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Old 06-20-2020, 03:44 PM   #21 (permalink)
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Does your truck have a backlift?

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Old 06-20-2020, 05:40 PM   #22 (permalink)
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Look into Coanda nozzles.

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Old 06-20-2020, 05:55 PM   #23 (permalink)
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Quote:
Originally Posted by freebeard View Post
Why? What impels the air to flow outward through the gap?

I presume the profile is similar to the 'wake boards' in that illustration (I don't stand behind any of that. I'd like to see 'passive porosity' explained for instance). The wake boards appear to be the start of a boxed cavity. They could be angled or inset [citation needed].
Me too on "passive porosity".

Seems to me, the first three representations illustrated mainly fall into the category, "Can't hurt", (much?)

Anything less then a proper angled bobtail, needs to Kamn it, or go box cavity.
I don't see any magic silver bullets here.
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Old 06-20-2020, 06:35 PM   #24 (permalink)
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I recall an example of putting round holes instead of horizontal slots in cattle trailers and how it reduces suffocation of the livestock!

It's on EcoModder somewhere. I don't trust Google to find it though.

Speed holes

https://i.kinja-img.com/gawker-media...jzd2eihqoc.jpg
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Old 06-20-2020, 11:53 PM   #25 (permalink)
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@jakobnev, no, there's no lift so three of the four sides are 'clean'.

@freebeard, j-c-c, on passive porosity, maybe that's something like the porous 'silencers' on some pneumatic systems which allow air to pass through for (eventual) pressure equalisation, but not tumble, shear etc.

I think those are metal or ceramic, so not very practical for the back of a truck. No reason though why other materials shouldn't work. There seem to be some papers out there if anyone wants to dig.

@freebeard, I've also seen dimples and riblets for flow adjustment. Too many holes in your vehicle can lead to unexpected and fatal results!

Back to the o.p., I'm inclining towards vanes with v.g.s
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Old 06-21-2020, 12:44 AM   #26 (permalink)
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The example was extreme. The holed fenders were probably for racing class rules. A less extreme example would be punching holes in the back bumper of a Honda.



All beside the point of course. Rear wheel spats and a diffuser would help a lot.
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Old 06-21-2020, 01:32 PM   #27 (permalink)
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Some truck aerodynamics links:

NASA: https://spinoff.nasa.gov/Spinoff2008/t_3.html

https://www.rtsinc.com/guides/how-be...d-fuel-savings

VG Paper: https://www.sciencedirect.com/scienc...16302537#f0005

(There are a lot of links to other scientific papers in that last link)
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Old 06-22-2020, 09:20 AM   #28 (permalink)
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There's also another great report by Maragno & Visser, May 2003 with a lot of plots.

Comments then please on a vane system, 3 or 4 sides of box, say 6 in / 15 cm radius, no inset, v.g.s?

90 degree, 90 degree perforated (after first 10/15 degrees), or increase the radius and reduce the arc length?

Other combinations?
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Old 06-24-2020, 10:42 AM   #29 (permalink)
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no deflector

Quote:
Originally Posted by Engeu1 View Post
@JSH, yes that's the sort. Side skirt is in the pipeline, but priority has to be rear now because of a forthcoming non-aero change. Anyway, looks like U-Haul didn't fit a deflector on the overcab part of the box and left it square.

@freebeard, I hadn't seen the vortex traps (though not applicable) or strakes.
Wolf-Heinrich Hucho made a comment about commercial vehicles which pertains to the deflector.
At the large scale, reflected in commercial trucks, a leading edge radii equal to 5% of the square-root of the frontal area is all that is necessary to achieve flow 'SATURATION.'
No additional softening of the leading edges will result in lower drag.
If true ( highly probable considering the source), then the NASA / U-HAUL box configuration would yield drag characteristics on par with a box with deflector.
A stagnation bubble travels ahead of the box, of which the surrounding air deflects around as if it were a solid, bulbous structure. Smoke images from wind tunnels clearly illustrate this phenomena, labeled ' Prandtl surfaces of discontinuity,' after Ludwig Prandtl's aerodynamic research which 'discovered' them.
Since the front of a truck constitutes only 5.8% of the overall drag, the gaps, sides, belly, and rear become the target for drag reduction.
Leyland DAF, in 1988, had a box truck with Cd 0.325, by Besco Bodies, Northhampton, England.
In 1986, Renault had a semi-trailer,'V.I.R.A.G.E.S.', with Cd 0.254, with cab-to-van gap-filler and 32-inches ( 813 mm ) of boat-tail.
If you simply mimic'd what NASA had done by 1980, your truck could have the drag of the 2020 Porsche Taycan Turbo S.
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Old 06-24-2020, 10:44 AM   #30 (permalink)
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punching holes

Quote:
Originally Posted by freebeard View Post
The example was extreme. The holed fenders were probably for racing class rules. A less extreme example would be punching holes in the back bumper of a Honda.



All beside the point of course. Rear wheel spats and a diffuser would help a lot.
Those holes have been scientifically proven to INCREASE drag!
I'd avoid them like the plague.

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