Vortex Generators - hopefully a rational discussion.

[Joenavy85]

so much for rational discussion

[Blu3Z3rg]

I wonder if this is more effective or less effective than the wings on SUV's and mini-vans? With the large dead space behind, perhaps VG's will fill that hole somehow?

[Frank Lee]

Instead of wondering, why don't you check out what's been said.

[dcb]

Originally Posted by Joenavy85

so much for rational discussion

A big part of the problem is that aerodynamics is largely trial and error, FOR EACH AND EVERY VEHICLE, and can more easily make things worse than better when you are adding surface area like with vortex generators.

But most folks (myself included) want working solutions, so they stop reading after seeing that they worked some infinitely small amount (after large amounts of work and validation), without understanding that the configuration may or may not work on their own vehicle. Then they randomly stick some randomly shaped tabs on their vehicle and drive off into placebo land.

maybe this video will help explain the situation

http://www.youtube.com/watch?v=XVV_COOey0E

At which point homer decides to go DIY and takes a pick axe to the family car.

[UFO]

Originally Posted by dcb

You must understand that any chance of improvement will have to come with very vehicle specific VG tabs and installation instructions (and data to document the testing of the tabs on a previous particular vehicle configuration). And even then it sounds like the gains of VG on a car are on the slim side (i.e. .006 CD with a whole lot of tuning I'm sure). I do not find such a large effort for a small (if any) gain very appealing myself Maybe I'm wrong.

I think I do have an understanding of what you are saying. That minimal gain was on a very differently shaped vehicle than the one I wish to experiment on, the VW beetle. And the effort is not large when compared to a tail or kamm. The Beetle has a particularly large problem with the constantly increasing angle back, so it seems likely that keeping that airflow attached to the base of the rear glass, where it can be straightened by a Bonneville-type spoiler may have greater effect than an Evo that likely was intended to have more gains in downforce than drag reduction.

[UFO]

Originally Posted by ChazInMT

I can't believe these resurfaced. They're gimmicks in 99% of the applications. My 04 Honda Civic is in fact and absolutely ideal candidate for these things, the back window is just a little too steep to maintain attached flow. But flying pigs would have to be waging the snowball war of the century in hell before I'd ever put VG's on my car.

You seem determined to send this thread to the corral. I'd prefer you don't comment if you have nothing more rational than that to say.

[UFO]

Originally Posted by Frank Lee

For the sake of argument let's just say they work. The factory boys with supercomputers and windtunnels and teams of experts and a big budget were able to lower Cd by .006... and that's on specific, limited applications. Would they not utilize their hard-earned knowledge and put VG on all their models if it was worth it?

.006.

Doesn't that right there tell you how much this is worth obsessing over?

I think you are assuming drag reduction was the goal. It may not have been; I suspect the VGs were designed to increase downforce, and perhaps just to look cool....

[dcb]

.006 CD is background noise for even the most stringent coast down testers here. Its a virtual unicorn unless you bring in sophisticated testing equipment to the discussion (or maybe some 3d airflow modeling software and 3d data aquision software). Unless the discussion moves from wishful thinking to something more resembling science, it should get corraled.

[MetroMPG]

Originally Posted by euromodder

Looking at what gliders use - zig-zag tape and turbulator tape - all VGs I've seen here are way too big, too tall, and as a result, too draggy despite there being too few of them.

Apples & oranges.

Gliders can use low profile VG's and tapes because they are carefully designed to maintain laminar flow much further back along the vehicle/wings, so the boundary layer thickness is likely very small where they're applied.

On the other hand, flow is turbulent much earlier on typical automobiles., so at the locations where VG's have been placed by OEM's, the boundary layer thickness is in the neighbourhood of tens of millimeters thick (~30 mm at the rear roof area, according to Mitsu, if I recall).

A VG needs to extend through a significant portion of the boundary layer to work, thus their size. Mitsu tested several heights (15-25 mm) for the EVO application.

http://www.mitsubishi-motors.com/cor...004/16E_03.pdf

Though he didn't see a reduction in fuel consumption, when MetroMPG tested a few of the big AirTab VGs, they did smooth the airflow over the rear window.

The key point is: the difference they made to fuel consumption was undetectable in A-B-A testing.

The deflector on this Gremlin would also "smooth" the airflow over the rear window. And it would increase net drag.

[ChazInMT]

OK , How's this for rational.

Whereas my 04 Honda Civic Sedan is a car very likely to benefit from the use of airtabs, the VW Beetle would be the least likely to see an improvement. I believe this is due to the question of simply where on the Beetle would you put the tabs in the huge transition area, then what effect would they have on what appears to me to be a very large area where the air actually does separate from the back of the car. And if you don’t think I’ve given the Beetle aerodynamics much thought, you obviously have not researched my previous postings.

Scientifically, VG’s serve on aircraft as control surface devices, they don't help the plane fly faster or further on a tank of gas.

I question how something sticking up only 1 inch or so on a car, which is devoid of laminar/orderly flow on its surface due to the unaerodynamic forms poking up everywhere, could possibly effect the relatively huge amount of air flowing within the 24 inches or so of the body which has probably 80% of the effect on aerodynamics, and out to 20 feet for the remaining 20%. In other words, it is like saying you're going to walk up to a 24" hole at the bottom of the Hoover Dam and plug it with a wine bottle cork. To the rational among us, that makes no sense. And some of us rational people speak out when they hear such absurd things. Because that's how we are.

I hate pseudoscience with a passion, that's where someone takes a small truth and then horribly mixes in a bunch of technobabble to make it sound like it might work. Call me Prejudice if you will, I prefer Skeptic. If you think air tabs might work, than I'd guess Skeptic is a word very far down on long list you would use to describe yourself with Sucker, Gullible, and Dupe being very high on the list. Nothing personal, we all have to be something, it's just how we're wired.

Airtabs are usually touted as a huge energy saving device and the companies selling them are essentially snake oil/HHO generator salesmen, they are people who probably know what they are selling is worthless, but they look at it as “simply a way to make a living.” They think, “If you wanna be a sucker and not look into what I’m saying, than that’s your problem. If they don’t work as advertised for you, I can always blame it on misapplication on your part.” Yeah, I have a problem with people who think like that.

VG’s don't work 99% of the time. There is not a .01% of the time where VG’s are going to have a vast improvement on the fuel efficiency of a car in the real world. However, 99% of the time they “Can improve efficiency by up to 15%” in ad copy. In the rare instances when they do work, results are minute.

Hey reminds me.....
A guy walks into a bar with a newt on his shoulder. "What do you call that?” asks the bartender. "I call him Tiny, because he's my newt!"

Now that's rational, it isn't agreeable, but thats not what you asked for.