Dear Liu,
Sorry for my late reply. Yes, when you are using the CGP method, you are creating particles that are l times larger in diameter than the original particles but behave in a similar manner as the original size ones.
I am sorry, but I did not have time to go deeply in the code to check on the drag implementation. There is no discussion on the scaling laws for drag in CGP DEM, so I assume the scaling law is correct and is l^3 for the body forces (drag and gravity).
In any case I have just done a quick search in the code and I believe the drag is calculated using the original particle diameter. I see just before calling to the drag laws something like this:
IF(CGDEM) DPM = 2.0d0*DES_CGP_RPR(NP)
and then the drag laws are called using DPM as the diameter. Then, somewhere in the code:
DES_CGP_RPR(PIP) = DES_RADIUS(PIP)/DES_CGP_STW(PIP)**(1.0d0/3.0d0)
which means that, just before calling the drag laws, the diameter is unscaled, drag is calculated, and then upscaled again by l^3 when multiplying by the volume. At least that is what I have just deduced. Please correct me if you have reached a different conclusion.
Cheers!
Eduardo