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if you search three's lots of physics around the behavior of air in wheel wells. from what I've read the air in front of the wheel actually travels upwards - fast moving air traveling under the car hits the front of the wheel and flows upwards. it meets air drawn up the back of the wheel at a high pressure area at the top of the wheel just in front of the wheel axis. this is why you often see wheel well vents in this location. effect is heightened by the fact that the airspace in the wheel well is larger than that under the car so air is drawn into the well by the resultant pressure difference. without a vent in the fender the air will try to push out the sides and create additional friction.
regardless, out the sides is better than straight down. I read someplace that venting the centre rad on the gt3 upwards instead of downwards created some huge amount of downforce that was said to be a big part of the initial gt3's handling prowess.
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