[Cd]

We have probably all seen it : a guy driving along in his pickup truck with a box fan in the bed spinning full speed.
I wonder what this does for the trucks aerodynamics.

Since the fan spins because of the fan being within the vortex of air spilling off the cab, I'm guessing the fan adds velocity to the air facing towards the front of the truck.

I would think this would make the drag even worse.

But what if that fan were spinning the other direction ?
I know it won't, but just ignore that for the moment.

If a free spinning fan blade could be placed within a duct facing into the wake of a vehicle, would it draw in more air into the wake ?
Would the drag of the fan blade cancel out any benefit from forcing more air into the wake ?

Is that just a really stupid question ? One that begins to sound like perpetual motion nonsense ?

( I should mention that I was just thinking of adding more air into a wake. I didn't mean to imply any sort of free energy )