CFD Online Discussion Forums - udf not working properly

i have a domain for which i need to find co-ordinates of maximum stream function at all flow time values. for this i wrote a code as follows. but it worked fine for on time piston travel of my domain but for off time (i.e., after the end of piston movement the flow continues) the udf is giving same output values for all the flow time intervals. i have been working on this for a month can any one please tell me where i was wrong. the code is as follows

Code:

#include "udf.h"

#include "metric.h"

#include "mem.h"

#include <math.h>



#define ID 12 /* Zone ID (in boundary condition panel) on which node data is to be obtained */



char *what = "stream-function"; /* field variable name for which data is to be extracted */

char *what1 = "q-criterion"; /* field variable name for which data is to be extracted */

char *what2= "vorticity-mag"; /* field variable name for which data is to be extracted */



DEFINE_ON_DEMAND(on_demand_calc_stream_fun_node)

{

Domain *d; /* declare domain pointer since it is not passed as an argument to the DEFINE macro */

FILE *fp;

real tem,tmax=0.;

real x, y,x1,y1;

real z[ND_ND];

int n;

real flow_time=CURRENT_TIME;

Thread *t;

cell_t c;

Node *v;



d = Get_Domain(1); /* Get the domain using ANSYS Fluent utility */

t=Lookup_Thread(d,ID);



Node_Function_Values(d, what);



  /* Initalize all node marks on the thread == 0 */

  begin_c_loop(c, t) 

  {

    c_node_loop(c, t, n) 

        {

      v=C_NODE(c,t,n);

      NODE_MARK(v) = 0;

    }

  }

  end_c_loop(c, t);

/* Loop over the nodes and get the data */

  begin_c_loop(c, t) 

  {

    c_node_loop(c, t, n) 

        {

      v = C_NODE(c,t,n);



      x = NODE_X(v);

      y = NODE_Y(v);

           

      if(NODE_MARK(v) == 0) /* if not already visited */

          {

                 C_CENTROID(z,c,t);

                 

               if ((z[0]<=0.17967881) && (z[1]<=0.042617384))

          {

         tem=NODE_VALUE(v);

                 if (tem > tmax) 

                 {                         

                 tmax = tem; 

                 x1=x;    

                 y1=y;                

                 }        

          }                 

         NODE_MARK(v) = 1;          

          }

    }

  }

  end_c_loop(f, t);

  fp = fopen("c_1_stream_fun_max_node.dat","a");

  fprintf(fp,"stream_fun_max_node = %g\t x= %g\t y= %g\t flow_time = %g\n",tmax,x1,y1,flow_time);

  fclose(fp);  

}