Air deflectors on trucks/lorries

[aerohead]

Originally Posted by Piwoslaw

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.

[freebeard]

I hadn't noticed the A-pillar deflectors. The one across the windshield is [apparently] translucent. It could be a sun shade for the driver.

I think the locomotive deflectors are done right, although I suspect it was for mitigating the black smoke plume. They will capture more air stagnating on the front and pressurize it.

[JulianEdgar]

Caught up with air guide vanes on a Mercedes.




Inlet....



...versus outlet.



Note how duct converges.

Edit adition: these relative openings (three fingers entrance versus two fingers exit ) are what I settled on with my Edgarwit external air curtains on my Gen 1 Insight. I did a fair bit of throttle-stop testing before settling on those figures.

[j-c-c]

I don't see any comment on the trucks vertical turning vanes cross section uniformity?

[JulianEdgar]

Originally Posted by j-c-c

I don't see any comment on the trucks vertical turning vanes cross section uniformity?

There has been discussion on other threads about whether such ducts should be converging, diverging or of constant internal cross-section.

[j-c-c]

Yes, I am aware of that, but you (?) shared that the truck pictured shared the approx similar converging you used on your test vehicle, but your test vehicle had a varying cross section air foil, and a turning vane in many cases I believe does not have an inherent need for a typical airfoil wing crossection, as it by design has to add drag for little gain, when used symmetrically opposed vertically on the front of a vehicle.

So my question still stands, were the vertical turning vanes on the front corners pictured recently in this thread on the front corners of the semi truck, of a uniform cross section?

[JulianEdgar]

Originally Posted by j-c-c

Yes, I am aware of that, but you (?) shared that the truck pictured shared the approx similar converging you used on your test vehicle, but your test vehicle had a varying cross section air foil, and a turning vane in many cases I believe does not have an inherent need for a typical airfoil wing crossection, as it by design has to add drag for little gain, when used symmetrically opposed vertically on the front of a vehicle.

So my question still stands, were the vertical turning vanes on the front corners pictured recently in this thread on the front corners of the semi truck, of a uniform cross section?

Yes

[JulianEdgar]

Originally Posted by j-c-c

... and a turning vane in many cases I believe does not have an inherent need for a typical airfoil wing crossection, as it by design has to add drag for little gain, when used symmetrically opposed vertically on the front of a vehicle.

I think you are underestimating the low drag of an aerofoil. A good example of equivalent drag of a circular rod and streamlined shape:



Even when producing lots of lift, the GOE222 aerofoil has drag that is tiny (around 1/70th of the lift force).

I haven't done comparative testing of the aerofoil Edgarwits against curved flat plates but I'd be surprised if drag was high.

(I did do comparative testing of an aerofoil section against a curved flat plate for A-pillar guide vanes, and the curved flat plates didn't work. The aerofoils did.)

But as I always say - I don't have a monopoly on testing. I'd love to see others try some external guide vanes (of whatever design) and report on their drag results.

[freebeard]

I suspect that the leading and trailing edges are significant, but thickness mid-chord isn't.

[Piotrsko]

Gotta stop using those fat ugly gottengen style windmill blade profiles and at least use something like a Selig 737 tailfeather airfoil which are less sensitive to angle of attack and have a lower cd.