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Old 05-20-2020, 02:48 PM   #11 (permalink)
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Originally Posted by Piwoslaw View Post
I've seen air deflectors on the front corners of trucks for years and have always wondered whether they really work.

The above picture shows deflectors not only on the sides, but also above the windshield. Does this help by deflecting towards the rear air that would normally be pushed to the side, thereby gaining forward momentum (sort of like a sail), or is this outweighed by the penalty of extra drag? I've also seen deflectors/scoops on the rear of vehicles, scooping air into the lowpressure zone immediately behind the truck. Would this make sense, or would it act more like a parachute?

I'll add that trucks aren't the only places I've seen side deflectors:

German-built Ty2-50 steam loco.
The forebody is in the most favorable pressure regime of the whole truck. And we know that leading edge radii constituting a very small percentage of the square-root of the frontal area is adequate to achieve fully-attached flow. Very little rounding will 'saturate' it,with any additional radii returning zero drag improvement.
I don't know what the premise for the deflectors are.They certainly are not necessary for the lowest drag.

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