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up July 6, 2001 16:18

UDS DIffusivity...

I'm a having a difficulty with UDS modeling. I'm trying to define the UDS Diffusivity with a UDF, and it gives me the following error.

Error: UDS_Diffusivity: unrecognized diffusivity method for UDS-0: 198172680. Error Object: ()

I've changed the UDS Diffusivity to a constant value and defined it from materials panel, but i'm still getting the same error. Since manual does not cover anything in detail about UDS modeling, can you please help me?

I'd appreciate your help...


Greg Perkins July 8, 2001 21:05

Re: UDS DIffusivity...
I suspect that you must return a value for each index that fluent calls in your UDS. From memory:

DEFINE_DIFFUSIVITY(my_uds,cell,thread,k) {

if (k==my_uds) return my_uds_value;

else return 0.0; }

this will return the value for uds k with diffusivity defined by you. All others will be zero. When you use this fucntion you will need to return the diffusivity for all uds's by checking k as above. Sometimes you may need to return a small non-zero value 1e-9 instead of 0.0 - depedning upon what other models are activated etc.

UP July 15, 2001 02:58

Re: UDS DIffusivity...
Thanks Greg for your reply;

But the thing is I think I have a different problem, even if I try to use a constant diffusivity it's giving the same error to me.

Is there anything i'm missing? I just define one uds from uds panel, then i set the bc's and diffusivity from materials panel.

But whether i select a udf to define the diffusivity or a constant it's giving me the same error.

By the way, do i need to define uds before any iterations?

Thanks a lot...

Greg Perkins July 15, 2001 23:25

Re: UDS DIffusivity...

stick with a constant for the moment. As you pointed out if you can't get this to go, its not likely to be your user code.

Check bc in the bc panel. Pick somthing simple.

Also what have you chosen for your mass flux?? Usually its either none or the fuid phase mass flux.

I assume its steady too.

Can you perform any its. If not, it must be in your setup - so keep checking those - and maybe trying different options.

If you get some iterations, then in my experience, you can run into problems if convection is very large compared to diffusion and you have strong 'shock' like gradients in the solution. If this is the case try increasing the diffusion, at least until you start getting solutions.

Generally I found user scalars work pretty well - and are very flixble, but they do take a bit of effort.


UP July 16, 2001 08:11

Re: UDS DIffusivity...
Thanks for you response Greg;

However, the funny thing it's not working with a constant either. I really wonder is there anything I'm missing, because I tried the same thing several times, it's not working at all...

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