[borhan_sd@yahoo.com]

Hi to all,I want to write an uds for mass and the scalar is concentration,my unsteady term is -dC/dt that C is concentration(=density *mass fraction)

I studied udf manual about unsteady term udf for UDS,but I didn't understand its example,Should I write my udf as that example as below?
Is this example general?
Why this udf is used everywhere and for any uds????????
Where and how the variable of Concentration is specified in this udf??

#include "udf.h"

#define rho 1000

DEFINE_UDS_UNSTEADY(uds_unsteady, cell, thread, i, apu, su)

{

real time_step, vol, rho, phi_old, phi_curr;

time_step= RP_Get_Real("physical-time-step");

vol = C_VOLUME(cell,thread);

*apu = -rho*vol/time_step; /*central coefficient part*/

phi_old = C_STORAGE_R(cell,thread,SV_UDSI_M1(i));

phi_curr = C_UDSI(cell,thread,i);

*su = rho*vol*phi_old/time_step; /*source term part*/

}
Shouldn't I change any general thing in this udf?(I don't mean details as rho or etc.)
Help me plz