2.2.9 DEFINE_ON_DEMAND

Description

DEFINE_ON_DEMAND is a general-purpose macro that you can use to specify a UDF that is executed "on demand'' in ANSYS FLUENT, rather than having ANSYS FLUENT call it automatically during the calculation. Your UDF will be executed immediately, after it is activated, but it is not accessible while the solver is iterating. Note that the domain pointer d is not explicitly passed as an argument to DEFINE_ON_DEMAND. Therefore, if you want to use the domain variable in your on-demand function, you will need to first retrieve it using the Get_Domain utility provided by ANSYS FLUENT (shown in the example below). See Section  3.2.6 for details on Get_Domain.

Usage

DEFINE_ON_DEMAND( name)

Argument Type	Description
symbol name	UDF name.
Function returns
void

There is only one argument to DEFINE_ON_DEMAND: name. You supply name, the name of the UDF.

Example

The following UDF, named on_demand_calc, computes and prints the minimum, maximum, and average temperatures for the current data field. It then computes a temperature function

and stores it in user-defined memory location $0$ (which is allocated as described in Section  3.2.3). After you hook the on-demand UDF (as described in Section  6.1.6), the field values for $f(T)$ will be available in drop-down lists in postprocessing dialog boxes in ANSYS FLUENT. You can select this field by choosing User Memory 0 in the User-Defined Memory... category. If you write a data file after executing the UDF, the user-defined memory field will be saved to the data file. This source code can be interpreted or compiled in ANSYS FLUENT.

/**********************************************************************
   UDF to calculate temperature field function and store in           
   user-defined memory. Also print min, max, avg temperatures.             
***********************************************************************/
#include "udf.h"

DEFINE_ON_DEMAND(on_demand_calc)
{
   Domain *d; /* declare domain pointer since it is not passed as an  
                 argument to the DEFINE macro  */
   real tavg = 0.;
   real tmax = 0.;
   real tmin = 0.;
   real temp,volume,vol_tot;
   Thread *t;
   cell_t c;
   d = Get_Domain(1);     /* Get the domain using ANSYS FLUENT utility */

   /* Loop over all cell threads in the domain */
   thread_loop_c(t,d)
     {

     /* Compute max, min, volume-averaged temperature */

     /* Loop over all cells  */
     begin_c_loop(c,t)
       {
         volume = C_VOLUME(c,t);   /* get cell volume */
         temp = C_T(c,t);          /* get cell temperature */

         if (temp < tmin || tmin == 0.) tmin = temp; 
         if (temp > tmax || tmax == 0.) tmax = temp; 

         vol_tot += volume;
         tavg += temp*volume;

       }
     end_c_loop(c,t)

     tavg /= vol_tot;

     printf("\n Tmin = %g   Tmax = %g   Tavg = %g\n",tmin,tmax,tavg);

     /* Compute temperature function and store in user-defined memory*/
     /*(location index 0)                                            */

     begin_c_loop(c,t)
       {
         temp = C_T(c,t);
         C_UDMI(c,t,0) = (temp-tmin)/(tmax-tmin);
       }
     end_c_loop(c,t)

     }
}

Get_Domain is a macro that retrieves the pointer to a domain. It is necessary to get the domain pointer using this macro since it is not explicitly passed as an argument to DEFINE_ON_DEMAND. The function, named on_demand_calc, does not take any explicit arguments. Within the function body, the variables that are to be used by the function are defined and initialized first. Following the variable declarations, a looping macro is used to loop over each cell thread in the domain. Within that loop another loop is used to loop over all the cells. Within the inner loop, the total volume and the minimum, maximum, and volume-averaged temperature are computed. These computed values are printed to the ANSYS FLUENT console. Then a second loop over each cell is used to compute the function $f(T)$ and store it in user-defined memory location $0$. Refer to Chapter  3 for a description of predefined macros such as C_T and begin_c_loop.

Hooking an On-Demand UDF to ANSYS FLUENT

After the UDF that you have defined using DEFINE_ON_DEMAND is interpreted (Chapter  4) or compiled (Chapter  5), the name of the argument that you supplied as the first DEFINE macro argument (e.g., on_demand_calc) will become visible and selectable in the Execute On Demand dialog box in ANSYS FLUENT. See Section  6.1.6 for details.