Vortex generators - where's the solid data for hatchback/van benefits?
 

Info given to me by Ron Davidson at Airtabs. (Posted with permission)

• 3rd party tests showing utility of tractor/trailer "gap" VG application at MIRA (British Motor Industry Research Association) test track, by IRTE/BTAC (Institute of Road Transportation Engineers / Brewing Transport Advisory Committee), 1994.
  http://www.box.net/public/sbj6b0xsrk
  
  
• Airtabs background info on the MIRA/IRTE/BTAC report.
  http://www.box.net/public/e192gezve1

AND most relevant to my situation:

• Images showing wind tunnel results (base pressure reduction, ie. improvements to low pressure zone at the back of the car ) on a Civic hatchback model with VG's installed only on the sides. Click to zoom.

http://images6.theimagehosting.com/a...vic-one.th.gif

http://images6.theimagehosting.com/a...vic-two.th.gif

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The Australian AutoSpeed web site recently ran a 4-part series on VG's. Some of the articles are open access, some require payment:

http://www.autospeed.com/cms/search/...vortex&x=0&y=0

The writers have done quite a few excellent articles on DIY aero mods before, and they're normally pretty rational and scientific in their approach (ie. decent methodology, healthy skepticism). I've paid the $5 to access a few of their articles in the past, and haven't been disappointed.

They claimed they "tested" VGs on the rear of an Insight and a 1st generation Prius in the last instalment of the 4-part series, so I bought access to one of the articles to read the results.

Save your money: there were no "results" because they didn't properly test them!

They did show a beneficial improvement in airflow down the rear window of the Prius via photographs of tuft tests, but they didn't quantify the results re fuel consumption changes.

http://us1.webpublications.com.au/st...0/3061_5lo.jpg

http://us1.webpublications.com.au/st...0/3061_3lo.jpg

For the Insight, they didn't even attempt to do controlled back-to-back testing - they just drove their regular commute over several weeks and then compared the results there to (apparently) a single commute with the VGs attached. That methodology sucks!

And it doubly sucks because this is the application I'm most interested in: ie do VG's have the ability to reduce wake / base pressure beyond the trailing edge of a hatchback-style vehicle?

http://us1.webpublications.com.au/st...0/3061_8lo.jpg

Still, they somehow feel confident enough to conclude that:

Quote:
(emphasis added.)

I've had a box of airtabs for quite some time, but I refuse to just stick them on the car and pretend they work.

I've prepared to do a test: attached the airtabs on flexible/removable plastic sheets that I can remove in a jiffy at the road side.

I just need to make another couple of sheets to complete the sides, and double-check with the distributor that the placement is OK.

click/zoom...


http://images6.theimagehosting.com/s...-orange.th.jpg

Above: I increased the spacing between the tabs by 50% to compensate for the 45 degree angle on the rear window.

I may not be able to place an uninterrupted line of them down the side of the car due to contours that they won't stick to. But I'll add a few more at least.

http://images6.theimagehosting.com/3-4.5c5.th.jpg

Unfortunately, I've been too busy to actually DO the test yet.

VG freeway stats
 

At last a site for milage OCD!

I drove from Vancouver to San Antonio and back in a 1995 VW Eurovan camper at the local posted speed limit. Gas auto otherwise unmodified typical 16 mpg. I would remove/install these VG's at fill ups and check mileage per tank. Considering the cost (2 yogurt containers & a meter of white electrical tape) the approx. 7% mileage increase (~$50 saved - $85 if I'd had them on the whole trip) was acceptable. Definitely a TLAR installation. Would be nice to do wind tunnel/tuft/rainy day video testing to optimize location.
A similar installation on a VERY heavy U-Haul trailer showed a 2% improvement.

Damn, I was hoping to see the bit of info I've been after..... How far from the leading edge to place :p That variable will be one of the most important ones...

Hi bryan - welcome to EM!

OK, I have to say I don't know that one. What's TLAR??

I agree. And good to hear from someone who gets the importance of data + testing.

That said... back to your anecdotal info... I'd be interested to hear more details about your experience with the things on your van & the trailer - E.G. how many on vs. off comparisons you made; whether the weather, road conditions & terrain were similar from tank to tank.

There's no shortage of customers claiming great things about airtabs in the company testimonials. But you have to take that with such a huge grain of salt, it's not really worth very much.

Again, not saying they don't work.

But I would like to see some more convincing data for my application. Either that or I need more warm months in the year to gather the info myself.

randomization
 

Considering that during this 15,000 km test the combined vehicle weight was 2500 to 5500 kg, speed limits vary from 45 to 130 km/h, altitudes from sea level to 10,000 ft and a prevailing westerly wind I just alternated on/off at each tank ~500 km. This then gave me, say, 500 km on and 500 km off at the Colorado Plateau, same at 80 mph Texas freeway speeds. Once I had convincing data I just left them on for the last 3000 km or so, since by then I was driving in heavy snow & mountain passes which gets a bit too variable.
TLAR: That Looks About Right :D . These are about the size & angle of attack of aircraft VG's (400 km/h) at half spacing, installed at about the location AirTabs recommends. I didn't do the reynold's number conversion to see if the size was appropriate. I will bet double density would help.

Since then I also have tried premium gasoline which gives an 18% improvement (!) for a 10% cost increase. This is city driving and probably type-specific.

Per tank mileage variability was 9 to 25 l/100 km so the Signal to Noise Ratio here is pretty crappy. The fuel flow meter in the Saskatchewan Metro Videos would have speeded this up considerably.

I'd say if you don't have a sizeable 20-45 degree slope towards the center of the vehicle in the area after you place the air tabs, you'd be just as well off writing a check out to me as buying air tabs. But don't worry, I won't rip you off, you'll be getting the most premium magic beans sourced at great expense from exotic locations around the world.

I have some pictures, somewhere, of my buddy's rowing oars... He got them recently and have vortex generators on them (on leading edge)... I believe his immidiate comment on the first stroke was something like "These new oars are F'ing sweet" or something like that :p

The Mitsu Evo has similar fins on it.

http://www.scoobyblog.com/wp-content...006/10/011.jpg

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