[a.lynchy]

Hi Kartn
The problem with the previous code that I posted was in getting the value for the variable “flow” from the DEFINE_EXECUTE_AT_END section and using it in the DEFINE_PROFILE. The way it was set as a global variable was okay but what was happening was it was been set to zero after the DEFINE_EXECTUE_AT_END loop so the DEFINE_PROFILE function was seeing a value of flow=0. Sorry if what I am saying is hard to follow, but I have attached the working code below and maybe from comparing the two you will be able to understand.
The only real different is in introducing the flow_tot variable as the global variable instead of using the flow variable.

#include"udf.h"
real flow_tot;
real p ;

DEFINE_EXECUTE_AT_END(funny)
{
Domain *d;

real flow;
cell_t c;
Thread *t;
face_t f;
d = Get_Domain(1);
t= Lookup_Thread(d, 12);

begin_f_loop(f,t)
{
flow+=F_FLUX(f,t);
}
end_f_loop(f,t)
p = C_P(c,t);
printf("static pressure = %g\n", p);
printf("MASS Flow Rate: %g\n",flow);
flow_tot =flow;
flow=0.;
}

DEFINE_PROFILE(pressure_out,t,i)
{
real set_point = 1;
real pressure;
face_t f;




printf("MASS Flow Rate: %g\n",flow_tot);
if (flow_tot < set_point)

pressure = p - 1;

elseif (flow_tot > set_point)

pressure = p + 1;

else
pressure = p;



begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = pressure;
}
end_f_loop(f,t)
printf("Adjusted pressure = %g\n", pressure);
}

Hope this helps!

[mdzubairmanipal@gmail.com]

I am trying to figure out how to write a udf for applying a transient mass flow rate at the inlet. In my case study the mass flow rate varies with time from 0 to 40L/min and then comes down to zero in one cycle that ranges from 0 to 4 sec.

If anybody have a ready made udf, i wud be grateful if u could plz post it in this link or pass on through my email id.

I admit i am have a bad programming knowledge but since it is bit urgent, i have to seek ur opinion and help without having to take the pain to learn it myself.

Plz do help

[aleisia]

Originally Posted by a.lynchy

Hi Kartn
The problem with the previous code that I posted was in getting the value for the variable “flow” from the DEFINE_EXECUTE_AT_END section and using it in the DEFINE_PROFILE. The way it was set as a global variable was okay but what was happening was it was been set to zero after the DEFINE_EXECTUE_AT_END loop so the DEFINE_PROFILE function was seeing a value of flow=0. Sorry if what I am saying is hard to follow, but I have attached the working code below and maybe from comparing the two you will be able to understand.
The only real different is in introducing the flow_tot variable as the global variable instead of using the flow variable.

#include"udf.h"
real flow_tot;
real p ;

DEFINE_EXECUTE_AT_END(funny)
{
Domain *d;

real flow;
cell_t c;
Thread *t;
face_t f;
d = Get_Domain(1);
t= Lookup_Thread(d, 12);

begin_f_loop(f,t)
{
flow+=F_FLUX(f,t);
}
end_f_loop(f,t)
p = C_P(c,t);
printf("static pressure = %g\n", p);
printf("MASS Flow Rate: %g\n",flow);
flow_tot =flow;
flow=0.;
}

DEFINE_PROFILE(pressure_out,t,i)
{
real set_point = 1;
real pressure;
face_t f;




printf("MASS Flow Rate: %g\n",flow_tot);
if (flow_tot < set_point)

pressure = p - 1;

elseif (flow_tot > set_point)

pressure = p + 1;

else
pressure = p;



begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = pressure;
}
end_f_loop(f,t)
printf("Adjusted pressure = %g\n", pressure);
}

Hope this helps!

I think about it again, and found this in your code:
begin_f_loop(f,t)
{
flow+=F_FLUX(f,t);
}
end_f_loop(f,t)

Here the F_FLUX(f,t) is a vector, I believe, so what if you want to get the mass flow rate, instead of the flux, can I write:
begin_f_loop(f,t)
{
flow+=F_YI(f,t,i)*F_FLUX(f,t)*F_AREA(A,f,t);
}
end_f_loop(f,t)

or

real A[ND_ND], flux(ND_ND)
begin_f_loop(f,t)
{
flux=F_FLUX(f,t);
F_AREA(A,f,t);
mf=F_YI(f,t,i);
flow+=mf*NV_DOT(flux, A);
}
end_f_loop(f,t)

As I understand, F_YI(f,t,i), F_FLUX(f,t) and F_AREA(A,f,t) just get one patch of the surface 'f' corresponding to the thread 't', so the area 'A' is the area of that patch, that's why we need the begin_f_loop and end_f_loop, am I getting this right?

thanks!

[kartn]

Originally Posted by aleisia

I think about it again, and found this in your code:
begin_f_loop(f,t)
{
flow+=F_FLUX(f,t);
}
end_f_loop(f,t)

Here the F_FLUX(f,t) is a vector, I believe, so what if you want to get the mass flow rate, instead of the flux, can I write:
begin_f_loop(f,t)
{
flow+=F_YI(f,t,i)*F_FLUX(f,t)*F_AREA(A,f,t);
}
end_f_loop(f,t)

or

real A[ND_ND], flux(ND_ND)
begin_f_loop(f,t)
{
flux=F_FLUX(f,t);
F_AREA(A,f,t);
mf=F_YI(f,t,i);
flow+=mf*NV_DOT(flux, A);
}
end_f_loop(f,t)

As I understand, F_YI(f,t,i), F_FLUX(f,t) and F_AREA(A,f,t) just get one patch of the surface 'f' corresponding to the thread 't', so the area 'A' is the area of that patch, that's why we need the begin_f_loop and end_f_loop, am I getting this right?

thanks!

Hi Aleisia,

F_FLUX(f,t) by itself outputs the mass flow rate entering or leaving a face in a cell. It is a scalar quantity and it outputs the result of the dot product of the mass flux vector and area normal vector.
So all u have to do in order to calculate the mass flow rate of a particular species is find the product of total flux of the mass flux vector and the mass fraction.

begin_f_loop(f,t)
{
flow+=F_FLUX(f,t)*F_YI(f,t);
}
end_f_loop(f,t)

[aleisia]

Thanks a million Kartn!

So, if I have some fluid entering into a surface, F_FLUX should be a negative scalar, right?

I remembered I've seen it somewhere, the result we see from the GUI values are actually the opposite of the F_FLUX , because they have different definitions of the 'normal' direction.
What I mean, from GUI, the value should be positive, because the fluid is entering into the surface, so it should be positive, but according to the definition of F_FLUX, it should be negative. Am I getting this right?

Thanks!

Originally Posted by kartn

Hi Aleisia,

F_FLUX(f,t) by itself outputs the mass flow rate entering or leaving a face in a cell. It is a scalar quantity and it outputs the result of the dot product of the mass flux vector and area normal vector.
So all u have to do in order to calculate the mass flow rate of a particular species is find the product of total flux of the mass flux vector and the mass fraction.

begin_f_loop(f,t)
{
flow+=F_FLUX(f,t)*F_YI(f,t);
}
end_f_loop(f,t)

[elcino]

Good morning everybody,

I was reading your post and I think I've the same problem as all of you. I'm analyzing journal bearings. The problems seams easy but i can't have the convergence if the "pressure-inlet" vs "pressure-outlet" are imposed. If I try to impose "mass flow inlet" vs "pressure-outlet" i can reach a good convergence. The problem is: I don't know the mass flow!!!! My idea is: impose a mass flow, calculate the total pressure (known data) and then recompute the mass flow at any iteration.

I've to write an UDF like yours:

1. Calculate the total average total pressure on the inlet and with an algorithm calculate the next mass flow (this is easy like a bisection algorithm will work sure, remember i had an aimed total pressure).

2. Input the new mass flow.

So the first UDF must be an DEFINE_EXECUTE_AT_END, to calculate for example "mass_flow", and the next will be an DEFINE_PROFILE.

My question is: how to calculate the average total pressure on a face (of course Ptot=Pstat+0.5*rho*v^2) but i don't know how to implement it as an average on the boundary.

when u do this:

begin_f_loop(f,t)
{
flow+=F_FLUX(f,t);
}
end_f_loop(f,t)

means that fluent is going to give u "flow" as the summatory of all the face fluxes of the boundary with the same ID?

thanks for your help!

[mm.abdollahzadeh]

Originally Posted by a.lynchy

Hi Kartn
The problem with the previous code that I posted was in getting the value for the variable “flow” from the DEFINE_EXECUTE_AT_END section and using it in the DEFINE_PROFILE. The way it was set as a global variable was okay but what was happening was it was been set to zero after the DEFINE_EXECTUE_AT_END loop so the DEFINE_PROFILE function was seeing a value of flow=0. Sorry if what I am saying is hard to follow, but I have attached the working code below and maybe from comparing the two you will be able to understand.
The only real different is in introducing the flow_tot variable as the global variable instead of using the flow variable.

#include"udf.h"
real flow_tot;
real p ;

DEFINE_EXECUTE_AT_END(funny)
{
Domain *d;

real flow;
cell_t c;
Thread *t;
face_t f;
d = Get_Domain(1);
t= Lookup_Thread(d, 12);

begin_f_loop(f,t)
{
flow+=F_FLUX(f,t);
}
end_f_loop(f,t)
p = C_P(c,t);
printf("static pressure = %g\n", p);
printf("MASS Flow Rate: %g\n",flow);
flow_tot =flow;
flow=0.;
}

DEFINE_PROFILE(pressure_out,t,i)
{
real set_point = 1;
real pressure;
face_t f;




printf("MASS Flow Rate: %g\n",flow_tot);
if (flow_tot < set_point)

pressure = p - 1;

elseif (flow_tot > set_point)

pressure = p + 1;

else
pressure = p;



begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = pressure;
}
end_f_loop(f,t)
printf("Adjusted pressure = %g\n", pressure);
}

Hope this helps!

Dear Adrian

Thanks For sharing your work. Could you please help me.
I want to calaculate the mass flow rate on a surface diffrent that the outlet boundary and then i want to modify the pressure at outlet according to that mass flow.
I dont have that surface as a boundary seprately. I just creat that surface as an isosurface in Fluent and I was calculataing the mass flow in GUI.

I appriciate your help.

Best
Mehdi

[asal]

Originally Posted by a.lynchy

Hi Kartn
The problem with the previous code that I posted was in getting the value for the variable “flow” from the DEFINE_EXECUTE_AT_END section and using it in the DEFINE_PROFILE. The way it was set as a global variable was okay but what was happening was it was been set to zero after the DEFINE_EXECTUE_AT_END loop so the DEFINE_PROFILE function was seeing a value of flow=0. Sorry if what I am saying is hard to follow, but I have attached the working code below and maybe from comparing the two you will be able to understand.
The only real different is in introducing the flow_tot variable as the global variable instead of using the flow variable.

#include"udf.h"
real flow_tot;
real p ;

DEFINE_EXECUTE_AT_END(funny)
{
Domain *d;

real flow;
cell_t c;
Thread *t;
face_t f;
d = Get_Domain(1);
t= Lookup_Thread(d, 12);

begin_f_loop(f,t)
{
flow+=F_FLUX(f,t);
}
end_f_loop(f,t)
p = C_P(c,t);
printf("static pressure = %g\n", p);
printf("MASS Flow Rate: %g\n",flow);
flow_tot =flow;
flow=0.;
}

DEFINE_PROFILE(pressure_out,t,i)
{
real set_point = 1;
real pressure;
face_t f;




printf("MASS Flow Rate: %g\n",flow_tot);
if (flow_tot < set_point)

pressure = p - 1;

elseif (flow_tot > set_point)

pressure = p + 1;

else
pressure = p;



begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = pressure;
}
end_f_loop(f,t)
printf("Adjusted pressure = %g\n", pressure);
}

Hope this helps!

I tried to use this UDF initially and modify according to my case. Prior to any modification, I got the following error after the first iteration on "DEFINE_EXECUTE_AT_END".

==============================================================================

Node 0: Process 13808: Received signal SIGSEGV.

==============================================================================

==============================================================================

Node 1: Process 17288: Received signal SIGSEGV.

==============================================================================

==============================================================================

Node 3: Process 17844: Received signal SIGSEGV.

==============================================================================

==============================================================================

Node 999999: Process 16332: Received signal SIGSEGV.

==============================================================================

==============================================================================

Node 5: Process 12176: Received signal SIGSEGV.

==============================================================================

==============================================================================

Node 6: Process 17444: Received signal SIGSEGV.

==============================================================================
MPI Application rank 0 exited before MPI_Finalize() with status 2
 The fl process could not be started.

How can I fix this error?!!
Thanks.
PS: I interpret the UDF.