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 wind December 4, 2009 10:11

Problem of SOURCE term gradient in UDS

Dear all,
I meet problems of defining source term in UDS. I want to solve a UDS, and the scalar equation has four terms. Beside the unsteady term, the convective term, the diffusion term, and a extra term expressed as -ə(Aφ)/əxi is left, where φ is the unknown variable to be solved, A is the constant. My questions are:

1. whether the extra term -ə(Aφ)/əxi could be treated as the source term of UDS?

2. If so, how to give the source and ds[eqn] in UDF?

source= -A*(C_UDSI_G(c,t,0)[0]+C_UDSI_G(c,t,0)[1]+C_UDSI_G(c,t,0)[2]); /*is it right?*/
dS[eqn] = ?;

thanks!

 gearboy March 2, 2010 02:58

Quote:
 Originally Posted by wind (Post 238744) Dear all, I meet problems of defining source term in UDS. I want to solve a UDS, and the scalar equation has four terms. Beside the unsteady term, the convective term, the diffusion term, and a extra term expressed as -ə(Aφ)/əxi is left, where φ is the unknown variable to be solved, A is the constant. My questions are: 1. whether the extra term -ə(Aφ)/əxi could be treated as the source term of UDS? 2. If so, how to give the source and ds[eqn] in UDF? source= -A*(C_UDSI_G(c,t,0)[0]+C_UDSI_G(c,t,0)[1]+C_UDSI_G(c,t,0)[2]); /*is it right?*/ dS[eqn] = ?; thanks!
You can put this item into the convection term. The convection term then becomes:
ə[(rho*Ui+A)φ]/əxi
You should write the UDS flux "rho*Ui+A" in your udf.

 wind March 4, 2010 07:32

Quote:
 Originally Posted by gearboy (Post 248031) You can put this item into the convection term. The convection term then becomes: ə[(rho*Ui+A)φ]/əxi You should write the UDS flux "rho*Ui+A" in your udf.
Dear gearboy, thank you very much for your reply, and I will try it. Could you kindly give me your email? so that I could contact you directly. thank you!

 gearboy March 8, 2010 03:51

Quote:
 Originally Posted by wind (Post 248531) Dear gearboy, thank you very much for your reply, and I will try it. Could you kindly give me your email? so that I could contact you directly. thank you!
I will visit this forum frequently. Just show your problem on this forum.

 wind March 11, 2010 08:50

Quote:
 Originally Posted by gearboy (Post 248924) I will visit this forum frequently. Just show your problem on this forum.
Deat Gearboy,
Could you help me check whether the following UDS especially for the convective term is right? thanks!

if the equation is expressed as below:
--------------------------------------------
əφ/ət+uj əφuj/əxj- ə[mu(əφ/əxj]/əxj = -əAφ/əx3
--------------------------------------------

UDF according to Fluent help:

#include "udf.h"
static real A_constant= 0.20;

DEFINE_UDS_FLUX(Vf_uds_flux, f, t, i)
{
cell_t c0, c1 = -1;

real NV_VEC(psi_vec), NV_VEC(A), flux = 0.0;
real dens;

c0 = F_C0(f,t);
F_AREA(A, f, t);

/* If face lies at domain boundary, use face values; */
/* If face lies IN the domain, use average of adjacent cells. */

if (BOUNDARY_FACE_THREAD_P(t)) /*Most face values will be available*/
{

dens = F_R(f,t); /* Set dens to face value if available */
else
dens = C_R(c0,t0); /* else, set dens to cell value */

NV_DS(psi_vec, =, F_U(f,t), F_V(f,t), F_W(f,t)+A_constant, *, dens);

if (ND_ND==2) /*for 2D simulation*/
psi_vec[1]=psi_vec[1];
else if (ND_ND==3) /*for 3D simulation*/
psi_vec[2]=psi_vec[2];

flux = NV_DOT(psi_vec, A); /* flux through Face */
}
else
{
c1 = F_C1(f,t); /* Get cell on other side of face */

NV_DS(psi_vec, =, C_U(c0,t0),C_V(c0,t0),C_W(c0,t0)+A_constant,*,C_R( c0,t0));
NV_DS(psi_vec, +=, C_U(c1,t1),C_V(c1,t1),C_W(c1,t1)+A_constant,*,C_R( c1,t1));

if (ND_ND==2)
psi_vec[1]=psi_vec[1];
else if (ND_ND==3)
psi_vec[2]=psi_vec[2];

flux = NV_DOT(psi_vec, A)/2.0; /* Average flux through face */
}

return flux;
}

DEFINE_UDS_UNSTEADY(Vf_uns_time, c, t, i, apu, su)
{
real physical_dt, vol, rho, phi_old;
physical_dt = RP_Get_Real("physical-time-step");
vol = C_VOLUME(c,t);

rho = C_R_M1(c,t);
*apu = -air_density*vol / physical_dt; /*implicit part*/
phi_old = C_STORAGE_R(c,t,SV_UDSI_M1(i));
*su = air_density*vol*phi_old/physical_dt; /*explicit part*/

}

DEFINE_DIFFUSIVITY(Vf_diffusivity, c, t, i)
{

return C_MU_T(c,t); /*turbulent viscosity*/

}

DEFINE_SOURCE(Vf_source, c, t, dS, eqn)
{
dS[eqn] = 0;
return 0;
}

 yashganatra June 21, 2013 05:39

Hi,

I have a similar problem. The energy equation in FLUENT is in form of enthalpy h and I want to solve it in terms of temperature. So i need to define a UDS.
The equation goes as:

∂ρT/∂T+ ∇(ρuT)= ∇(k∇T)+ ∂/∂t(ρ(lfrac)∆H+ ∇(ρu∆H)

the scalar variable is temperature.

u- velocity;
The last two terms are the source terms and i can use DEFINE_SOURCE, but they are not dependent on T, so can i include them? In the above thread the source term was dependent on 'phi'

Thanks
Yash

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