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-   -   about define_diffusivity (http://www.cfd-online.com/Forums/fluent/40476-about-define_diffusivity.html)

shuqin April 7, 2006 03:54

about define_diffusivity
 
I am modeling the transport of multi-conponents along two different zones,and the following code for setting different diffusivities : DEFINE_DIFFUSIVITY(mass_diff, c, t, i) {

if (THREAD_ID(t)==ZONE_1)

D = 12345;

if (THREAD_ID(t)==ZONE_2)

D = 67890;

return D; }

then what does the value D stands for,is it the diffusivity for species i in the mixture?if it is the case ,i suppose the Fick's law approximation is adopted by default,how should i set the binary diffusivity with UDF if i want to model in full multicomponent model?


Jun April 12, 2006 22:30

Re: about define_diffusivity
 
Hi, Shuqin,

YOu need to hook the mass_diff to the diffusivity of the species i.


shuqin April 13, 2006 08:06

Re: about define_diffusivity
 
hi,jun thanks for you attention,can you tell me how should i to hook the mass_diff to the diffusivity of the species i?


Jun April 13, 2006 22:36

Re: about define_diffusivity
 
Hi,

Follow these steps:

Define => Materials => Mass Diffusivity => user-defined => mass-diff

Enjoy

shuqin April 13, 2006 23:00

Re: about define_diffusivity
 
Jun,I am afraid you have not understand my problem. Thanks anyway! my question is about the UDF;

DEFINE_DIFFUSIVITY(mass_diff, c, t, i) {

if (THREAD_ID(t)==ZONE_1)

D = 12345;

if (THREAD_ID(t)==ZONE_2)

D = 67890;

return D; } does the value D stand for Di,m(the diffusivity of species i in the mixture)?if it is the case ,how shall I define Di,j which I can access through C_DIFF_L(c,t,i,j)?


Jun April 14, 2006 01:57

Re: about define_diffusivity
 
Hi Shuqin,

The value D stand for a single Di,m. I think the same value will be used for the diffusivity of each species in the mixture.

You cannot define the several Di,j with UDF. This might be the limitation of the FLUENT, and FLUENT will improve this in next version with my opinion.

Instead, you could find other methods which the FLUENT have already had to input Di,j.

This forces me learn more about the FLUENT.

Thanks

Jun

(the diffusivity of species i in the mixture)?if it is the case ,how shall I define Di,j which I can access through C_DIFF_L(c,t,i,j)?


shuqin April 17, 2006 08:40

Re: about define_diffusivity
 
by the way,jun.I got a problem when defining the diffusivity and actually I did post before,unfortunately I have not yet got the answer. I use following code to define two different diffusivity in two different fluid zones,but In Fluent ,since the two neighbouring cells have different diffusivities with a step change.there is a gradient between the two zones,and automatically Fluent will use interpolation between the face diffusivities. will this disarrange the species flux at the interface(defined as interior face) between the two zones ,if it is the case ,how shall I remove the gradiant? thanks ! /*********The code***********/ DEFINE_DIFFUSIVITY(uds_diff, c, t, i) {

real D;

if (THREAD_ID(t)==ZONE1)

D = 12345;

if (THREAD_ID(t)==ZONE2)

D = 67890;

return D; }


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