UDF function diverging
 

hello all,

I am trying to implement a source term on the x and y momentum equations that equals Q (space charge) x E_i (electric field)

so I have defined two User defined scalars, one for the space charge which is composed of a unsteady function, a diffusivity function and a scalar flux function. All of which I effectively took out of the udf manual so i know there are no mistakes there. The electric field UDS only has a diffusion term (diffusion of the voltage), and a source term which equals Q/epsilon...and I have tested it on its own and it works so no mistakes there. However, Every time I run these two UDSs with the momentum field on (which I need to calculate the scalar flux term in the UDS) the mass conservation residuals diverge, and so I then get a floating point error.

I think the problem is with the gradient of the scalar term, i.e. the source term for x-momentum = -C_R(c,t)*C_UDSI_G(c,t,0)[0]*C_UDSI_G(c,t,1). There is no C_UDSI_RG macro as far as I am aware... please note that the field=-(gradient of the voltage). which is why I need to calculate a gradient of a user defined scalar...

If anyone knows if there is a better way I can implement the user defined gradient macros I would really appreciate it...

Im attaching the momentum source UDFs which I believe are the problem.

3. MOMENTUM SOURCES


#include "udf.h"

DEFINE_SOURCE(xmom_source,c,t,dS,eqn)
{
real source;
source = -C_R(c,t)*C_UDSI(c,t,1)*C_UDSI_G(c,t,0)[0];
dS[eqn] = 0;
return source;
}

DEFINE_SOURCE(ymom_source,c,t,dS,eqn)
{
real source;
source = -C_R(c,t)*C_UDSI(c,t,1)*C_UDSI_G(c,t,0)[1];
dS[eqn] = 0;
return source;
}

Thanks in advance
AK

Dear ak,
Do you have solved this question. I have meet the same problem as you have meet.
The gradient of the scalar term! How good method you have to treat it?

thank you very much!