The question is not clear. What are the input parameters and what values are determined from Fluent?  Assuming that pressure drop is obtained from Fluent, we can determine the friction factor from the Darcy's definition.     

If the pressure drop is obtained from Fluent, we can determine the friction factor from the Darcy's definition.  Estimate the average velocity from the velocity profile.  The density of the fluid, the tube diameter and length are known.  Substituting the values in Darcy equation, the friction factor can be estimated. 

Dear Professor K. V Sharma thanks for the explanation.

Actually I am trying to add the friction factor in velocity profile using user defined function (UDF). A little explanation is as following

/* Friction Velocity */

frict_vel=sqrt(darcy_ff*pow(vmed,2)/8.); /* m/s */

begin_f_loop(f, thread)

{

F_CENTROID(x,f,thread);

r = sqrt(pow(x[1],2)+pow(x[2],2));

if(((rmax-r)*frict_vel*dens/visc)>=11.63505667)

F_PROFILE(f,thread, position)=frict_vel*(2.5*log((rmax-r)*frict_vel*dens/visc)+5.5);

else

F_PROFILE(f,thread, position)=(rmax-r)*pow(frict_vel,2)*dens/visc;

}

end_f_loop(f, thread)

}

Here "darcy_ff " is Darcy,s friction factor.  Rest is log law implementation in velocity profile.

My question is, Is this the right way to introduce the friction factor in velocity profile?

Regards.

it appears you use equations for the estimation of friction factor available in the literature.  If the determination of friction factor is a section of a larger analysis, it can be estimated in this manner.  If it is fundamental in nature to determine friction factor, It should be determined in the procedure stated earlier.

Dear Professor K. V sharma.

Thanks a lot for the valuable suggestions and guidance.

With profound regards