[engloly]

in my heat transfer model i want to write a udf for the energy source term
S y = - density * dH\dt
where H= the total enthalpy , t= time

my udf is ;

#include "udf.h"
#define dens 800.0
DEFINE_SOURCE(heat source,cell,thread,dS,eqn)
{
real source ;
source= - C_H-G(cell,thread)*dens ;
dS [eqn] =0 ;
return source ;
}


can any one help me to , i dont know if the cell enthaly gredient is equal to the chane of enthalpy with time?

[engloly]

im waiting for ur help???

[Philipp_Sch]

No, the gradient is a spacial derivative. Besides, it is a vector. You need a time derivative, which YOU have to provide.

[engloly]

yes u are right
but how can i write the dh/dt?

[Micael]

I did this before. I don't have access to it right now, but here is the idea (I AM NOT SURE OF THE EXACT SYNTAX, I just write it roughly by memory):

1- compute/store H in a UDM with a "Adjust" UDF
2- then derive it in your Source UDF, something like that (first order in that example):
time_step = "macro for time step goes here";
dHdt = (UDM(0) - UDM_M1(0)) / time_step; // suppose you put H in UDM(0)

For what I remember, it was not that easy to implement, but the idea is there. Among all, make sure to understand exactly when "Adjust" is executed.

This line:
dS [eqn] =0 ;
can be a problem. Make sure to understand what it means, look at UDF guide. You may need to derive your source term to ever converge (depend on your problem). I always derive my source term, it is either safer or downright necessary (numerically).

Keep motivation high, this problem will be challenging if you are beginner, but it is feasible.

[ComputerGuy]

Originally Posted by engloly

yes u are right
but how can i write the dh/dt?

Perhaps something like the following:

Make sure you enable a user-defined memory location... Also, I'm assuming you want the density at the current time step, not the average density between the two timesteps (current and previous). Also, enthalpy has a spatial gradient (x/y/z), but you want the gradient w.r.t. time.

This is a crude approximation of a derivative. Ideally, you should do some higher order approximations.

Finally: there is an "if" statement in there to ensure that, at the beginning of the simulation where you have no derivative information, no source term is calculated.

--ComputerGuy

#include "udf.h"
DEFINE_SOURCE(heat_source,cell,thread,dS,eqn)
{
	real source;
	source=0.0;
	real dHdt;
	dHdt=(C_H(cell,thread)-C_UDMI(cell,thread,0))/CURRENT_TIMESTEP;
	if(CURRENT_TIME>0)
	{
		source= -dHdt*C_R(cell,thread);
	}
	C_UDMI(cell,thread,0)=C_H(cell,thread);
	dS[eqn] =0.0;
	return source;
}

[Micael]

Originally Posted by ComputerGuy

Perhaps something like the following:

Make sure you enable a user-defined memory location... Also, I'm assuming you want the density at the current time step, not the average density between the two timesteps (current and previous). Also, enthalpy has a spatial gradient (x/y/z), but you want the gradient w.r.t. time.

This is a crude approximation of a derivative. Ideally, you should do some higher order approximations.

Finally: there is an "if" statement in there to ensure that, at the beginning of the simulation where you have no derivative information, no source term is calculated.

--ComputerGuy

#include "udf.h"
DEFINE_SOURCE(heat_source,cell,thread,dS,eqn)
{
    real source;
    source=0.0;
    real dHdt;
    dHdt=(C_H(cell,thread)-C_UDMI(cell,thread,0))/CURRENT_TIMESTEP;
    if(CURRENT_TIME>0)
    {
        source= -dHdt*C_R(cell,thread);
    }
    C_UDMI(cell,thread,0)=C_H(cell,thread);
    dS[eqn] =0.0;
    return source;
}

I don't think that can work. The reason is that DEFINE_SOURCE should be called more than once per time step (many iterations to solve a given time step). Hence, this line should be remove:

 
C_UDMI(cell,thread,0)=C_H(cell,thread);

C_UDMI(cell,thread,0) should be compute only once per time step and an DEFINE_EXECUTE_AT_END (or was it an DEFINE_ADJUST? - I don't remember) can do this. I do remember that it was tricky, I could find those files this weekend.

Keep in mind that dS[eqn]=0 could lead to convergence problem, but it is ok to try it first like that.

Otherwise, ComputerGuy is right, DEFINE_SOURCE should looks similar to that.

[ComputerGuy]

Originally Posted by Micael

I don't think that can work. The reason is that DEFINE_SOURCE should be called more than once per time step (many iterations to solve a given time step). Hence, this line should be remove:

 
C_UDMI(cell,thread,0)=C_H(cell,thread);

C_UDMI(cell,thread,0) should be compute only once per time step and an ADJUST can do this. I do remember that it was tricky, I could find those files this weekend.

Hmmm, you're right. Ok... Something like this ought to record the enthalpy once per timestep.

ComputerGuy

#include "udf.h"

DEFINE_EXECUTE_AT_END(ThisRunsAtEndOfTimestep)
{

  Domain *d;
  Thread *t;
  cell_t c;
  d = Get_Domain(1);   /* mixture domain if multiphase */

 thread_loop_c(t,d)
    {
     if (FLUID_THREAD_P(t))
       {
        begin_c_loop(c,t)
          {
           C_UDMI(c,t,0)=C_H(c,t);
         }
        end_c_loop(c,t)
       }
    }
}


DEFINE_SOURCE(heat_source,cell,thread,dS,eqn)
{
    real source;
    source=0.0;
    real dHdt;
    dHdt=(C_H(cell,thread)-C_UDMI(cell,thread,0))/CURRENT_TIMESTEP;
    if(CURRENT_TIME>0)
    {
        source= -dHdt*C_R(cell,thread);
    }
    dS[eqn] =0.0;
    return source;
}

[Micael]

Yes, that's it ComputerGuy!

Only left is the special case of the very first time step and the whole thing is adressed.

[ComputerGuy]

Originally Posted by Micael

Yes, that's it ComputerGuy!

Only left is the special case of the very first time step and the whole thing is adressed.

I think, because the code runs at the end of the time step, it should be fine. I've also addressed this in the define_source macro, where I check to see that the time is > 0. If it isn't, it wouldn't be possible to obtain a derivative, and thus I set the source to zero.

engloly: Let us know if these work

ComputerGuy

[Micael]

Still one detail, the UDM should be initialized so that during the first time step there is a value for it.

[mehrandadsetan]

Dear all,

I wrote a udf for energy equation, but I think it depends on the magnitude of sigma_r.
For sigma_r equal to 3e-1, udf works well , but if you change it to 3e-3, udf doesn't work.
can anybody help me?

UDF code:

#include "udf.h"
#define PI 3.14159265
#define ei 40e-3
#define sigma_r 3e-3
#define sigma_t 0.5e-6
#define t0 0
DEFINE_SOURCE(Energy_source, c, ct, dS, eqn)
{
real source;
real pos[ND_ND];
real centre[ND_ND];
real flow_time;
real r;
NV_D(centre,=,0.8,0,0); /* Set a vector */
flow_time = CURRENT_TIME; /* Special Fluent macro */
C_CENTROID(pos,c,ct);
NV_VV(pos,=,pos,-,centre); /* Vector arithmetic */
r = NV_MAG(pos);
source = (ei/(4*pow(PI,2)*pow(sigma_r,3)*sigma_t))*exp(-0.5*pow((flow_time-t0)/sigma_t,2))*exp(-0.5*pow(r/sigma_r,2));
dS[eqn] = 0;
return source;
}