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Originally Posted by SD987
My understanding of aerodynamics is that the air comes over the windshield, above the cabin and then around, hitting the deflector from behind as it were. So I'm not sure if hole-less deflectors would be more inclined to blow off, as some comments below imply.
That being said, I've never seen a windscreen element on any car, that crossed the width of the vehicle, not have "holes", including BMW (the six series using a small solid windbreak), Merc, Audi, Porsche. This leads me to believe there is a reason not to have unperforated wind deflection. I'd be curious if any of the engineer types on the board would know why that is.
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You're right, in general, as to what's happening with the airstream. The blockers keep the air that's made it's way over your head and separated from the body of the car from coming back and hitting you from behind. The passenger's are basically in a "dead air pocket". (FYI: This happens to be the same behavior that makes it more aerodynamic to keep the liftgate on a pickup truck up. Dropping it down or adding an "Airgate" hurts drag coefficient or at least doesn't help, but that's another story).
The airstream constantly changes with speed as you might imagine. The faster you go, the lower the "pocket" gets. Back behind the windscreen, the air is making big circles as it's spinning between the tonneau, the windscreen, and the free moving air above the screen.
The key here is that faster moving air has a lower pressure than slower moving air and non-moving air will be at the highest pressure. (Bernoulli's Principle) So there will be a pressure difference between the non-moving air in the cabin and the swirling/moving air behind the screen. If the screen is big enough, this pressure can turn into a fairly large force pushing the screen out of the cabin. The holes in the windscreens help to reduce the presure difference by letting some air from the cabin "bleed" into the air behind the screen and also reduce the overall area of the screen, both reducing the force on the screen.
Because the holes are small, the overall screen is still doing a good job of blocking the backflowing air. Without the holes, not only would the roll bar screens have more of a tendancy to pop out at speed, but the overall pressure in the cabin would be higher and would tend to push the middle screen off as well.
As to the ones without holes. They are either held in place better or the airflow is such that this phenomena is less pronounced.