Turbulence - Friend or Foe? |Here's a lot of Aero info, not spread in several threads

[Snax]

Or perhaps a wing like Subaru did with the STi.

[ZugyNA]

Originally Posted by Silveredwings

I think it's a term for the bubbles you create while sitting in the bathtub.

That is flatulence. The French use wine corks for this problem.

[ZugyNA]

Originally Posted by trebuchet03

It would be great if someone had some tuft photos of a prius' rear glass at speed. The Prius hatch is closer to a streamlined shape (as is the 1L and UFE-III). So if the separation point is really far down the glass, or not until a sudden change in body shape (rear cut off), there's more benefit of keeping laminar flow and dealing with skin friction.

I dug around autospeed a little more, and found some more information http://www.autospeed.com/cms/A_3061/article.html. They specifically tested the air tab product and they did have promising results - similar, in effect to my tuft testing while behind a semi.

There are four articles there...good reading. But with their last test on the Honda Insight...why weren't the airtabs placed up above the rear glass as with the Toyota Prius right above? Separation not expected? Not a fair test...who would expect improvements on an aero tweaked shape?

They need to test something like the green Honda SUV.

[Snax]

I think it would have made sense for them to evaluate the Insight at significantly higher speeds. But as it was, the whole exercise appeared to be pointless on that car.

[trebuchet03]

Originally Posted by ZugyNA

There are four articles there...good reading. But with their last test on the Honda Insight...why weren't the airtabs placed up above the rear glass as with the Toyota Prius right above? Separation not expected? Not a fair test...who would expect improvements on an aero tweaked shape?

They need to test something like the green Honda SUV.

As far as "fairness" is concerned... They weren't comparing vehicle to vehicle. They were comparing a concept to another concept. The Prius has a much steeper angle compared to the insight (separation is not expected - as can be seen while driving in the rain). Doing the "wake fill" test on the Prius wouldn't really help if flow has already separated.

As far as the whole "wake fill" idea....

But what about the idea that vortex generators can pull extra air into the low pressure wake, increasing its pressure and so decreasing drag? As far as we?re aware, there is no independent scientific evidence for this idea (as opposed to the energizing of boundary layers, where there is 50+ years of experience on aircraft!).

[GasSavers_Erik]

OK, I am sold on the idea of using a turbulator to trip the boundary layer ahead of my hatchback so I don't get held back by suction at highway speeds.

I am going to put a thin zig zagged layer of clear silicone caulk across the roof of my civic hatchback. (I can always scrape 99% of it off later with a razor blade)

The question is, how far should I put the caulk "trip" strip ahead of the hatchback? 100mm like Mitsubishi did?

[DRW]

Originally Posted by Erik

OK, I am sold on the idea of using a turbulator to trip the boundary layer ahead of my hatchback so I don't get held back by suction at highway speeds.

I am going to put a thin zig zagged layer of clear silicone caulk across the roof of my civic hatchback. (I can always scrape 99% of it off later with a razor blade)

The question is, how far should I put the caulk "trip" strip ahead of the hatchback? 100mm like Mitsubishi did?

In Mitsu's test, the VG were placed 100mm ahead of where the airflow separated from the rear glass. Similarly, in the link to the glider modification, the zigzag tape on the wing was placed ahead of the point of separation. The first step is to find where airflow separates from the hatch. I think a good way to determine where flow separates on your car is to do some tuft testing at various speeds.

I did some tuft testing on my car. There's side view pic of my car in my profile for reference. At 60mph flow started to separate at about 2/3 of the way down the rear hatch, or about 6 inches away from the rear deck. The angle of the glass at the point of separation is about 21 degrees. The flow wasn't completely turbulent, the tufts were still pointed downstream, but they wiggled noticeably more than the tufts just upstream.

Now you've got me thinking of using clear silicone caulking to make a zigzag strip!

Edit: oh wait, I just saw your car pic in your profile. It's pretty clear where the air would separate.

[GasSavers_Erik]

DRW- Thanks for the info/help, but my hatch glass is pretty close to vertical. Maybe a 15 degree slope downward (a station wagon would be 0 degrees). So it seems like I need to put the caulk strip on the roof- right?

Its gonna be hard for me to do tuft testing with tufts on the roof unless I can find a volunteer to hang out the window and watch them at 55 mph

[DRW]

Ah! I edited my post at the same time you wrote yours!

Erik, the trailing edge of the roof of your car is similar to the Insight. They both have a gently tapered rooflines terminating with a sharp edge. This is done to give the air a definite point to separate from the body. See part one of the vortex article here www.autospeed.com/A_3058/cms/article.html
where they state:

" Cars with a two-box shape (eg hatchbacks and wagons) are always stuck with flow separation at the end of the roof, so creating a larger wake. In those cars, and also at the trailing edge of the boot in three-box cars, the separation should be clean – ie the flow shouldn’t wrap around the end of the roof or the boot lid.


On hatchbacks and wagons, roof extension spoilers achieve this clean separation, as do sedans by means of the sudden transition from horizontal to vertical at the trailing edges of the boot lid."

My interpretation of what that article is saying is that your car already has a nice aero shape at the top of the roof/hatch junction. Sure, it's a large wake, but it has clean separation. VG's might reduce the wake on your car, but it would also make the wake more turbulent.

[GasSavers_Erik]

DRW- Thanks, that was a great article. I am going to try it just to see if ther eis any change in my mileage, but it looks like a Kamm back is the only aero option to help the back end of my car