Aerodynamic Brick Thread: 1990 Vanagon Carat

[aerohead]

Hucho has mentioned that,so far,no wing-like structures have proven effective in ground proximity.Only box-cavities,and boat tails.

[dhehnke]

Originally Posted by aerohead

Hucho has mentioned that,so far,no wing-like structures have proven effective in ground proximity.Only box-cavities,and boat tails.

I'm not sure what you mean by this. As far as improving aerodynamics on a blunt rear end of a car, are you saying a wing-like structure doesn't help? I find this hard to believe when you can find small versions of this on a huge portion of modern SUVs and other boxy rear ended cars. I can't see them spending money on this if it didn't work.

[aerohead]

Originally Posted by dhehnke

I'm not sure what you mean by this. As far as improving aerodynamics on a blunt rear end of a car, are you saying a wing-like structure doesn't help? I find this hard to believe when you can find small versions of this on a huge portion of modern SUVs and other boxy rear ended cars. I can't see them spending money on this if it didn't work.

Many of today's SUV rear extensions merely elongate the roofline profile,allowing a little more pressure recovery before separation occurs.They also provide some shade and keep snow off some of the backlight.
As to airfoil-like structures,Wolf H. Hucho,who ran Volkswagen's climatic wind tunnel for a decade,and who continues to publish textbooks on road vehicle aerodynamics,has reported that airfoil-like structures which DID reduce drag on aircraft,or projectiles,failed to reduce drag when placed on road vehicles.Not even NASA, will attempt to use wing sections on a road vehicle.
Again,the only structures which have proven drag reducing capabilities,are either box cavities, boat-tails,or blown/suctioned slots.
And since,statistically,a road vehicle will be driving in 7-mph wind spectra most of the time,you cannot rely on the drag characteristics of a fixed angle-of-attack airfoil,operating in a dynamic flow environment.
A look at Abbott and Von Doenhoff's book on airfoil sections will show clearly reveal the evidence.
Hucho is adamant about only simple,separation-free elongation of the aft-body being effective towards streamlining.

[freebeard]

^^
He's right, you know.

If you want to know more go [citation needed] on Bertone BAT, VW Golf GTI 660 W-12, Ford GT40, or many recent Ferrari.

If the opening slot was horizontal, not vertical, drag wouldn't be added. You'd do better to remove the rear side windows and the back window and add internal ducts. Black expanded metal mesh would make it 'unobtrusive'.

[dhehnke]

Believe me, I agree with most of what you are saying.

First of all though, I'd like to take a picture (and I'll work on this) of the type I'm talking about. I've only seen it on fairly new vehicles. It follows a similar idea to the shape that I'm giving a try.

As far as Abbot, yes I agree that airfoils don't act very similar with a 10% angle of attack difference from a reference. I'd guess though that something this close to the top of a car will not often see that much range.

It is a horizontal "slot", if that is what you are talking about.

Another word about Abbot, and also about NASA and NACA data. Most of that data (and most of which is from the '60s or earlier) was simply obtained by many many many hours of testing and by trial and error.

I simply couldn't find proof of anyone building a structure like this one on the back of a square van. So talk all you want, I'm trying it out. It very likely won't do alot of good, maybe it will need some refining, maybe it will do some good. It cost me very little to prototype, and I'm here to post results on what happens.

[westygo]

I think we are all thankful that you are trying this out. I like your build, the rc plane builder in me appreciates the strong and clean design. Also like that you've sparked a discussion I haven't heard before; We are all going to learn something here!
I have a '98 Odyssey that I am contemplating the Kamm back design for. Your results will help point me in the right direction, so thank you!

[westygo]

Oh, and I just noticed that, no matter the results from this test, you could easily extend the sides down to the bumper, creating a combined Kamm back and airfoil diffuser thingy! It could all be attached to the hatch. Might want to get stronger struts to handle the weight. That would be combining "known to work" technology with experimental. Who knows, it might jus be hugely effective when built this way. Nobody has tried it!

[freebeard]

It is a horizontal "slot", if that is what you are talking about.

My thinking is the the height above the back of the van will be pretty critical, sizing this gap. Right now I have it about 1.5" I think (will check later) above the back, which has it still sitting lower than the high point on the top of the van, so no additional frontal area. It's not too hard to adjust this gap though.

One of these things is not like the other. Being below maximum camber doesn't mean much, the air will follow down to the back edge, the encounter an 1 1/2" slot that leads to an internal duct with interference drag.

So talk all you want, I'm trying it out.

Maybe someday you will have done some fraction of the work aerohead has done. Which informs his comments.

[dhehnke]

Originally Posted by westygo

Oh, and I just noticed that, no matter the results from this test, you could easily extend the sides down to the bumper, creating a combined Kamm back and airfoil diffuser thingy! It could all be attached to the hatch. Might want to get stronger struts to handle the weight. That would be combining "known to work" technology with experimental. Who knows, it might jus be hugely effective when built this way. Nobody has tried it!

Yes, that was my thought as well. I can also lower the structure down until it meets the roofline if I want, and then I essentially have a nice shape for a proven design.

[dhehnke]

Originally Posted by freebeard

One of these things is not like the other. Being below maximum camber doesn't mean much, the air will follow down to the back edge, the encounter an 1 1/2" slot that leads to an internal duct with interference drag.



Maybe someday you will have done some fraction of the work aerohead has done. Which informs his comments.

Yes, and I have thought about all this as well. And I still think it's worth trying. As we all know, fluid flow is not very intuitive. The point of this experiment was to combine some tried and true concepts with a concept that some PHD's developed that reduced drag in computer simulations. As we also know, cfd does not always translate to real life. Thus the "trying it" part!