Help choosing vortex generators

[ConnClark]

Originally Posted by ChazInMT

Show me a real scientific study that shows airtabs have helped with fuel efficiency on anything.

Although hosted on their site the test was not done by airtabs http://www.airtab.com/MicrosoftWordHondaWTReportV6.pdf

to quote the inventor who worked for nasa "This application for VGs should be tested on a hatchback or van where the base area is a very large fraction of the car's frontal area."

here is a Nasa document on the successful spin off of the airtab technology http://ntrs.nasa.gov/archive/nasa/ca...2009001813.pdf

[slowmover]

I offered the link as given, not as proof. There is always the hope that the "wind tunnel test" may be offered at some point (other participants). Truck aero, wake vortexes and the rest will continue to be of interest. Takes very little improvement on a truck to make real percentage gains in comparison to cars. Truck handling, trailer behavior in winds is also a real concern for operators. This should also be quantified in some fashion if such devices are to have merit.

[euromodder]

It still amazes me that the not-so scientific testing methods under uncontrolled conditions as used by site members are accepted on face value, while tests and claims by others are discarded as being invalid for not being scientific, independent, or controlled enough ...

[MetroMPG]

Referring by any chance to the tests I did?

Because I'll defend the A-B-A Airtab tests done on the Corolla sedan and on the GM minivan as "as controlled as possible" (following the best practices outlined in http://ecomodder.com/forum/showthrea...ery-11445.html )

[euromodder]

Originally Posted by MetroMPG

Referring by any chance to the tests I did?

Not just these, but many of the A-B-A tests I've seen on here.

Because I'll defend the A-B-A Airtab tests done as "as controlled as possible"

You can defend them as whatever you wish, but they're just not controlled enough to accurately measure minor differences - even less so when testing on an already fuel efficient vehicle.

I'm not saying the testers were cheating or whatever, I'm just saying we can't sufficiently control the test conditions to get really reliable data.
There's far too many factors beyond our control out on the road.

I'm not saying none of the tests were useful, for they helped proof some of the more effective mods.

But when I see differences within "A" or "B" being more than the difference between "A" and "B", it's going to take a lot more than 3 runs to get a reliable claim that "A" and "B" really have different averages.


AirTab claim their customers get 2-4% better FE.
If I use my own FE of 5L/100km, that translates into 0.1 or 0.2 L/100km.
0.1 L/100km is the smallest difference my ScanGauge or OBC will show ...

From your testing, I recall they smoothed out the airflow.
So they are doing something.

[dcb]

Vortex generator - Wikipedia, the free encyclopedia
"Objective testing has shown that fitting after-market VGs may reduce cruise speed slightly"
The implication here is that they INCREASE drag. Cars don't have to worry about control surface effectiveness so much.

Darin is as good as it gets when it comes to eco testing methodology BTW.

[MetroMPG]

Originally Posted by euromodder

You can defend them as whatever you wish, but they're just not controlled enough to accurately measure minor differences

I agree it's true that the hardest mods to test are those with the smallest results.

I'm not at all saying my testing was/is perfect. I've done some crap tests in the past - but the Airtabs ones weren't among them.

So, I'll continue to defend...

even less so when testing on an already fuel efficient vehicle.

Depends on the units you're using on your fuel consumption display:

- Using L/100 km gives you the poorest resolution in an efficient car.
- Change it to MPG (US) for testing purposes and you immediately have far better resolution.
- Change it to MPG Imperial and you have even more.
- KM/gallon Imperial, more still.

I'm just saying we can't sufficiently control the test conditions to get really reliable data. There's far too many factors beyond our control out on the road.

That depends on the effort you're willing to put in to doing a test. I believe I was able to control for the major factors that would affect results:

- waited for a windless day (by being patient);
- performed runs (in the middle of the afternoon, because I can) with no other traffic around (no confounding aero effects);
- averaged bidirectional runs on a very straight & level stretch of road;
- cruise control was set only once for all runs (driver's right foot removed from the equation);
- all comparisons were done immediately following one another (eliminating major potential changes in the vehicle/atmospheric factors).

The result of attempting to control these factors was I think I pretty respectable (low) standard deviation in the results: .31 mpg (US) for the A set and .20 mpg (US) for the B set in the Corolla tests. Using Airtab's claims, A 2-4% improvement in fuel economy improvement would have shown up as a ~1-2 mpg (US) difference on this 49 mpg vehicle (at the speed tested), well above the statistical "noise" in this case.

From your testing, I recall they smoothed out the airflow.
So they are doing something.

I agree they did something on the Corolla. I believe they directed flow through what would otherwise (in the absense of the VG's) be a separation bubble at the mid/base of the rear window. Yet there was no measurable effect on fuel economy, probably owing to the energy required to form the vortices which redirected the flow.

Of course on-road testing sucks in general compared to lab tests. But with patience & effort it's possible to control for more than you might think and get good data on the road.

[ /end-defending-my-honour ]

[slowmover]

Personally, I'd settle for a commercial operator with plenty of experience and understanding of record-keeping to see if this (or other) was a worthwhile addition. Truck & trailer behavior against winds (all sorts, natural or man-made) alone might make this device worthwhile even though it doesn't answer the question about them, per se.

I suspect that since Wal-Mart is now aero-equipping it's trailers that this device would have shown up. Not to my knowledge, or not at this point. It's cheap enough, after all.

This problem of testing is hardly confined to mpg. On one of the RV boards is the proposal of a test of a weight-distribution hitch's "levelling bars" in one-inch increments (different bar types; different "weight" ratings, etc), and the contention is dozens of posts long well before results are posted.

We'll do the best we can (is how I see it), and submit to "peer review". What we need be mindful of is "consensus" becoming a chokehold (have seen more than one site go to #&$% over "consensus"), as, often, some very interesting voices pop up at one point or another which get discouraged or lost.

.

[MetroMPG]

Good points, slowmover.

I can think of an instance where VG's would help a 3-box sedan: as was pointed out by another member, if the car decklid is high/long enough that there is flow reattachment by its trailing edge, there may be little to gain in terms of wiping out the separation bubble via VG's

But if the deck is a little too low/short for flow to reattach after leaving the roof, and VG's could get the flow to reattach, then I think you'd have something worth testing further.

I can't think of too many cars that fit that profile though. Designers don't make aero mistakes like that anymore (complete detachment at the trailing edge of the roof on a 3-box vehicle).

[AeroModder]

Originally Posted by dcb

Vortex generator - Wikipedia, the free encyclopedia
"Objective testing has shown that fitting after-market VGs may reduce cruise speed slightly"
The implication here is that they INCREASE drag. Cars don't have to worry about control surface effectiveness so much.

Darin is as good as it gets when it comes to eco testing methodology BTW.

The problem here is that article is for VGs used on airplane wings. The VGs are placed closer to the leading edge on wings, where VGs on cars are on the trailing edge.