DEFINE_DIFFUSIVITY

jim

hi Using DEFINE_DIFFUSIVITY, I want to compute the mean age of air. I see DEFINE_DIFFUSIVITY in fluent manual, but i don't understand.

exemple in flunet manual DEFINE_DIFFUSIVITY(mean_age_diff, c,t,i) { return C_R(c,t) * 2.88e-05+C_MU_EFF(c,t)/0.7;}

Why DEFINE_DIFFUSIVITY is used for mean age of air?... and I want to know the process..(ex, step1->steady solver,step->interprete udf...)

Thank's for your attention.