Box Cavity for a pick-up truck

[aerohead]

Originally Posted by kach22i

This is totally a KISS method, might try it on my own truck. Is it crazy or what?

Modified a random image off the Internet, should work on any pick-up truck.

Cant corrugated plastic in 10-degrees from top and both sides, with tailgate down.

Automobile 2 - Odds And Ends Photos by kach22i | Photobucket


Might have to put a cargo net back there, could be trouble otherwise.

Inspiration was this:
Automotive Aerodynamics: Drag Area – Size Matters

I think that it will definitely show at the gas pump.
As long as the trailing edges are round about where Mair's boat-tail profile would be,the air will separate at the tailgate area,then swirl into a captured-vortex;stuck against the tail,with the outer air streaming by.
It's that outer air which will have a chance to decelerate to a higher pressure before it does break away,which will raise the base pressure back there.

[euromodder]

Originally Posted by Frank Lee

I'm not seeing how that will do anything.

Well, I fail to see how a rather small/short box cavity will work on regular cars - yet it does ...

[Frank Lee]

I did explain, right? I don't think it'll interact with the free stream air and I think to be effective it should. I'm not seeing how it works on the Audi either, and when I load the pdf the stupid pics land on top of the cd charts so I can't read them. How effective is the box by itself?

[freebeard]

Frank -- here is my understanding of box cavities, FWIW.

They come in two types, stepped and canted. The proposal combines the two. Compare the Vanagon example. The 10° angle supposes the air will follow the contour ala Coanda. The stepped box should have its free edge right where the canted version's is.

The open box works (maybe) because it reduces the cross-section of the wake, or because the air whips around and pushes on the back of the body. Witness the half-tonneau on a pickup. Why does that work?

The interesting experiment would be to test full- half- and no-tonneau cover versions. The half-tonneau is optimized by a wall at the front for a fourth variation.

In any case limiting it to above the lowered tailgate leaves 33% of potential gain on the table.

[Frank Lee]

That's the key part, having that free edge end up where it intersects flow. I'm not seeing that in these dinky boxes.

[California98Civic]

I posted pics of my "box cavity" prototype in my modding thread: http://ecomodder.com/forum/attachmen...p;d=1373843754

[JeffD]

Had to drive to Atlanta last weekend ......
I had moved the half bed cover aft 4" to try out the box cavity. What I neglected to do was relocate the hook and eyes to make sure it was secured at the rear. This had not been a problem as the cover is wood and stays in place very well and doesn't even rattle. But...I pulled over on the shoulder of the interstate to call 911 for a car that ran into the ditch and a large semi/car carrier whipped by very fast and close. BANG, the half bed cover flipped up and over (rotated about the hinges) and hit the the cab (ok, it covers half the bed +4 inches). Just about scared me to death. I guess that box cavity isn't so good if wind is from the rear! Anyhow, it cut thru the Naugahyde covering in 4 places (2 in front and 2 in back), so I think I'm gonna move it back into it's original position. Hope to put a sloping extension on next weekend and try again (with the back well secured). Based on this thread, not sure the cavity idea will survive as a long term mod though.

[kach22i]

Originally Posted by Frank Lee

That's the key part, having that free edge end up where it intersects flow. I'm not seeing that in these dinky boxes.

Maybe the air steps down as it does with stepped louvers?

equationsinmotion - Equations In Motion

Originally Posted by JeffD

........a large semi/car carrier whipped by very fast and close. BANG, the half bed cover flipped up and over (rotated about the hinges) and hit the the cab (ok, it covers half the bed +4 inches).

That sucks, but at least you are alright, could have been worse. It's very dangerous to pull over on the side of a highway.

[aerohead]

Originally Posted by Frank Lee

I did explain, right? I don't think it'll interact with the free stream air and I think to be effective it should. I'm not seeing how it works on the Audi either, and when I load the pdf the stupid pics land on top of the cd charts so I can't read them. How effective is the box by itself?

Hucho and Emmelmann researched these at VW around 1978.In his chapter on commercial vehicles he says they're good for a 5% drag reduction.And that's as a simple extension,straight back.
Around 1991,Continuum Dynamics won an SBA DARPA research grant for a staggered box cavity which,as you say,had its free end intersecting along an imaginary line where Mair might have had his boat tail contour.
The air immediately separates at the beginning of the cavity but does re-attach at the back,capturing a vortex on top,both sides,and bottom.
As with the tonneau cover on a pickup,the outer flow skips over the stationary vortices and as it exits the cavity,its flow direction is now cambered inwards,vectored into the wake,running slower and at a higher static pressure,which increases the base pressure,lowering the pressure drag.
This 'staggered' configuration showed a 10% drag reduction on an 18-wheeler inside NASA's Ame's Research Center wind tunnel at Palo Alto,California.
Magazines like Pop Sci carried photos of the rig and also a colorized pressure histogram of the wake flow.
They're 'quick and dirty' although now,probably eclipsed by the 'Trailer Tail' like Mesilla Valley Trucking and others are running.
In order for the cavity to function it must be airtight.

[Sven7]

Originally Posted by aerohead

This 'staggered' configuration showed a 10% drag reduction on an 18-wheeler inside NASA's Ame's Research Center wind tunnel at Palo Alto,California.
Magazines like Pop Sci carried photos of the rig and also a colorized pressure histogram of the wake flow.
They're 'quick and dirty' although now,probably eclipsed by the 'Trailer Tail' like Mesilla Valley Trucking and others are running.
In order for the cavity to function it must be airtight.

Might be related:
Semi Boat Tail & Box Cavity Patent

(Includes several illustrations)