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Burley Wang March 9, 2000 21:31

about UDF,VOF, surface reaction
1. any kind guy could pls expalin that why cannot I change the density and the specific heat using UDF, I may need to change the specific heat or the volumetric heat capacity (product of the density and specific heat)

2. FLUENT's limitation says that coupled heat transfer cannot be modeled when using VOF. however, if I wanna do so, any possibilities by using UDF?

3. I don't think suface sorption/adsorption can be modeled by species transport w/ surface reaction, what do you think about it?

Thanks a lot for any suggestions or explaination


Arthur Valais March 9, 2000 21:38

Re: about UDF,VOF, surface reaction

Can you not change C_R(c,t) and C_CP(c,t)? What errors does it produce? [p.33 of udf manual.]

---Arthur Valais

Burley Wang March 9, 2000 21:54

Re: about UDF,VOF, surface reaction
Thanks Arthur,

I mean FLUENT said that " you can customize the property definition (except density and specific heat)" (UDF mannual, Chapter 1, Overview, text-line 9)

or did I misunderstand its meaning?


Greg Perkins March 9, 2000 22:05

Re: about UDF,VOF, surface reaction
I wouldn't advise changing these within a UDF - I'm pretty sure when Fluent wants the values for these properties it will call its internal routines before setting the matrix values. Thus, it will probably overide your settings (haven't checked this though).

You can't link a UDF to specific heat because in determining heat release in reactions you need to integrate the expression over a temperature range. That's why Fluent only provides a limited range of polynominal expressions etc.

In my case I'd love to modify the specific heat using dynamic (rather than constant) co-efficients in the polynominal expression. This would still allow an analytical expression for the integral, but could be flexible - ie could account for a composite fluid, solid like in the standard porous media model but could depend on space and time.

Hopefully this will be in the next release of Fluent.

Arthur Valais March 9, 2000 22:06

Re: about UDF,VOF, surface reaction
Just saw that. I guess that density is something that is solved for, so you would need to specify the other variables such as mass, pressure. My only experience is in reading the variables and setting the reaction rate.

---Arthur Valais

Arthur Valais March 9, 2000 22:08

Re: about UDF,VOF, surface reaction

So, of all the values on p.33, do you know what can one change and what should be left alone?


Burley Wang March 9, 2000 23:02

Re: about UDF,VOF, surface reaction
Thanks Greg,

I also hope (and actually if it could, it is what I want)just modify the specific heat dynamiclly using polynomial expression.

you said hopefully,it seems Fluent doesn't provide the function. but you also mentioned "that's why Fluent provides a limited range of polynominal expressions etc." so whether it can be modified or not?

Thanks a lot


Greg Perkins March 9, 2000 23:14

Re: about UDF,VOF, surface reaction
You can't modify the co-efficients of the polynomial expressions dynamically using a UDF as far as I know.

If you find out how to do this, please let us all know.


Sung-Eun Kim March 10, 2000 15:55

Re: about UDF,VOF, surface reaction
FLUENT 5.3 now allows you to specify density as arbitrary function of any other FLUENT variables. As long as the sensitivity of density to the change of the variables is reasonably (not too high), you can customize density without much problem.

Specific heat (Cp) still is not and will not be specifiable as an arbitrary function of arbitrary variables. One of the main reasons is that the energy equation has to rely upon enthalpy which is basically evaluated from integral of Cp in the temperature space. As you can immediately see, we run into a serious problem of relating those arbitrary variables and temperature.

I hope you understand why Cp can't enjoy the UDF customization.

Greg Perkins March 10, 2000 19:30

Re: about UDF,VOF, surface reaction
Sung-En Kim.

I understand the argument that you require an integrable expression for the specific heat.

However, while full UDF custimisation may not be possible, it would be possible to have the co-efficients of the analytical expression (polynominals) to be set dynamically.

In my problem I'd like to account for solid and fluid in a cell in a similar way as the existing Fluent porous media model, with the addition that the amount of solid changes as a function of some reaction. In this case the porosity changes and thus the composite specific heat is a function of the solution.

I have got around this so far, by using two energy equations, one for the fluid (FLUENTs) and one for the solid, implemented with a UDS. But this adds processing time to an already complex problem and means I have to deal with interphase heat transfer as well.


Joakim Majander March 13, 2000 08:08

Re: about UDF,VOF, surface reaction
I can't see any reason why Cp couldn't be given trough UDF. It is done e.g. in CFX by giving an average and local Cp. The average can be used for calculating temperature from enthalpy or vice versa. I'm not very happy to input all the data of 12 species into fluent while I allready have the routines to calculate Cp.

How can density be given in 5.3? The UDF-manuals still say it can not be done.


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