Aero Experts - Radius for roofline/Kamm back curve?

[brucepick]

Originally Posted by Bill in Houston

Brucepick, it seems like the back of your wagon already has a pretty sharp transition, so maybe there is nothing to be gained. As opposed to the Metro or a Saturn wagon, which has a smooth transition that keeps the low pressure air attached longer... Know what I mean?

Yes, as built it's a pretty sharp transition. Roofline drops a bit at the very end but just a little. The sides taper inward, also just a little.

So is there some gain in having the separation occur a bit further back? Either like what's in the pic in the first post (or longer front to rear as I plan), or maybe just a horizontal fin going straight back 2-3 inches off the last bit of roof edge?

[CO ZX2]

CO ZX2 writes: If you do not already know about the trailer testing, you really should read what they have to say. Let me know and I can get it to you.

Well, Bruce here you are. Notice how the top of their extension is the same angle as the roof, sides tapered. I have another file with more pics if you need it.

Trailer Aero Testing link: SAE 2000-01-2209
http://permanent.access.gpo.gov/webs...00-01-2209.pdf

[brucepick]

Zounds. That article is an eyeful.

Their "boattail" seems to be parallel to the truck's body lines. However the panels are all "stepped" inward, that is the distance between the two sides is narrower than the truck's width, and the height of the top panel is less than the truck's roof. Almost like jwxr7's concept except due to the size and proportions they were able to use a stepped transition rather than a smooth one, and apparently it worked even with the step.

I'm going to have another look at the article, maybe at work where I can print it out on paper and share it with someone I know who may understand some of the bits that I don't get. I took a decent physics course but we didn't cover aerodynamics. I'll have another look but I don't know if I'll ever be able to say honestly that I understand what is going on there.

[jwxr7]

I don't know if you've checked out metro's kammback section so here's a link. http://www.metrompg.com/posts/boat-tail-prototype.htm
It shows the back of a semi trailer with angled boattailing.

[Bill in Houston]

Originally Posted by brucepick

Yes, as built it's a pretty sharp transition. Roofline drops a bit at the very end but just a little. The sides taper inward, also just a little.

So is there some gain in having the separation occur a bit further back? Either like what's in the pic in the first post (or longer front to rear as I plan), or maybe just a horizontal fin going straight back 2-3 inches off the last bit of roof edge?

Ya, I don't know those answers. My Element and Odyssey have a sharper edge at the top, and rounded edges on the sides. So, like you, I am wondering if there is an advantage to having a little plate that extends backwards 2 or 3 or 5 inches on either the top or the sides.

[brucepick]

Originally Posted by Bill in Houston

Ya, I don't know those answers. My Element and Odyssey have a sharper edge at the top, and rounded edges on the sides. So, like you, I am wondering if there is an advantage to having a little plate that extends backwards 2 or 3 or 5 inches on either the top or the sides.

I sure can have fun speculating. Next I'm going to fully read the metrompg pages quoted two posts up by jwxr7, and get some education.

I think the decision behind the Element and Odyssey design is that consumers can accept a "spoiler" at the rear roof edge but would think the same thing would look quite odd if used on the sides. (I"m assuming the sharp top edge is kind of a mini spioler or horizontal fin).

Also if the car has rear lights going up the D pillar they would be blocked by a vertical fin unless the fin had additional lighting inside. Extra cost to the mfg.

[trebuchet03]

Here is what I would do..... Get some tape, and some red string (for contrast). Put a whole bunch of tufts just before the kamm, on the kamm, and on the rear windscreen. Then, drive at a variety of speeds and have someone take photos from another car.

Then, do the same thing without the modification. If the strings on your rear windscreen suddenly point up, you've found a less desirable trait of your design.
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Now, from research for the HPV - a teammate found a whole lot of information on diffusers and optimal angles. We found 8 degrees off a flat plane to be the best for minimizing wake.

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Unless you do a CFD model, the best way is by guess and check (considering the low cost of a prototype). Try several iterations and see what shows the least amount of disturbance.

All of that said - I've always wanted to try taking big PVC, quartering it and seeing the results. I recall one boat tail design (for trailers) that had rounded trailing edges.

[MetroMPG]

Bruce: my 2 cents...

- a curved shape at the rear, both in profile & plan views is best if possible (look at the EV1; Prius etc.) but not required (see the truck trailer prototypes, both stepped & straight angled)

- as for what angle or radius, you're safe if you go for something that doesn't exceed those cars' shapes at the rear. The figure often thrown around is 10 degrees, but taken from what tangent is the question