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Old 01-15-2008, 12:02 AM   #7 (permalink)
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Originally Posted by Frank Lee View Post
Perforated screens are velocity sensitive; at low speeds they let in about what you'd expect- say, a 30% open screen will let in 30% of what an unobstructed vent would- and at higher speeds they "close up" to the oncoming flow and appear to the air to be more "solid".
Without experimenting, I believe this is completely correct. Fluids have a tendency to adhere to a surface as they sheer off it. As you increase your speed there will be a build up of air around the surface of each hole's entrance that is an area of localized slow or stagnate air. So as you increase velocity, more air will be blocked from entering due to this.
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