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Old 06-19-2020, 06:08 PM   #11 (permalink)
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You have a truck like this?



If so the first thing I would do is cover side the protection bars with plastic panels to keep air from going under the box.

Like Aerohead said, Uhaul has done something similar with their box trucks

Everything listed above works. I can ask the aero / wind tunnel guys if they have tried vortex generators. My hunch is if vortex generators worked they would be on our trucks today.

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Old 06-19-2020, 06:45 PM   #12 (permalink)
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Have you seen this one?

Without the 20% lost in the gap, the other percentage are higher. Not mentioned so far are vortex strakes and high-momentum mudflap. I've never seen that one elsewhere.
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Old 06-20-2020, 11:10 AM   #13 (permalink)
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@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.
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Old 06-20-2020, 11:23 AM   #14 (permalink)
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Uhaul puts large radius corners on their box all the way around but they don't have a deflector. I believe part of that is because they have 3-4 different chassis that they fit the same box to.

Then there is the cost / benefit analysis. A deflector would increase cost to save fuel. Uhaul doesn't pay for fuel so why add cost? Most of what they have done is optimizing parts they have to have to be better not adding additional parts.
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Old 06-20-2020, 11:35 AM   #15 (permalink)
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I've also seen something like this, but don't remember where right now...

Ok, no images allowed then, ASCII is too difficult, hope the thumbnail gets through.

There is a right angled, radiused profile tucked behind the rear corners. Air flows from the back of the truck through the gap between profile and rear and joins the flow from the sides as it reaches the end of the side panel.


Thoughts on this appreciated.
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Old 06-20-2020, 12:19 PM   #16 (permalink)
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Quote:
Air flows from the back of the truck through the gap between profile and rear and joins the flow from the sides as it reaches the end of the side panel.
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].
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Old 06-20-2020, 12:27 PM   #17 (permalink)
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Well, explanations are where non-experts can start contradicting themselves, but it looks like a venturi effect. No doubt a simulation of the whole flow system would be necessary to understand it.
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Old 06-20-2020, 12:36 PM   #18 (permalink)
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I can see an argument that where air curling in at the back becomes chaotic and turbulent, giving it somewhere to continue flowing (through this gap) might reduce the turbulence somewhat.
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Old 06-20-2020, 02:03 PM   #19 (permalink)
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Quote:
...giving it somewhere to continue flowing (through this gap)...
Turbulence doesn't work like that. Your posited flow would have to flow 'uphill' as it were.

IMHO of course. Subject to testing.
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Old 06-20-2020, 02:59 PM   #20 (permalink)
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There may have been more to it and I'm wondering now if it was maybe pneumatic. I'll keep looking for now.

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