How to access the local residuals through UDF

In the TUI of Fluent, type: solve/set/expert and answer 'yes' to 'Save cell residuals for post-processing?'

You can now access the residuals in the post-processing.

It is also possible to access the residual values in a UDF. An example is provided below.

In this example, a first DEFINE_ON_DEMAND stores the residual values of Pressure (= Mass Imbalance) X,Y and Z velocity, and the temperature in 5 UDMs.

A second DEFINE_ON_DEMAND use a general way to access any post processing variable with C_POST_VAR. In this second macro, "temperature-residual" can be replaced by the name you would enter in the TUI if you were to use TUI commands to produce e.g. a contour plot...

#include "udf.h"
DEFINE_ON_DEMAND(report_residuals)
{
    cell_t c;
    Thread *ct;
    Domain *d=Get_Domain(ROOT_DOMAIN_ID);
    
    if(RP_Get_Boolean("save-cell-residuals?"))
    {
        Message("\nSaving Residuals in UDMs...\n");
        thread_loop_c(ct,d)
        {
            begin_c_loop(c,ct)
            {
                C_UDMI(c,ct,0) = C_STORAGE_R(c,ct, SV_P_R);
                C_UDMI(c,ct,1) = C_STORAGE_R(c, ct, SV_U_R);
                C_UDMI(c,ct,2) = C_STORAGE_R(c, ct, SV_V_R);
                C_UDMI(c,ct,3) = C_STORAGE_R(c, ct, SV_W_R);
                C_UDMI(c,ct,4) = C_STORAGE_R(c, ct, SV_T_R);
            }
            end_c_loop(c,ct)
        }
    }
    else
    {
        Message("\nResiduals are not set to be saved.\n");
    }
}
DEFINE_ON_DEMAND(alternative)
{
    Domain *domain = Get_Domain(1);
#if !RP_HOST
    Thread *t;
    cell_t c;
#endif
    Cell_Function_Values(domain, "temperature-residual");
#if !RP_HOST
    if (RP_Get_Boolean("save-cell-residuals?"))
    {
        Message0("\nSaving Residuals in UDMs...\n");
        thread_loop_c(t, domain)
        {
            begin_c_loop(c, t)
            {
                C_UDMI(c, t, 4) = C_POST_VAR(c, t);
            }
            end_c_loop(c, t)
        }
    }
    else
    {
        Message0("\nResiduals are not set to be saved.\n");
    }
#endif
}