Del Sol Aerodynamics

[krousdb]

So in another thread there was a discussion about the Del Sol and the relatively high drag (CD 0.378) that is caused in part by the targa design...

Originally Posted by MetroMPG

about the del sol... i wouldn't be surprised if that Cd is right.

the car's rear window design is a killer. airflow separates at the trailing edge of the roof, and likely doesn't re-attach by the end of the rear deck because it's fairly short. so you've got a relatively large trailing wake as a result.

to the wind it looks like a mini short-bed pickup truck with a tonneau cover. a "del camino"?

the "buttresses" on each side of the back window probably help to some extent (helping contain a bubble of air that the over-roof airflow can ride over without going instantly turbulent).

but if i were the betting type, i'd say the del sol is a particularly good candidate for aero mods in that area that would have a measurable impact on hwy mpg.

So I am stuck with a design that isn't optimal. But I love the look and its nice to have a removeable top and a rear window that opens.... which got me to thinking....

If I cruise with the side windors cracked open and the rear window open fully, might that decrease drag by providing an air flow through the rear, effectively bursting the bubble? Maybe the added drag caused by opening the windows would be mitigated by reduction of drag in the rear? A nice flow of air on a hot summer day?

And what about when the top is off? Leave the side windows up and the rear window down? That would provide more distance for the air flow to reattach at the rear? Both times I have had the roof off I was impressed on how quiet it was. With the side windows up and the rear window down, there was definately a flow of air coming over the front windshield and through the rear window. No turbulence inside of the cabin.

Am I on to something here?

[MetroMPG]

Originally Posted by krousdb

If I cruise with the side windors cracked open and the rear window open fully, might that decrease drag by providing an air flow through the rear, effectively bursting the bubble?

easy enough to test. do some before/after wool tuft runs.

i suspect you'll see turbulence all the way down the rear deck under closed-car driving. if you can achieve attached flow at the rear of the deck under some combination of window/roof opening, then you know you're on to something.

maybe all you need to do is put your heat/ac blower on MAX and blast the air out the back window cracked open a hair. like the renault concept car

[Matt Timion]

now that you have the supermid you can test this on the fly. If you manage to find a flat piece of land just give it a shot.

[krousdb]

Originally Posted by Matt Timion

now that you have the supermid you can test this on the fly. If you manage to find a flat piece of land just give it a shot.

Yes, thats the problem. Nowhere flat. I guess I could try a long stretch of highway, but without cruise control, I can't be sure that I applied the same pedal pressure up and down the hills. I need to think about this awhile. First I need to setal some of my wife's red yarn for the trunk lid. :-)

[krousdb]

Well I havent gotten around to taping yarn on my trunk lid and watching for turbulence with the windows closed and then with the top off and the rear windows down... But I did have some interesting results today....

Every weekend I drive to the inlaws house to check on it while they are in Florida. It is about a 25 mile round trip. The elevation is about 100ft lower at thier house than at mine.

So the trip out was 60.11 MPG, starting with the engine fairly warm, about 130F.

Before leaving for the trip back I removed the top and rolled down the rear window. Temp was 54F and the winds light and variable. While driving I put my hand out the rear window, over the trunk lid and could feel the air flow, so there is a possibility that the air flow could actually help the areodynamics by smoothing out the turbulence on the trunk lid due to the targa roof. Hopefully it offsets the drag created by having the roof off.

The trip back home was 62.7 MPG at a 100ft+ elevation change.

While this is not a scientifically valid test (really need to do two round trips, one with and one without the roof) it seems evident that driving with the roof off at 25-40MPH speeds does not significantly hurt FE.

Current tank, which also includes the trip back from the gas station...

32 miles, 59.5 MPG. :-)

W00t!

[Matt Timion]

That's amazing. I'm wondering if you have dimishing margin returns depending on the speed used. This would be an interesting thing to test at three different speeds

[krousdb]

So I finally got around to taping yarn on my trunk lid. One row at the end of the lid, one across the middle and one across the front, near the rear window. It didn't take much speed to see what is going on back there. The rear ones flail about toward the rear of the car. But the front and middle rows immediatly flip over and flail about toward the front of the car. No matter what speed, same results. Rear window open with side windows open, no change. Rear window open with roof off, no change. One hell of a lot of turbulance.

So I headed toward I-79 to take a highway test run to see what sort of mileage I can get now that I am SuperMID equipped. Temp was 60F with 8 MPH tailwind to start. Of course on the way back ot was a headwind. Terget speed 55 but I bled off speed going up hill down to 50, and let speed increase to 60 on the downhills.

Results:
- 22 miles with an 8MPH tailwind, 58.4 MPG.
- 22 miles with an 8MPH headwind, 56.5 MPG.
- 44 mile RT, 57.4 MPG.

What a pleasant surprise. I was only expecting 50MPG. The Prius is good for 65 MPG under the same conditions.

Lots of errands today also.

Current tank, 144 miles, 58.8 MPG.

[krousdb]

So I am wondering what the turbulence looks like on by trunk lid. Is it a swirling tornado with a horizontal axis is it more chaotic than that. I was thinking about a few options for smoothing out the turbulence, both which would leave my trunk unuseable.

First I could install a clear flat plastic plate that would close in the area between the side butresses, the roof and the trunk lid. This would give me some transition from the roof to the trunk lid instead of the cliff that I have now.

Or I could build a wedge shape out of clear plastic that would sit on the trunk lid, between the side butresses and extend from the roofline to the end of the trunk.

Can anyone draw me a picture of what the airflow would look like in each of the three setups, stock, flat plate and wedge?

Also, I am considering cutting vents into the rear bumper cover. There is an area just above the black lip that would make the vents very difficult to see. Any thoughts as to the benefit of this?

[SVOboy]

Also, I am considering cutting vents into the rear bumper cover. There is an area just above the black lip that would make the vents very difficult to see. Any thoughts as to the benefit of this?

I have heard this doesn't do too much, and I thinking that the better route would be to model yourself a rear diffuser, I will try to get some pictures and what not of DIY rear pans/diffusers for your car later today, or mehbe just general if there are none for del sols.

[SVOboy]

http://www.geocities.com/frontsidesk8er/delsolaero.xls

There ya go, should be hosted well and stuff, I will test download it again to make sure the link works in here.