There's probably plenty of radius around the grille opening to keep the flow attached,since the whole area is in a favorable pressure gradient,courtesy of all the attacking air in front of the car.
The grille opening itself should have some radius itself,or you can have what is called a vena contracta entry loss,which can invisibly choke down the flow.
Here's an extreme example
The yellow region is nothing but turbulence and a real energy loser.Pure drag.
