Learning Forum

[Silversea]

Hi,

I wrote a simple UDF to get the previous time step values in a transient model in Ansys Fluent V2022. It worked fine when I used current time macros, e.g., C_R(c,t) or previous time step macros, e.g., C_R_M1(c,t). However, when I tried to get the second previous time step macros, no matter which one, e.g., C_R_M2(c,t), C_U_M2(c,t), or C_T_M2(c,t), I always get Segmentation fault error.

Does fluent store the second previous time step values by default?

Should I do something in fluent before accessing the second previous time step (_M2) macros?

 

[Atharva Nagarkar]

Hello,

A segementation error is usually related to incorrect allocation of memory or errors in writing the UDF.

Can you please share your UDF?

Thanks!

[Silversea]

Hi, 

Thanks for the reply.

This is the code I tested:

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

/*

Calculate the average velocity at outlet and print it.

*/

 

#include "udf.h"

 

DEFINE_EXECUTE_AT_END(pre_time)

{

Domain *d;

Thread *t, *t_f;

cell_t c;

face_t f;

real V, sum = 0., sum_V = 0.;

real ave;

int zone_ID = 7; // outlet ID

 

d = Get_Domain(1);

 

t_f = Lookup_Thread(d,zone_ID);

begin_f_loop(f,t_f)

{

t = THREAD_T0(t_f); // get cell thread

c = F_C0(f,t_f); // get corresponding cell

V = C_VOLUME(c,t);

sum_V += V;

sum += V*C_U_M2(c,t);

}

end_f_loop(f,t_f) 

 

ave = PRF_GRSUM1(sum/sum_V);

 

if I_AM_NODE_ZERO_P

{

Message("ave = %g\n",ave);

}

}

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

 

It is a simple code that calculates and prints the average velocity at a boundary. I used it on both 2D and 3D models (very simple models such as 2D laminar flow in a pipe).

First, I run the model for a few time steps, e.g. time steps =10. Then I apply the UDF, and continue the simulation.

 

When I change the line "sum += V*C_U_M2(c,t);" to  "sum += V*C_U_M1(c,t);" or "sum += V*C_U(c,t);" it perfectly works without error and prints the average velocity. But with C_U_M2(c,t) (or any other _M2 macros) it encounters the segmentation error. 

[Silversea]

Update:

I tried it on older versions of Ansys, such as 18 and 19, with the same outcome.

 

I also used THREAD_STORAGE(t, SV_...))) to test whether the memory has already been allocated as follows:

if (THREAD_STORAGE(t,SV_U_M2) == NULL)
{
    Error("error THREAD_STORAGE SV_U_M2");
}

and it produced the error. So, it means that memory is not allocated for this macro.

 

A question, Am I the only one that gets this problem? Would somebody please confirm that any _M2 macros work?

 

[Selawe97]

 

Hello

I am getting the same problem also, But i read in the documentaion that data from C_T_M1 is available only if user-defined scalars are defined. and i did not understand what that mea!

 

[Rob]

It means you'll need some scalars (look in User Defined tab in the solver). They're extra scalar fields we can use to solve "stuff" as required.