Confused about Drag Coefficient calculation

Dear All

I read in this forum about calculation of Drag coefficient. I see a method known as Momentum exchange, given by Ladd (i think the Ladd introduced this method at the first, if i am wrong please correct me) and a algorithm here written by Khan for calculation of this parameter but as a matter of fact at the end after calculation of drag and lift force I don’t know which velocity and density I should use to calculate drag coefficient, I should add this one that I use HF-BB on walls and periodic inlet outlet and body force as external force.
by the way, I will be appreciated if another method for calculation of drag force will be recommended.

Thanks in Advance