalars are for specifying diffusivity only?

Scalars as i know means property that is not affected by direction, for example, temperature, pressure, speed, diffusivity etc, unlike velocity which has both magnitude and direction.

So the user-defined scalars in Fluent actually means "diffusivity" only and not other scalar properties?

Then why do we need to put so many scalars for just one property-diffusivity? When do we need more than one scalar?

I'm sorry if my question sounds stupid, but I can't seem to understand this.

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Old   July 18, 2002, 08:30
Default Re: UDS-diffusivity
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hi, ok as far as i have understood your problem and Fluent, I think that in fact : uds-0, uds1 , etc is a list of diffusivity of scalar . I mean, if you have defined the transport of more than one scalar in Define/User define/scalar, you have to specify one diffusivity for each scalar you may use. uds-0 is diffusivity for your 1st scalar, uds-1, is the diffusivity for your second scalar, if you are using another one. As you are using one scalar, i think you have just to specify the diffusivity for this one. I think this is the explanation.
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Old   July 15, 2010, 10:35
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I read this thread and as soon as you seem you can solve my problem I'll ask you. I can't understand whether using m^2/s as units for diffusivity is right, while next to the box in the drop-down list in materials properties the units are kg/m-s like viscosity. I thought I have to convert the diffusivity value in m^2/s, as Fickian diffusivity value for my species, to these units but it's a kind of freakingly strange thing I hope you can help me and i thank you as well!
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Old   December 16, 2014, 03:05
Lorenzo Mazzei
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Because Fluent is asking you the value of rho x D, not D.
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