I removed the brake discs from the front of my CRG based kart with VEN05 brakes. When I reinstalled them, I am not sure I put them on facing the right way. Should the fins point forward or lay backwards when viewed from the side? Does it even matter?
Ventilated rotors behave essentially like radial fans. Air pumps, like Steen says. If the internal vanes are straight, obviously it does not matter which way they spin. But if the vanes are angled, the ends near the center should lead the outside ends in the direction of rotation.
Something that has always bugged me is seeing aluminum wheels on cars, which have spokes shaped like they might be able to move some air. But they are almost always identical no matter which corner they are on. To me that makes it look like they are backwards on one side of the car.