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I wouldn't say airflow over the top grill is "good" vs. "bad" over the lower. Let me explain.
What we're seeing in the first part of that video is a map of the velocity of the particles close to the vehicle surface. The CFD model does not have open grill areas (no airflow through them); instead, those surfaces are closed in order to generate a velocity or pressure map (they're related) over an idealized solid surface.
The purple area on the lower grill illustrates the stagnation point, where the energy of the airflow is almost entirely converted to pressure (velocity near zero), while the green over the upper grill area shows higher velocity/lower pressure. You'll get more airflow through the radiator (by having a greater pressure differential in front of/behind it) by leaving the lower grill open, while blocking the upper grill won't affect radiator efficiency as much but should improve drag by giving the airflow there a surface to flow over.
Radiator efficiency can be further improved by building a sealed duct from the grill to the radiator and, if you're really ambitious, a duct behind as well--but usually there's no room to do that without moving or angling the radiator itself.
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