The drag of a flat plate opening will be akin to a flat plate (Cd 1.11),with massive flow separation which actually chokes off the airflow within the passageway,(known in fluid mechanics as a vena-contracta entry loss) worse than with the cuboid (far left) below
Even with a sharp-edged pipe opening we get the vena contracta loss (the yellow regions of turbulence are choking down the flow)
If the inlet is bell-mouthed (like inside the throat of a carburetor's venturi) the loss is completely eliminated,leading to the most aerodynamically-efficient inlet.
