[MASOUD]

Hi folks,

I've got a quick question; here it is:

How can I impose a Robin (or maybe Mixed) boundary condition in internal or external boundaries, for a gas species in?

If writing a UDF is a solution, then where can we hook it up?

Any feedback is highly appreciated.

Masoud

[Amir]

Hi,
first separate the value term from the others, write a UDF for other terms, then hook it in the value boundary condition:
e.g. u+du/dn=cte ---> u=cte-du/dn ---> .....

[MASOUD]

Thanks Amir.

First off, I would assume this is possible for 'external' boundaries. How about for internals?

Next, I wrote the following UDF which works fine. But as soon as I change

'0.72'

to

F_PROFILE(f,t,i)=((0.1*F_YI_G(f,t,i))+0.48);

and re-compile it, get this error:

UDFsconv.obj : error LNK2019: unresolved external symbol _F_YI_G referenced in function _AGC_MASS_TRAN
libudf.dll : fatal error LNK1120: 1 unresolved externals

Any idea why this is happening?

======

DEFINE_PROFILE(AGC_MASS_TRAN,t,i)
{
face_t f;
begin_f_loop(f,t)
{
F_PROFILE(f,t,i)=0.72;
}
end_f_loop(f,t)
}
=======

[Amir]

Hi,
if you mean internal walls (which need boundary conditions) it doesn't have any differences with this procedure!!
about your code, there are many problems in it, I provide a template for you; change that for your cases:

#include "udf.h"
DEFINE_PROFILE(AGC_MASS_TRAN,t,i)
{
face_t f;
cell_t c;
Thread *t0;
begin_f_loop(f,t)
{
t0=THREAD_T0(t);
c=F_C0(f,t);
F_PROFILE(f,t,i)=((0.1*C_YI_G(c,t0,i)[0])+0.48);;
}
end_f_loop(f,t)
}

[MASOUD]

Thanks. But what is the purpose of

t0=THREAD_T0(t);

and

c=F_C0(f,t);

[Amir]

Originally Posted by MASOUD

Thanks. But what is the purpose of

t0=THREAD_T0(t);

and

c=F_C0(f,t);

As you know, some commands are not described in UDF manual; instead, you may find them in src folder, but we don't have F_YI_G(f,t,i) , at least I couldn't find that, so we have to use cell gradients instead. For more information about these functions take look at UDF manual.

Regards,

Amir

[MASOUD]

I can admit the fact that I'm totally confused with this feedback from Ansys customer support! Does this make sense to you at all? Here it is:

-------------------

A convective boundary condition cannot be imposed on internal or external boundary (wall boundary). For a wall either a zero diffusive flux or a constant value (which can be modified using a UDF) can be specified.

You need to use source terms on either sides of the interior surface to model the convective boundary condition.

Another note, the Robin condition might come out of the flow solution itself as the differences in concentrations between the electrode surface and bulk can drive diffusion in the domain.

DEFINE_SOURCE is hooked to the fluid zones. Inside the macro you need to check for the cells that are next to the interior boundary and impose the sources or sinks there. For the rest of the domain zero sources can be specified.

The sources should be added to one side and the negative value of it should be added to the other side to conserve mass. The unit of mass sources are in kg/m3-sec. The flux needs to be converted to that unit to be specified to the cells adjacent to the boundary.

[Amir]

Originally Posted by MASOUD

I can admit the fact that I'm totally confused with this feedback from Ansys customer support! Does this make sense to you at all? Here it is:

-------------------

A convective boundary condition cannot be imposed on internal or external boundary (wall boundary). For a wall either a zero diffusive flux or a constant value (which can be modified using a UDF) can be specified.

You need to use source terms on either sides of the interior surface to model the convective boundary condition.

Another note, the Robin condition might come out of the flow solution itself as the differences in concentrations between the electrode surface and bulk can drive diffusion in the domain.

DEFINE_SOURCE is hooked to the fluid zones. Inside the macro you need to check for the cells that are next to the interior boundary and impose the sources or sinks there. For the rest of the domain zero sources can be specified.

The sources should be added to one side and the negative value of it should be added to the other side to conserve mass. The unit of mass sources are in kg/m3-sec. The flux needs to be converted to that unit to be specified to the cells adjacent to the boundary.

I think that these notes generally concern about modeling convecting b.c which certainly cannot perform on walls. for example, these BCs can use for simply modeling a fan, but I believe that there is easier procedure for this purpose which is using fixed values in domain! Actually, using these procedures depends on cases' nature.

[MASOUD]

Well, my case is a Fuel Cell. Attached shows the geometry. The boundary which needs convective mass transfer b.c. is the interface between chanel and electrode which is highlighted.

Unfortunately, I couldn't understand from your reply that I should stick to what Ansys says, or I can use what I have been doing (specified mass fraction derived from the convective b.c.). Please clarify.

[MASOUD]

Also, how can fixing the mass fraction turn in to convective b.c?

[Amir]

Hi,
Maybe it's better that you clearly specify what you you want to do (the analytic relations or governing eq.s), then I'll try to help you handling it.

PS. : fluent have some capabilities and modules in fuel cell modelling.

[MASOUD]

Amir,

Could you send me your email address? I'll drop you a descriptive email then.

[Amir]

Originally Posted by MASOUD

Amir,

Could you send me your email address? I'll drop you a descriptive email then.

you can do that in this forum; both private and E-mail massages !!!

[serene6390]

Originally Posted by Amir

Hi,
if you mean internal walls (which need boundary conditions) it doesn't have any differences with this procedure!!
about your code, there are many problems in it, I provide a template for you; change that for your cases:

Can anyone tell me what does the C_YI_G mean? Is it applied to indicate the gradient of u? I'm also working on a problem with mixed boundary condition. I'm still confused after reading these.

[Amir]

Originally Posted by serene6390

Can anyone tell me what does the C_YI_G mean? Is it applied to indicate the gradient of u? I'm also working on a problem with mixed boundary condition. I'm still confused after reading these.

This is gradient of species mass fraction!
Please take a look at the UDF manual for more info...

Bests,

[serene6390]

Originally Posted by Amir

Hi,
if you mean internal walls (which need boundary conditions) it doesn't have any differences with this procedure!!
about your code, there are many problems in it, I provide a template for you; change that for your cases:

Hi, Amir,

I'm trying to build a mixed boundary condition for temperature profile, I used the same code you provided except changing the C_YI_G(c,t0,i) to C_T_G(c,t0),but there's some error come out after I initialize.

My code is:
DEFINE_PROFILE(Temperature_profile,t,i)
{
face_t f;
cell_t c;
Thread*t0;
real x[ND_ND];
real y;

begin_f_loop(f,t)
{
t0=THREAD_T0(t);
c=F_C0(f,t);
F_CENTROID(x,f,t);
y=x[1];
F_PROFILE(f,t,i)=(200-20*(1-y)+C_T_G(c,t0)[0]/0.002)/(100+9.8*(1-y);
}
end_f_loop(f,t)
}

My current physical model is very simple, just a 2D rectangular area.
Thank you so much.

PS: I don't know why but the program sometimes works and sometimes not, but even if it could run successfully, the result seems not correct.

The error looks like this:
MPI Application rank 0 exited before MPI_Finalize() with status 2
The f1 process could not started