Tested: vortex generators (AirTabs) on a Toyota Corolla

[SlideWRX]

The Mitsu already had flow re-attaching to the decklid; it was trying to get cleaner air around the wing. I'll bet the corolla also has airflow re-attaching at the decklid, so the airtabs weren't helping much. The decklid shape/size is determining the primary drag effect.

[MetroMPG]

Valid points, Slide.

Yes, tuft testing showed the Corolla had attachment at the end of the decklid.

[The Rooster]

I'm still not clear if VG's cause drag because of the frontal area, or the drag is from the vorticies themselves.

[MetroMPG]

Drag is the product of both frontal area and Cd. A VG with half the projected area, but the same Cd as another would have half the total drag. (Ignoring the downstream effect of the vortex, if any.)

[b939625]

I read a paper awhile back that reminded me of a few concepts and would love to run them by the group. Bare with me because I don't remember the majority of this paper, but I did highlight a few interesting things when I originally went through it.

idea: Co-rotating or Counter Rotating VG's, and Sub Boundary VG's (advantages)
idea: The use of VG's under the vehicle.



"The experiments claim that the vortex generator should be placed three (3) to ten (10) heights upstream of the separation line.......Vortex Generators submerged in the boundary layer give the best compromise between effectiveness in reducing separation and device-drag......" pg 14.

"... also reported that counter-rotating VG array can be as efficient as co-rotating arrays in preventing separation when the blade spacing is greater than three times their height." pg 15

"Compared to a single vortex case the circulation from each vortex is about 20% stronger... probably due to the impermeable constraint imposed as vortices act as mirror images of each other..." pg 15

"....show that small vortex generators are just as effective in preventing separations as the normal sized devices... low-profile VG's should be applied when the detachment point is relatively fixed and the VG's can be positions close the the separated region.... the maximum vorticity generated increases as the angle of attack increases for the small VG, but decreases with angle of attack for the large VG due to stall." pg 17 and 18

Logdberg, Ola. "Vortex Generators and Turbulent Boundary Layer Separation Control". Technical Reports from Royal Institute of Technology, KTH Mechanics; Stockholm, Sweden. October 2006.

[BillyBob251]

https://www.truckinginfo.com/1011841...o-small-to-see

I realize this is a very old thread, but just wondering what you guys think of the above article, where they say that sometimes trucking companies only see gains from various mods after 1,000 miles, or 6 months of testing. How do EcoModders in general address the issue of gains which are too small to show up in A-B-A testing but which nevertheless are gains?

An example I can think of is installing a shaft seal in my car which turns more easily on the torque converter shaft than two other seals for the same application, which I also happened to have on hand. The seal that turns with less friction will get better MPG, even though it is a microscopic gain and can never even be measured.

So is there a way to mathematically calculate the positive impact of the Airtabs as opposed to their negative effects? I guess I just don't believe their positive impact is *exactly offset* by the total drag created by the Airtab's vortices... Rather, I think there is probably a *net impact* going on which is only measurable over a large amounts of distance, perhaps?

[Piotrsko]

Well I was sitting this morning and started thinking: what EXACTLY are the tufts testing? The somewhat obvious answer is that they demonstrate the kinda boundary layers but do not show what is going on much above that. For me, then, this does not tell me anything useful in the big drag picture because the layers arent like a cake layer they are kind of fluid in transition.

[aerohead]

My opinion is, that the Airtab ought to be used as Gary Wheeler originally intended. On a notchback car.
Stuck at the back of a semitrailer, with nothing behind to affect flow reattachment, it violates the logic of the function.
Even if the wake is 'reduced' in perimeter dimension, it says nothing about the pressure at the separation line, which could be lower than originally, leading to lower base pressure, higher pressure drag, higher overall drag.
I talked with Nish, of The iconic Nish Family, 'Royal Purple Streamliner at one of Mike Cook's 'Shootouts on the Salt', at Bonneville. They have Airtabs at the rear of the streamliner. Nish's explanation for their inclusion had to do with drag-chute deployment success / efficiency, not drag. They've shredded parachutes immediately upon deployment, initiating up to a 12-mile coast, to and from 'Floating Mountain.'