[annan]

Hello everyone,

I want to compute the distance between cells in order to get dx and dy, so I thought I could use :

C_CENTROID(x0,c0,t0);
C_CENTROID(x1,c1,t1);

real dx = x1[0]-x0[0];
real dy = x1[1]-x0[1];

But it doesn't give me the right values I should get. I also tried using connectivity macros like INTERIOR_FACE_GEOMETRY(f,t,A,ds,es,A_by_es,dr0,dr1 ) to get ds, but still don't get the right values.

Thank you in advance for your help.

[AlexanderZ]

how do you define c0,t0,c1,t1 here?

Best regards

[annan]

Thank you AlexanderZ for your reply.

I define them like the following :

t0 = THREAD_T0(t);
c0 = F_C0(f,t);

if (THREAD_TYPE(t)==THREAD_F_INTERIOR)
{
t1 = THREAD_T1(t);
c1 = F_C1(f,t);

... <the code I wrote in the first message is in here>

}

The overall is within a face loop.

[AlexanderZ]

in chapter 2.7.3.3. Example from UDF manual you can find something that can be used for your case

/**********************************************************************/
/* 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;
}

Here t0 and t1 are defined in other way

t0 = F_C0_THREAD(f,t);
t1 = F_C1_THREAD(f,t);

You may try to use this approach

Unfortunatelly, I can't test it

Best regards

[annan]

Thank’s for your answer. I’ve tried to implement your suggestion, but it didn’t change anything to the obtained values...

Best regards

[AlexanderZ]

try this code

#include "udf.h"

DEFINE_ON_DEMAND(calc_distance)
{
	Thread *t,*t0, *t1 = NULL;
	cell_t c0, c1 = -1;
	face_t f;
	Domain *domain;
	real x0[ND_ND];
	real x1[ND_ND];
	int n = 0;
	real dx = 0.0, dy = 0.0;
	domain = Get_Domain(1);
	Message("Start on demand calc_distance\n");
	n = 0;
	thread_loop_f(t,domain)
	{
		begin_f_loop (f,t)
		{
			c0 = F_C0(f,t);
			t0 = F_C0_THREAD(f,t);
			if (BOUNDARY_FACE_THREAD_P(t))
				{}
			else
				{
				n+=1;
				t1 = F_C1_THREAD(f,t);
				c1 = F_C1(f,t);
				C_CENTROID(x0,c0,t0);
				C_CENTROID(x1,c1,t1);
				dx = x1[0]-x0[0];
				dy = x1[1]-x0[1];
				Message("dx = %f, dy = %f, number of pairs = %d\n",dx,dy,n);
				}
		}
		end_f_loop (f,t)
	}
	Message("End on demand calc_distance\n");
}

Best regards

[annan]

Thank you very much for taking the time to write the code
I just tried to run it, I get the right dx. However the dy is not right ... i get some few points right but the other values are equal to zero (everywhere)
My grid is quad, uniform in the x direction (dx=constant) and variable in the y direction ( dy increases gradually). It’s an easy case normally