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Old 07-27-2019, 03:22 PM   #1 (permalink)
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Still trying to learn stuff.........box cavity?

Hello again,

I'm still trying to learn about aero concepts, if even in general terms for now.

Can anyone give me a quick tutorial on the idea of a box cavity on the rear of a vehicle and how and why it works? Nothing in depth mind you. Just a simple explanation.

And as usual, thank you all.

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Old 07-27-2019, 05:17 PM   #2 (permalink)
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box cavity

Quote:
Originally Posted by hat_man View Post
Hello again,

I'm still trying to learn about aero concepts, if even in general terms for now.

Can anyone give me a quick tutorial on the idea of a box cavity on the rear of a vehicle and how and why it works? Nothing in depth mind you. Just a simple explanation.

And as usual, thank you all.
As viewed from the rear,the perimeter of the box would be inset from that of the body 'slice' onto which it's attached.
When viewed from the side,the trailing edge of the box would fall where an imaginary 'template' line would intersect that point in space.
Rather than a gentle curving surface,you'd just a rearward-facing step.
The flow would separate from the body as the downslope hit 22-23 degrees from horizontal,but re-attach right at the end of the step,capturing a vortex of circulating air.
Flow just outside the cavity would skim across this pool of captured,swirling air,and undergo decelerative pressure gain,such that when it did separate from the trailing edge of the cavity,it would be at a higher pressure than before,and behind a smaller wake,effectively increasing the base pressure,while reducing pressure drag.Not as good as a boat tail,but way out ahead of nothing!
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Old 07-28-2019, 01:30 AM   #3 (permalink)
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Review this: EcoModder Forum > EcoModding > Aerodynamics > 'Fluid Tail'




The box can be beveled as above, or stepped, as below.



My favorite would be the concave truncation.


http://jalopnik.com/the-renovo-coupe...-ft-1621969757

Here's a boxed back window over a difusser:

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Old 07-28-2019, 10:33 AM   #4 (permalink)
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Quote:
Originally Posted by aerohead View Post
As viewed from the rear,the perimeter of the box would be inset from that of the body 'slice' onto which it's attached.
When viewed from the side,the trailing edge of the box would fall where an imaginary 'template' line would intersect that point in space.
Rather than a gentle curving surface,you'd just a rearward-facing step.
The flow would separate from the body as the downslope hit 22-23 degrees from horizontal,but re-attach right at the end of the step,capturing a vortex of circulating air.
I'm assuming the trailing edge would be sloped about 12 (to be in-line with the template). But viewed from the top, would the rearward taper be the same 12 or something closer to 7? I think I remember when I was attempting my aerocap for my old Ranger, something had been said about the angle from front to back being more like 7 instead of 12. Or does the template keep curving more aggressively as you move rearward. Seems like I had planned my old aerocap at 3 down at the cab out for a foot. Then another 4 (7 total) for another 2 feet, and another 5 (12 total) for the remaining 3 feet. Or does the template keep curving more aggressively beyond about 12 as it moves rearward?

I guess I'm still not seeing how 12 down angle from the template would would ever intersect with 22 to give a point for the trailing edge of the box.
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Old 07-28-2019, 10:46 AM   #5 (permalink)
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Give me a bit. My mind is remembering pictures I have seen here that might help me get a better grip on this one.

Edit: Can't seem to find the pics I'm thinking of, but now my mind is seeing some other options for my truck. I may have some more truck specific questions later If I can get my head wrapped around how it might work.
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Old 07-28-2019, 11:39 AM   #6 (permalink)
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Very interesting reading freebeard.

All the pictures seem to represent vehicles where the box starts at the roof line. My mind is thinking something more for the rear of pick-up either in the bed (no aerocap) or behind the cab. Behind the cab would be where the box would meet the roof line (partial aerocap?) but would the horizontal edge of a bed cover be like a second "roof line" and have similar angles or (as I am thinking) more closely follow the template?
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Old 07-28-2019, 01:49 PM   #7 (permalink)
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Quote:
Can't seem to find the pics I'm thinking of, but now my mind is seeing some other options for my truck.

Quote:
I guess I'm still not seeing how 12 down angle from the template would would ever intersect with 22 to give a point for the trailing edge of the box.
The whole discussion of defining [a sector of circle] by the angle confounds me. It describes the axial angle not the central angle. The axial angle goes from 0 at horizontal to 90 at the vertical. You can't just go to 12 at max camber (the widest/tallest point) and hold it, that gives a straight line. [shrug]

That said, the curves converge to a point if the height is half the width. Taller and you get the Trophenwagen, shorter and you get aerohead's Baby Template.



https://ecomodder.com/forum/showthre...6-a-32673.html
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Old 07-28-2019, 02:50 PM   #8 (permalink)
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These were some of the pictures I was thinking of.

I was wondering if I could use a flip over bed extender to make box cavity of sorts.

Maybe the top of the red area covered in coroplast also to extend the surface of the bed cover to meet up closer to the template line?

It's just a quick MS Paint representation. I think the factory bed extender is tapered like in the third picture. The green coroplast sides would probably have to follow this taper but still extend enought to make a 12" deep cavity.
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Old 07-28-2019, 03:10 PM   #9 (permalink)
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I wish I knew how to place the template over a picture and move it. If you placed the template at the rear with the high point being the top of the tailgate in the up position, would the resulting shaded area be the shape of what I'm aiming for?

Or something like this........
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Old 07-28-2019, 03:18 PM   #10 (permalink)
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Everything I know about aerodynamics I learned on Ecomodder.

To me a box cavity has always been a fast and dirty boat tail. Not perfect, probably not good looking, but effective. I've seen some graphics of pretty ones that just needed some skin to be real boat tails, but even one panel hanging out back coming close to the template is good.

I can't find the post, but this was on an early version of The Anal Probe:



The setups on the backs of 18 wheelers these days are a good example. Crappy boattails, but way better than the unmodded truck:



It's not pretty, it's not optimal. It's a box.

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