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NACA did a lot of good work on ducting in the 1940s, with optimal inlet and outlet geometry.
The cooling air needs a chance to slow down and regain pressure (higher pressure air absorbs more heat) before it goes through the radiator, so the smooth inlet widens to that point. Inlet vanes help to distribute the cooling air evenly across the radiator, rather than just bunching up at the middle. Then, after it goes through the radiator and draws off the heat, the cooling air needs to be re-introduced to the airstream in a smooth, non-turbulent way, preferably as parallel to ambient flow as possible.
Ideally, a good inlet geometry takes advantage of the stagnation point at the nose for intake, coupled with a low pressure outlet along the side of the body, where the fast-traveling slipstream sucks the cooing air out.
There is a synergy here, if done right. Think: fish gill on a tuna or other fast swimmer.
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