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

[tylerc]

Originally Posted by justpassntime

Do race cars or speedboats have them? Nascar, F1, NHRA, to name a few.

Don't recall ever seeing any or an announcer making comment on them, and you know how NASCAR and NHRA guys like to talk.

If they did any good they would be used in race applications.

here is a pretty serious AI race car that uses the vortex generators
ww.mustang50magazine.com/featuredvehicles/m5lp_0804_sn96_mustang_race_car/index.htm
Also has some other fun little aereo tricks, vents on the fenders etc... lost of weight savings. Cool car and dear to the heart since one of my cars is a v8 mustang

[bilgladstone]

Link doesn't work for me.

[khafra]

Originally Posted by bilgladstone

Link doesn't work for me.

Try SN-95 Mustang AIX Race Car - Agent 47 Racer - 5.0 Mustang & Super Fords Magazine

Also, if anybody's got coastdown or cruise-control experimental data from VGs at the trailing edge of a hatchback, I'm interested.

[UFO]

Originally Posted by khafra

Try SN-95 Mustang AIX Race Car - Agent 47 Racer - 5.0 Mustang & Super Fords Magazine

Also, if anybody's got coastdown or cruise-control experimental data from VGs at the trailing edge of a hatchback, I'm interested.

Interesting car, but the aerodynamic work is not designed for drag reduction. The rear wing, the VGs, the fender scoops and the wider hood are all adding drag. With the kind of power that beast is making, aero stability is king in simply keeping it on the road, and drag reduction is most decidedly not a priority. There appears to be no underbody or diffuser work at all, and the VGs probably do nothing as the wing is way up out of the rear end turbulence.

What sort of racing does the car do? Top speed stuff like NASCAR? I'm curious.

[srt-cuatro]

Sorry to revive the thread, but does anyone know where to place VGs on a hatchback?

[MetroMPG]

As you have read in this thread, nobody has yet posted any solid, reliable evidence that VG's work at the end of a hatchback bodystyle to improve fuel economy.

I tested airtab style VG's , A-B-A, on both a sedan and minivan and could not detect any change in fuel consumption.

http://ecomodder.com/forum/showthrea...fect-6381.html

http://ecomodder.com/forum/showthrea...olla-2390.html



I wouldn't bother with them, except in very specific circumstances (eg: sedans with steep rear glass and short deck lids).

[srt-cuatro]

Thanks.

[seifrob]

I have found one more A-B-A test (AirTabs on Honda Fit): http://s89686473.onlinehome.us/airtabs-test.html

Personally, I am thinking of installing them on my Dacia Logan MCV (long station with vertical back end).


(image taken from Renault Car Rentals Renault Auto Rental)

I wonder if they will help as the rear back is somehow "tikered" - there are curvatures with sharp endings etc.

[MetroMPG]

Thanks seifrob!

I'll describe/quote the main parts of the test you linked (http://s89686473.onlinehome.us/airtabs-test.html ), for posterity (these things have a bad habit of disappearing from the web):

Date posted: 2007

Vehicle: Honda Fit

VG's used: AirTabs

Methodology: coastdown test, 100-80 km/h, on a nearly level road, by dipping the clutch at 100 km/h, manually starting a timer, then stopping the timer @ 80 km/h. Baseline (no VG's); then 8 VG's across the end of the roof; then baseline repeated; then 8 VG's across the roof + 4 VG's on each side.

Results: (time in hundredths of seconds)

A1 (baseline, no VG's): 1550, 1647, 1607
B1 (8 VG's taped to roof): 1691, 1678, 1700, 1706
A2 (baseline, no airtabs): 1628, 1623, 1649
B2 (8 VG's taped to roof, plus 4 taped on each side): 1597, 1662, 1722, 1671

16.17 seconds: average time of the "A" runs coasting from 100 - 80 km/h.
16.78 seconds: average time of the "B" runs coasting from 100 - 80 km/h.
00.61 seconds: difference

Experimenter's conclusion & comments:

Overall, the average time (in hundredths of a second) for both sets of test runs with Airtabs was 1678, 3.77% greater than the average times for the unmodified car of 1617. Seemingly not a huge difference, but it is very much statistically significant, according to this web t-test calculator, which claims that the probability of this event, assuming the null hypothesis, is 0.011.

The first run in set B2 was anomalously slow. I suspect this could be the result of the car cooling down (most importantly, I guess, the tires) while I took my time taping the Airtabs back on. Also, it started raining very lightly during the last run, again putting the Airtabs at a disadvantage.

[MetroMPG]

I'd also like to point out some reasons we should be skeptical of the results in the above test. (Note I'm not saying "dismiss" his results, but why we should still question them.)

Also: the guy who posted the test deserves a lot of credit for thoroughly describing what he did.

1) The tester did not "remove himself" from the experiment, exposing the results to the potential effects of experimenter bias.

Experimenter bias risk could have been avoided by recording speed digitally, which also would have been more accurate, since the sampling rate of the recording could have then been used to time the runs (see below).

The potential for bias exists at 4 points: in the "reading/interpreting" of the analog speedometer; depressing the clutch & starting the timer; reading the speedometer at the end of the run; stopping the timer.

2) The accuracy of the results is questionable due to reaction time of the experimenter on the timer. Average human reaction time on a similar "clicking" task is about 0.22 seconds. (eg. Human Benchmark - Reaction Time Stats ).

Consider the average difference of the runs was 0.61 seconds, and you can see why this is important.

The experimenter himself points out the difficulty of being accurate:

starting the timer exactly simultaneously with pushing the clutch pedal took me a while to learn.

3) The test is complicated by placing additional VG's on the side of the car for the second "B" set of runs. It's not really an A-B-A-B test, it's an A-B-A-C test.

If you compare the numbers as 2 separate tests, you may get different results. EG: why did doubling the number of VG's apparently decrease coasting time between the two B runs. (The experimenter suggests it's due to light rain at the end of the second test.)

4) The experiment was also run in the opposite direction, and the results turned out to be not statistically significant:

on the way back from each run, I measured the time in the other direction. In this other direction, whatever difference the airtabs made was not statistically significant, though [the raw data] still showed a slight gain.