[MayTheFlowBeWithYou]

Dear CFD-online community,

I am trying to make a UDF for transport of a UDS. I am planning to build this on the following example from the manual (basically I'm trying to add a flux based on the Nernst equation and diffusion to it):

/************************************************** ********************/
/* UDF that implements a simplified advective term in the */
/* scalar transport equation */
/************************************************** ********************/

#include "udf.h"

DEFINE_UDS_FLUX(my_uds_flux,f,t,i)
{
cell_t c0, c1 = -1;
Thread *t0, *t1 = NULL;
real NV_VEC(psi_vec), NV_VEC(A), flux = 0.0;
c0 = F_C0(f,t);
t0 = F_C0_THREAD(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*/
{
real dens;
/* Depending on its BC, density may not be set on face thread*/
if (NNULLP(THREAD_STORAGE(t,SV_DENSITY)))
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), *, dens);
flux = NV_DOT(psi_vec, A); /* flux through Face */ }
else
{
c1 = F_C1(f,t); /* Get cell on other side of face */
t1 = F_C1_THREAD(f,t);
NV_DS(psi_vec, =, C_U(c0,t0),C_V(c0,t0),C_W(c0,t0),*,C_R(c0,t0));
NV_DS(psi_vec, +=, C_U(c1,t1),C_V(c1,t1),C_W(c1,t1),*,C_R(c1,t1));
flux = NV_DOT(psi_vec, A)/2.0; /* Average flux through face */
}
/* ANSYS Fluent will multiply the returned value by phi_f (the scalar’s
value at the face) to get the ‘‘complete’’ advective term. */
return flux;
}

The thing about this example is that it uses the macro "NV_DS" which is not explained in the manual (the manual talks about ("NV_V", "NV_VV", "NV_V_VS" and "NV_VS_VS").

If I look at, for example:
NV_DS(psi_vec, =, C_U(c0,t0),C_V(c0,t0),C_W(c0,t0),*,C_R(c0,t0));
It is pretty obvious that it want to take the velocity in each direction for each adjacent cell and multiply by the density in that cell.

The question however is that, according to the manual chapter 3.4.3. "The NV Macros", the correct way to do this would be:
NV_VV(psi_vec, =, C_U(c0,t0),C_V(c0,t0),C_W(c0,t0),*,C_R(c0,t0));

So, what is the difference between NV_DS and NV_VV?

Additionally, Does this "DEFINE_UDS_FLUX(my_uds_flux,f,t,i)" loop over faces of over cells?

If it loops over cells, then what does "adjacent cell" mean? in a 3D space there would be 6 adjacent cells, does it automatically take the adjacent in the x,y,z-direction correctly for positions 0,1,2 in the vector?