Many people believe the round holes in the centre of canvas signs such as advertising or those big banners across the street advertising the coming town fair are to allow air to escape to reduce wind pressure.
Not so. As we said above, air passing over a sharp edge creates a vortex and large drag forces. With no centre holes the sign would bounce and flap as the air spilling over the edges created uneven vortices. By having focused, deliberate drag points by putting in holes it is the same as putting on anchor tags and ropes on the back of the fabric to stabilise the sign.
Ever wonder why a parachute has a hole in the centre? Same reason - it creates a focused drag point in the highest point by the air escaping, spilling over the hole edges and creating drag right in the centre to stabilise the chute. Parachutist have experimented with cutting holes in their parachutes to achieve a specific purpose such having the ability to track instead of just falling straight down. They get the same effect as somebody flying behind pulling strings to create drag in a certain direction.
Jet aircraft and drag car parachutes are a seemingly silly round ones full of holes, square or cross shape, how is that going to catch the air. It doesn't. The strange shape has as many edges as possible for the air to break away and spill over creating vortices and lots of drag.
Back in the early 70s I was working for Lindsay Brothers in their Sydney depot when they fitted their very first roof air foil. Remember those small ones about a metre or so across? After a few weeks trial, I asked Tom how it was performing. His exact words were, "Gives us about a quarter of a mile per gallon extra."
Sad thing is, I can't remember whether it was on a KW or one of their first Louisvilles, which were coming on line about that time.