[StefanR]

Hello everyone!

I am solving this steady state 2D problem: I have one solid, which has constant thermal conductivity. But this constant conducitivty changes in dependence of average temperature of this solid. So i wanted to write UDF, which takes average temperature of solid and set constant thermal conductivity after every iteration.

I have read forums and one was close to mine, but it doesn't work - thermal conducitivity is 0 W/mK.

Can someone help me please? I am not very good at programming in C and this is first time I am using UDF.

This is my code:

#include "udf.h"
real tavg;
DEFINE_ADJUST(average_temp)
{
Domain *d;
face_t f;
real temper = 0.0;
real A[ND_ND];
real area = 0.0;
real area_tot = 0.0;
int ID = 2; /*this is the ID of the boundary wall that I want to get the temperature from*/
Thread *t;
int zone_ID;
d = Get_Domain(1);
t = Lookup_Thread(d,ID);
tavg = 0.0;
begin_f_loop(f,t)
{
F_AREA(A,f,t);
area = NV_MAG(A);
area_tot += area;
temper = F_T(f,t); tavg += temper*area;
}
end_f_loop(f,t)
tavg /= area_tot;
printf("Tavg = %g area_tot = %g\n",tavg,area_tot);
}

DEFINE_PROPERTY(thermal_conductivity,f,t)
{
real keff;
keff=0.001*tavg;
return keff;
}

[pakk]

On my phone, so this is not complete, but you need just one UDF. Define a new face thread and rename some things. I hope to fix this code later.

#include "udf.h"

DEFINE_PROPERTY(thermal_conductivity,f,t)
{
real keff;
real tavg = 0.0;
real temper = 0.0;
real A[ND_ND];
real area = 0.0;
real area_tot = 0.0;
begin_f_loop(f,t)
{
F_AREA(A,f,t);
area = NV_MAG(A);
area_tot += area;
temper = F_T(f,t); tavg += temper*area;
}
end_f_loop(f,t)
tavg /= area_tot;
keff=0.001*tavg;
return keff;
}

[StefanR]

Thank you for quick reply.

I tried to complete the code. I added it to Fluent and it was without warrning or error. But when I try to initialize the case, the Fluent crashes with "recieved signal code SIGSEGV".

I know that there is problem with my code, but I am not sure where.

My code:

#include "udf.h"

DEFINE_PROPERTY(thermal_conductivity,f,t)
{
Domain *d;
face_t face;
real tavg = 0.0;
real temper = 0.0;
real A[ND_ND];
real area = 0.0;
real area_tot = 0.0;
real k_eff;
int ID = 2;
Thread *thread;
int zone_ID;
d = Get_Domain(1);
thread = Lookup_Thread(d,zone_ID);
begin_f_loop(face,thread)
{
F_AREA(A,face,thread);
area = NV_MAG(A);
area_tot += area;
temper = F_T(face,thread); tavg += temper*area;
}
end_f_loop(face,thread)
tavg /= area_tot;
k_eff=0.001*tavg;
return k_eff;
}

[AlexanderZ]

#include "udf.h"

real tavg;

DEFINE_PROPERTY(thermal_conductivity,f,t)
{
real k_eff;
k_eff=0.001*tavg;
return k_eff;
}

DEFINE_ADJUST(get_avg_temp,d)
{
face_t face;
real temper,A[ND_ND],area,area_tot;

Thread *thread;
int zone_ID = 2;
area = 0;
area_tot = 0;
tavg = 0;
temper = 0;
thread = Lookup_Thread(d,zone_ID);
begin_f_loop(face,thread)
{
	F_AREA(A,face,thread);
	area = NV_MAG(A);
	area_tot += area;
	temper = F_T(face,thread);
	tavg += temper*area;
}
end_f_loop(face,thread)
# if RP_NODE /* Perform node synchronized actions here. Does nothing in Serial */
	area_tot = PRF_GRSUM1(area_tot);
	tavg = PRF_GRSUM1(tavg);
# endif /* RP_NODE */
tavg /= area_tot;
Message0("************* tavg = %f *************\n",tavg);
}

[StefanR]

Thank you AlexanderZ.

I compiled it and ran calculation but there is some problem. The thermal conductivity is zero and doesn't change. So the temperature hits the limit 5000 K.

[AlexanderZ]

on each time step you should see in in console message with calculated tavg

************* tavg = SOME VALUE HERE *************

which in your case is linear with conductivity with factor 0.001

you may make small modification of line 3
was

real tavg;

to be

real tavg = 300;

which could help to avoid problems for the very first time step if there is one.

[StefanR]

I tried it, but it looks like the UDF doesn't change tavg. There is no *****tavg=something****** and thermal conductivity stays constant and doesn't depend on the average temperature of the face.

I tried to use this UDF also for transient calculation, but it is the same as I described above.

[pakk]

Did you hook average_temp?

[AlexanderZ]

if you don't have messages in console it means adjust function is not executed

as Pakk said, look for: "how to hook DEFINE_ADJUST in fluent"

[StefanR]

Thank you.

But after this, I found another problem.

1.png

I have a 2D problem, so am I right that I need cell temperature instead of face?

I tried to edit the code but Fluent crashes in the first iteration, so there is a problem with the code.

#include "udf.h"

real tavg=100;

DEFINE_PROPERTY(thermal_conductivity,c,t)
{
real k_eff;
k_eff=0.001*tavg;
return k_eff;
}

DEFINE_ADJUST(get_avg_temp,d)
{
cell_t cell;
real temper,A[ND_ND],area,area_tot;

Thread *thread;


int zone_ID = 1;
area = 0;
area_tot = 0;
tavg = 0;
temper = 0;

thread = Lookup_Thread(d,zone_ID);
begin_c_loop(cell,thread)
{
	F_AREA(A,cell,thread);
	area = NV_MAG(A);
	area_tot += area;
	temper = C_T(cell,thread);
	tavg += temper*area;
}
end_c_loop(cell,thread)
# if RP_NODE /* Perform node synchronized actions here. Does nothing in Serial */
	area_tot = PRF_GRSUM1(area_tot);
	tavg = PRF_GRSUM1(tavg);
# endif /* RP_NODE */
tavg /= area_tot;
Message0("************* tavg = %f *************\n",tavg);
}

[AlexanderZ]

you are using cell loop if the zone which you've defined through macro

int zone_ID = 1

is not the boundary
if it is boundary you are using face loop
check ID number in fluent gui

[StefanR]

Thank you both!

My problem is now solved, so here is the final code that I am using.

#include "udf.h"

real tavg=200;

DEFINE_PROPERTY(thermal_conductivity,c,t)
{
real k_eff;
k_eff=0.001*tavg;
return k_eff;
}

DEFINE_ADJUST(get_avg_temp,d)
{
cell_t cell;
real temper;
real volume = 0;
real total_volume = 0;

Thread *thread;
int zone_ID = 7;
tavg = 0;
temper = 0;
thread = Lookup_Thread(d,zone_ID);
begin_c_loop_int(cell,thread)
{
	volume = C_VOLUME(cell,thread);
	total_volume += C_VOLUME(cell,thread);
	temper = C_T(cell,thread);
	tavg += temper*volume;
}
end_c_loop_int(cell,thread)
# if RP_NODE /* Perform node synchronized actions here. Does nothing in Serial */
	total_volume = PRF_GRSUM1(total_volume);
	tavg = PRF_GRSUM1(tavg);
# endif /* RP_NODE */
tavg /= total_volume;
Message0("************* tavg = %f *************\n",tavg);



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