[behest]

Dear all,
I want to write a UDF for calculating the grid spacing in 3D. Actually, I want to campute delta_x,delta_y and delta_z for each cell. My study case is a 3 dimension problem around a wing. my quastion is which function should be used? How can I write this udf?

(delta_x means the length of a cell across x direction)

[pakk]

Originally Posted by behest

Dear all,
I want to write a UDF for calculating the grid spacing in 3D. Actually, I want to campute delta_x,delta_y and delta_z for each cell. My study case is a 3 dimension problem around a wing. my quastion is which function should be used? How can I write this udf?

(delta_x means the length of a cell across x direction)

I think you have an XY problem.
It is hard to imagine that the final output that you are interested in is the cell lengths. Probably you want to use this to calculate something different. (A gradient?)

You should explain what you want in the end. Not just to please me, but because the answer depends on what you want. Maybe you can avoid these calculations. Or if you really need them, what you need them for determines how you should deal with triangular meshes etc.

[behest]

Thank you for your response.
Acutally, I want to compute length scale of DES model and the formulation is given as:

L_s=sqrt(k)/(Bs*delta_max*C_des*w)
Bs=0.09 , C_des=0.61
k=turbulent kinitic energy
w=specific dissipation rate
delta_max=max(delta_x,delta_y,delta_z)

To honest, I am trying to write a UDF that computes this function.

Originally Posted by pakk

I think you have an XY problem.
It is hard to imagine that the final output that you are interested in is the cell lengths. Probably you want to use this to calculate something different. (A gradient?)

You should explain what you want in the end. Not just to please me, but because the answer depends on what you want. Maybe you can avoid these calculations. Or if you really need them, what you need them for determines how you should deal with triangular meshes etc.

[pakk]

Then I guess, from a physical point of view, what you really need is the maximum distance between two points in a cell. Since cells are convex, this is always between two nodes of that cell, so you could loop over all nodes.
Something like this:

 DEFINE_ON_DEMAND(calculate_grid_distance)
 {
  Domain *domain=Get_Domain(ROOT_DOMAIN_ID);
  Thread *c_thread;
  cell_t c;
  Node *node;
  int n;
 
  thread_loop_c(c_thread, domain) /*loops over all cell threads in domain*/
  {
    begin_c_loop(c, c_thread) /* loops over cells in a cell thread */
    {
      c_node_loop(c,t,n)
      {
        node = C_NODE(c,t,n);
        /* x position is NODE_X(node), 
           y position is NODE_Y(node), 
           z position is NODE_Z(node); 
           do some trick here to calculate distances 
           between all nodes of a cell and store the 
           maximum distance in a UDM 
           (C_UDMI(c,t,0)=maxdistance)*/
      }       
    }
    end_c_loop(c, c_thread) 
   } 
 }

This code is not finished, and therefore not tested, it is just a hint in the right direction.

[behest]

Thanks alot for the code. I have another question, how can I compute the distance? I mean that I have x,y,z coordinates of a node whereas I should have previous node coordinates too. In the cell node loop, I give just only one node coordinates. how does UDF save previous node coordinates?

Originally Posted by pakk

Then I guess, from a physical point of view, what you really need is the maximum distance between two points in a cell. Since cells are convex, this is always between two nodes of that cell, so you could loop over all nodes.
Something like this:

 DEFINE_ON_DEMAND(calculate_grid_distance)
 {
  Domain *domain=Get_Domain(ROOT_DOMAIN_ID);
  Thread *c_thread;
  cell_t c;
  Node *node;
  int n;
 
  thread_loop_c(c_thread, domain) /*loops over all cell threads in domain*/
  {
    begin_c_loop(c, c_thread) /* loops over cells in a cell thread */
    {
      c_node_loop(c,t,n)
      {
        node = C_NODE(c,t,n);
        /* x position is NODE_X(node), 
           y position is NODE_Y(node), 
           z position is NODE_Z(node); 
           do some trick here to calculate distances 
           between all nodes of a cell and store the 
           maximum distance in a UDM 
           (C_UDMI(c,t,0)=maxdistance)*/
      }       
    }
    end_c_loop(c, c_thread) 
   } 
 }

This code is not finished, and therefore not tested, it is just a hint in the right direction.

[pakk]

The UDF does not save the previous coordinates yet, this part you have to add

[behest]

yes, thank you very much, but counter of the loop is the main problem, each cell has several nodes and in a cell node loop, I want to save the the coordinates, but i do not know what is the counter (???). I am beginer in C program and I have worked with Fortran.

c_node_loop(c,t,n) {
node = C_NODE(c,t,n);
x[???]=NODE_X(node);
y[???]=NODE_Y(node);
z[???]=NODE_Z(node);
}

Originally Posted by pakk

The UDF does not save the previous coordinates yet, this part you have to add

[pakk]

Originally Posted by behest

yes, thank you very much, but counter of the loop is the main problem, each cell has several nodes and in a cell node loop, I want to save the the coordinates, but i do not know what is the counter (???). I am beginer in C program and I have worked with Fortran.

There is no counter yet, if you want one then you have to add it.

 int counter;
 float x[20],y[20],z[20]; //assuming each cell has 20 nodes or less
  
 ...
  
 counter=0;
 c_node_loop(c,t,n) {
  node = C_NODE(c,t,n);
  x[counter]=NODE_X(node);
  y[counter]=NODE_Y(node);
  z[counter]=NODE_Z(node);
  ++counter;
 }

This still does not calculate distances, you still have to add it.

[behest]

Thank you very much. I have just written the UDF as below:

/* This UDF is used to compute relative length scale in DES model*/

#include "udf.h"

DEFINE_ON_DEMAND(rls)
{
Domain *domain=Get_Domain(ROOT_DOMAIN_ID);
Thread *c_thread;
cell_t c;
face_t f;
Thread *tf;
Node *node;
int n,k,counter;
real delx,dely,delz,deltamax,maxx=0,maxy=0,maxz=0;
float x[10],y[10],z[10];

thread_loop_c(c_thread,domain) /*loops over all cell threads in domain*/
{
begin_c_loop(c,c_thread) /* loops over cells in a cell thread */
{
c_face_loop(c,c_thread,n) /* loops over faces in a cell thread */
{
f=C_FACE(c,c_thread,n);
tf = C_FACE_THREAD(c,c_thread,n);
counter=0;
f_node_loop(f,c_thread,n) /* loops over nodes in a face thread */
{
node=F_NODE(c,c_thread,n);
x[counter]=NODE_X(node);
y[counter]=NODE_Y(node);
z[counter]=NODE_Z(node);
++counter;
}
for (i=0; i<(counter-1); ++i)
{
for (k=i+1; k<counter; k++)
{
delx=fabs(fabs(x[i])-fabs(x[k]));
dely=fabs(fabs(y[i])-fabs(y[k]));
delz=fabs(fabs(z[i])-fabs(z[k]));
if (delx>maxx)
{
maxx=delx;
}
if (dely>maxy)
{
maxy=dely;
}
if (delz>maxz)
{
maxz=delz;
}
}
}
}
deltamax=0;
if (deltamax<maxx)
{
deltamax=maxx;
}
if (deltamax<maxy)
{
deltamax=maxy;
}
if (deltamax<maxz)
{
deltamax=maxz;
}
C_UDMI(c,c_thread,0)=sqrt(C_K(c,c_thread))/(0.09*0.61*deltamax*C_O(c,c_thread));
}
end_c_loop(c,c_thread)
}
}

But it is not worked and it gives an error in Line 20 (c_face_loop(c,c_thread,n)).
may I request you to make a comment about the UDF and the error?

thanks,

Originally Posted by pakk

There is no counter yet, if you want one then you have to add it.

 int counter;
 float x[20],y[20],z[20]; //assuming each cell has 20 nodes or less
  
 ...
  
 counter=0;
 c_node_loop(c,t,n) {
  node = C_NODE(c,t,n);
  x[counter]=NODE_X(node);
  y[counter]=NODE_Y(node);
  z[counter]=NODE_Z(node);
  ++counter;
 }

This still does not calculate distances, you still have to add it.

[pakk]

Hmm, I think I copied that from the Fluent manual, I can not access that now, I'm sorry...

One different thing: your calculation of distance is wrong:

delx=fabs(fabs(x[i])-fabs(x[k]));

should be

delx=fabs(x[i]-x[k]);

and similarly for y and z. Otherwise, if you are comparing for example -1 and 1, the code will think they have distance zero, while in reality it is 2.

[behest]

Many many thanks. I changed the delx,dely,delz, you are right. anyway, I recieve the error too. Please make a comment if you find any idea.
thanks again

Originally Posted by pakk

Hmm, I think I copied that from the Fluent manual, I can not access that now, I'm sorry...

One different thing: your calculation of distance is wrong:

delx=fabs(fabs(x[i])-fabs(x[k]));

should be

delx=fabs(x[i]-x[k]);

and similarly for y and z. Otherwise, if you are comparing for example -1 and 1, the code will think they have distance zero, while in reality it is 2.