Project: ECOfamily Civic and its Aero Hitch Box / Boat Tail

[christofoo]

Originally Posted by California98Civic

Thanks for the vids. I'm also involved in some cardboard aided design in my build thread. I have no special training. I just hang around this forum. And it looks to me like there is a larger than stock recirculation zone on the glass in clip #1 and that that zone grows very significantly in clips 2 and 3 with the kamm. I would think these all indicate down stream turbulence, but then the flow seems attached at the hitch box. Still couldn't that attached flow be just the air getting forced around the box, with turbulent wake above it and downstream of the hitch box?

Can you bring the hitch box closer to the rear bumper and close the gap, especially at the top? I bet their is a sort of bow wave at the front of the hitch box caused by the air rushing down from the roof and in from the sides down into that gap at the tail for the trunk.

EDIT: It occurs to me that if all you end up with is a cargo box that adds no big drag but does not reduce it, either, then you are still ahead of the roof mounted units.]

I actually think if the Aero Hitch Box fails to improve drag that I come out behind, since I already had that configuration with cheaper commercial components (rack and bag). But let's not get pessimistic.

My thoughts at this point:

1. The excess "stock" turbulence certainly is a mystery.
2. Tuft testing works well if the shape you're tufting is supposed to be an airfoil, i.e. where the body surface coincides with the relevant flow fields. I think tufts are intrinsically hard to interpret here because the shape I've got has gaps and so there are important flow fields that we can't see with tufts. A smoke wand would be much better.
3. I was dismayed to find the contrast of the tufts on top of the Kamm made them invisible in all of my youtube posts. Here's a still from the raw data. The only time you can see the tufts really well is when the car passes under a tree.
   What I wanted to know is whether the flow over the Kamm is smooth; I'm still not sure. It looks like parts of it are smooth, but I think that one weird tuft is real (not stuck on something).
4. The turbulence under the Kamm seems to be fed from the side of the car. I think once again we have higher pressure on the side. I'm still scratching my head about this, but it could simply be that the shape is 'astigmatic,' meaning the roofline curves before the side, as opposed to being rotationally symmetric. I also wonder if it has something to do with my grille block.
5. One theory is that the Kamm needs to have a lower angle. I might even have to raise the Aero Box lid to make the car's target profile more rotationally symmetric. My competing theory is that it just needs to be a full Kammback, from the roofline to the bumper. And of course for that to work properly it needs rounded edges.
6. No evidence of baffling having created an improvement. No evidence of a vertical air flow being a factor in any configuration.
7. No evidence of the side detachment garnish creating an improvement. Maybe I need to start the form further forward on the car and make the trailing edge flat like the box side instead of rounded like the car side.
8. The rear wheel skirt does help against the turbulence found at the side of the rear bumper, but doesn't affect the side of the box. I've always wondered whether the slick wheel cover and skirt are redundant, now I have my answer.

So there's more work to do.

[freebeard]

May I suggest flourescent-colored yarn?

...and a few on inverted golf tees or miniature (homemade) Eiffel Towers so you can see what's happening a few inches above the surface?

[Xist]

I do not suppose that you could actually generate smoke?

[California98Civic]

Maybe the simplest way around this now is to take advantage of the ease of dismounting the hitch box and try coast down testing, ABA. Find a big hill or a long straightaway and see how far it will coast from still (if on a hill) and from a specific speed if on a long flat straight. Measure the difference in speed attained or maintained (if a hill) and measure distance rolled (if a straight away).

[NachtRitter]

Sounds like we need a road trip to christofoo's place so we can all help him out!

[freebeard]

Originally Posted by California98Civic

...see how far it will coast from still (if on a hill)...

Aero effects start at about 20mph. When I get to testing, I have a hill nearby picked out and I intend to pass the crest at ~45mph and see if I can go from losing to gaining speed on the downhill side.

[christofoo]

Extended the Kammback (rear windshield cover) to the rear bumper, also I added a baffle to stop flow from the side, (also I found a less bumpy road).



So this configuration is:

• Full Kammback
• Rear wheel skirt
• Baffles between the car and the box, both horizontal and vertical.
• Aero Hitch Box
• EDIT: also I put the passenger side mirror back on, since I'm blocking 90% of the rear-view with the cardboard

Results:

I think that looks a lot better. The full Kammback always seemed like the solution that just had to work.

Video, cruising speed ranges a little from 30-40 MPH:

http://www.youtube.com/watch?v=rkPs_LXDC80

Well, I mean it still isn't perfect. I still need to find a form that fixes the turbulence on the side of the Aero Hitch Box, if possible.

But I think my next task is Kammback fab. (And duct tape gum removal... any pointers?)

EDIT: I forgot to mention; while I was scouting the road for this test this morning, I saw the sweetest full aero shell on a recumbent bike (velomobile). I wish I'd had time to snap a picture, but it was headed the other way. I think it was going at least 30 MPH on a very slight uphill. I wished I could have stopped the guy and asked if he made it himself, and if it was for a competition or not. I was so jealous.

[California98Civic]

Googone for the duct tape goo. I used it on my 1998 Civic paint job... no problems. In this vid, things look better. But I do see two things worth thinking about, in my mind. Looks like one tuft on the hitch box is being sucked down into the gap between the car and the box. Also looks like a few tufts at the end of the trunk lid are as well. Am I right? Also, gonna try a coast down test before building the Kamm? Seems prudent to invest that time before investing the time in fabrication.

But it's all really impressive! I'm hopefully going to get to do some testing myself today. I won't have video, but I hope to have interesting results and a pic of the rear box cavity I am testing.

Cheers!

[christofoo]

Originally Posted by Xist

I do not suppose that you could actually generate smoke?

Well, it's been on my mind... I'm not sure how practical or workable this is though.

My pet idea is salvage an ultra-sonicator from a humidifier, apply it to fluorescent chalk instead of water, get a powerful blower and duct it into a 1" diameter vacuum cleaner hose, or something like that. Strap it to the roof, run the strap around through window cracks so it can be manipulated from the cab. But the hose would alter the air flow. Routing along the base of the front windshield would be the best compromise for on-road test. Hmm. Maybe I could use a 12V air compressor and make it with smaller air hose instead of the blower and vacuum cleaner hose.

Better yet, if I could wait for a day with a 30 MPH steady wind and minimal gusts I could find a place to park the car, preferably a wide-open lot, and use nature's wind-tunnel, and put the 'smoke' generator on a wand. Also need a weather vane, and preferably an anemometer.

I can't see it coming together for me though. Which is a shame, the amount of duct-tape and cardboard optimization that could be performed in the space of an hour in a parking lot could be phenomenal.

Originally Posted by California98Civic

Googone for the duct tape goo. I used it on my 1998 Civic paint job... no problems. In this vid, things look better. But I do see two things worth thinking about, in my mind. Looks like one tuft on the hitch box is being sucked down into the gap between the car and the box. Also looks like a few tufts at the end of the trunk lid are as well. Am I right? Also, gonna try a coast down test before building the Kamm? Seems prudent to invest that time before investing the time in fabrication.

But it's all really impressive! I'm hopefully going to get to do some testing myself today. I won't have video, but I hope to have interesting results and a pic of the rear box cavity I am testing.

Cheers!

Coast-down is in the works. I'd have to work out logistics but I might try a cardboard Kamm A-B-A. TBD. I don't think we're talking about a ton of fab investment for the Kamm though, nowhere near as much for the AHB itself.

Back to the tufting at hand, I agree there is some air getting pulled down in between the box and the car. The bottom and right side were baffled, but the left side was not. Oops?

Seeing the air flowing into the gap might be an indicator that the full Kammback, in quenching most of the rear-windshield turbulence, was successful in increasing the base pressure across the whole area. Definitely something I'll be paying attention to moving forward.

The solution I'm gravitating toward is a vinyl gap filler around the bottom and sides, probably velcro attachment behind the taillights, the rear-belly-pan (when I get to that), and around the front of the Aero Hitch Box. Pending tufting... certainly that should annihilate all the flow through the gap, if I'm lucky the full side gap filler might also clean up the flow on the side of the AHB.

Final thought; my food tastes better today. Really.

[christofoo]

Am I seeing a little detachment on the aero box lid? Probably the transition curvature is too sharp, as Darin (MetroMPG) already pointed out. Also arguably an artifact of the mismatch between the trailing edge of the Kamm and the leading edge of the Aero Box (lack of a compound lid shape).