UDF for energy source term
 

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?

im waiting for ur help???

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

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

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.

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

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) 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.

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

ComputerGuy

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

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

Energy source term does not work!
 

Dear all,

I wrote a udf for energy equation, but I think it depends on the magnitude of sigma_r.:confused::confused:
For sigma_r equal to 3e-1, udf works well :cool:, but if you change it to 3e-3, udf doesn't work. :eek:
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;
}