Vortex Generators and snake oil

[JimIsbell]

I am about to try Vortex Generators on the Insight. The plan was to put some home made Vortex Generators on the car before my 400 mile trip to houston. But as life offten does, it threw me a curve and the wife decided she wanted to go in the Jaguar instead of the Insight. Oh well, I will try them on the Jag firtst and if they work out I will put them on the insight. A report follows:

Yesterday I decided to try Vortex Generators on the XJ40 to see if
they really worked. BUT....being a skeptic, I sure wasn't going to pay
$2.75 a piece for 40 or so of them to properly do the job untill I was convinced they would work.

The theory is quite sound and I have looked at several NASA papers and
several other writings on the subject. The original idea was that by
using Vortex Generators you could increase the lift of an aircraft
wing. The way it worked was by placing them at the leading edge of
the wing they effectively increase the thickness of the wing by laying
down horizontal vortices's (small tornadoes) on the top of the wing.
This increased the lift. However, increased lift comes with increased
drag, so that isnt very useful for a car. In the beginning people
were putting them on the front edges of the car and this just detached
the air from the car and made its drag higher.

Then they found that by placing them on the trailing edge of the car
body and anyplace where the surface departed from "streamlined" by as
much as 12 degrees, it would cause the vortex to fill in the area
behind the vehicle so that a vacuum didn't develop there. A vacuum
behind your car effectively pulls back on the car as drag.

It sounds good, but does it work? The sellers of Vortex Generators
make some very amazing claims for them and most of the claims are ridiculous snake
oil. Many kids put them on the cars because they "look cool". But one firm makes a product called AirTabs. But as I
previously noted, even though $2.75 is pretty cheap, 40 x $2.75 gets
expensive and they are needed every 4" of lineal edge that is being
outfitted. The areas best fitted are the leading edge of the wheel
wells (to help the air flow over the gap without causing drag) at the
back of the roof above the rear window and all around the rear end of
the car.

Well, I decided to make my own out of flashing and double sided tape.
$4.00 worth of material, 2 hours later, I had 42 vortex generators of
my own design fitted to the aft of the car and to the back of the
roof. I didnt try the wheel wells this time but I will on the Insight.

Yesterday I took off on the 400 mile round trip to Houston for a doctors
visit to MD Anderson Hospital. The trip was one I have made many
times so I knew the mileage to expect of this car. The tires were
aired up to 33 PSI (sidewall pressure), tank filled, air temp in high 60s, wind into my
face at 15 mph. Today I returned the last 200 miles with air in high
60s,wind at my tail, at 15 mph

Several major factors that can vary mileage are wind, fuel, car tune,
tire pressure and wear, elevation change and driving style. The wind
and elevation change were averaged out by a two way trip, fuel was the
same for both trips, the same car was used on two consecutive days,
tire presure was the same both days, and the same driver, trying his
best to drive the same both ways on a familiar route was used. Yes,
there are many other smaller variables, but the sum total of them
won't make as much difference as I found.

Much to my surprise I found an increase in mileage of 1 miles per
gallon. This is an increase of almost 3.8% Huge, way beyond any
expectation. In fact I didn't expect it to be measurable at all !!! An equivalent percentage change on the Insight would be about 3 miles per gallon!!!

On the way back I was cruising at 32 MPG for better than 50 miles and
it dropped off toward the end because of several small towns I have to
pass through!! The average was over 29 MPG coming back and 25 mpg
going up into the teeth of that 15 mpg wind. That averages out to 27
mpg for the whole trip and my usual trip on a GOOD day is only 26 mpg
and into the wind is often 22 mpg.

The insight is next!!!

[Insightful Trekker]

Originally Posted by JimIsbell

an increase of almost 3.8% Huge, way beyond any
expectation.

IMO reads like your already "snake bit". The "venom" just needs a bit more time to work. Such a _small_ % "increase" is well _within_ normal background noise levels reguardless of familarity of the route or 2 way averaging.

And your Jag's CD has more room for improvement vs. the Insight. Not to mention its frontal area.

Still, maybe some of the higher speed losses can be recovered. But the best MPG comes from keeping the areo drag factor low in the MPG equation i.e. slow down.

[JimIsbell]

Originally Posted by Insightful Trekker

IMO reads like your already "snake bit". The "venom" just needs a bit more time to work. Such a _small_ % "increase" is well _within_ normal background noise levels reguardless of familarity of the route or 2 way averaging.
.

Once again, you are totally wrong. The mileage I got is outside the range of ALL mileages on that trip for the past THREE years. Maybe you think its like global warming??? Cold means Hot??? NO, it is one mile per gallon ABOVE the highest mileage I have EVER gotten on ANY trip in three years of ownership of this car. Its outside of any noise background period. Yes, over a 3 mile run there will be higher and lower numbers, but over 400 miles the noise is averaged down almost out of sight. I am a scientist (two degrees) with 40+ years of experience and I know the scientific method. This is a HUGE percentage and that percentage I quoted is only the percentage above the HIGHEST mileage I have ever goten. The percentage above the mean is MUCH higher. You do comprehend the "mean" right?

[JimIsbell]

Your second two points are correct, however, I was NOT addressing those with this thread. I was addressing the viability of Vortex Generators, not saying they were the only way or even that they were the most important way, or even that they were the best way. Only that they do work.

[IamIan]

A properly designed system might work an alternative to other aerodynamic modifications like adding a "boat tail".

I wish I had free use of a wind tunnel to do some more detailed tests of the difference.

Depending on the amount of consistent increase for the vehicle and on the cost of a durable installation... this might be a viable option.

Way down the road I had on my list to eventually try to improve my Insight's Aerodynamics ... after I finish with other things... I might just have to look into this option...

I think the snake oil part only comes in for people who sell these things to people when they are not understood... if installed in the wrong place or the wrong way, they can make things worse... or they can result in no measurable effect.... Kind of like using Hydrogen injection to make a normal ICE run lean... it does work ( due to Hydrogen faster flame speed ) but, too often they promote it as something like run your car on water .... or they install it incorrectly... etc...etc.

[Insightful Trekker]

Back in the days of the first gas "crisis" there was a LOT of attention spent on aero factors. One Datsun (early Nissan) wind tunnel test showed that adding a rear spoiler helped on the order that Jim is reporting.

Using the same setup researchers did one further test. By adding 100lb in the rear seat the net improvement became a net loss of the same order. Aero is not an easy add-on modification.

IMO seat of the pants "improvements" cannot be gauged outside of controlled conditions. Its that complex, really.

[JimIsbell]

Being an engineer, this test was not "seat of the pants" as suggested.

I researched the subject, designed the generators according to accepted science from NASA and placed them, also according to that same accepted science.

They were placed every 3" along the surface and at alternating angles of 12 to 15 degrees from the axis. If you look at Picasa Web Albums - JimIsbell you will see 5 pictures that show the installation.

Iamian, I think these could be used in a permanent installation. The main change I would make on the Jaguar is to move the rooftop ones forward about 2" as I think they would be more effective if further into the main slip stream. Papers on the subject suggest that they can be as much as 12" forward of the break from horizontal so moving them 2" would be well within that boundary. I placed them where I did because I was doing it late, (I had to leave early the next morning) I was tired, and the top of the rear window made a nice line to use. But in the daylight I could see they were too far down the curve, beyond the 12 degree departure from horizontal.

Two additional things I would do is to curve the metal down on the leading edge of the base so that it protects the double sided tape, and I would use a better quality double sided tape. The stuff I used has held for 400 miles, BUT....it only cost $1 a roll so it cant be too good.

If they were then painted to match the car they would be almost invisible.

[JimIsbell]

Today I ran tests on the Cd and Crr of both the Honda STOCK with NO modifications and the Jaguar with the VGs installed. The results were interesting and revealing.

1988 Jaguar XJ40:
weight including passengers and fuel 1999Kilograms (full fuel tank and two passengers)
tires at 33 psi
est. Frontal area 2.05 sq meters
Air temp = 68F
Course, 3 times North and 3 times South...total 6 runs
Wind East 10 mph.
Dry Clear

Results: Cd = 0.368 and Crr = 0.015 These are totally within the box as expected.

2000 Honda Insight:
weight including passengers and fuel 1032Kilograms (full fuel tank and 2 passengers)
tires at 44 psi
est. Frontal area 1.99 sq meters
Air temp = 68F
Course, 3 times North and 3 times South...total 6 runs
Wind East 10 mph.
Dry Clear

Results: Cd = 0.281 and Crr = 0.010 Ooops, I guess that 0.25 Cd is advertising puffery..#

Tomorrow I will re run the tests with the VGs on the Honda and the Jaguar clean. That should tell us something. I found it interesting that the frontal area of the Jag is not much more than the Insight. The Crr being larger on the Jag was expected with the lower pressure and wider tires. Also expected was the advertising puffery of Honda. That advertised 0.25 Cd is probably with very carefully controlled lab conditions, no license plate, maybe no windshield wipers or radio antenna or mirrors, etc.

[JimIsbell]

I just found a website that states that the Cd for a 1988 XJ40 was 0.370. This then is truly amazing! I made my measurements without knowing this information. And the results? My measurements produced 0.368. That spread sheet seems to be very accurate just as it is!!!! I wonder if the .002 is the results of the vortex generators or just noise? We will know soon..

[JimIsbell]

Well, the results are in.

Vortex generators on the Jaguar XJ40 improved the Cd from .400 to .368 !!

BUT, Honda knew what they were doing when they designed the Insight body.
Vortex Generators RAISED the Cd from .282 to .304 !!

The Jaguar was not designed with any idea for economy so VGs help it to be a better shape. But Honda designed the Insight in a wind tunnel so they probably thought of everything at that time. Adding VGs to a superb design just screws it up.