passively variable angle VG; fixes crosswinds
 

Taken the idea from modern airliners' nacelle strakes which is basically a huge VG on the side of the engine parallel to the airflow in level flight. Let's say the angle between the airflow and lengthwise centerline of VG, θ is 0 here. At level flight, the θ is 0 so no vortex is generated and marginal amount of drag introduced.

Now imagine the aircraft still moving level, but pitched up ~20 degrees; the airflow remains the same but the plane is turned up, resulting in θ = 20.

NOTE : In case you didn't know, it took me years until I was able to finally grasp the reality that airplanes rarely fly in line with the centerline of fuselage outside cruise. They pitch up and still slide forward (and downward a bit when landing) to create more lift at less airspeed but less efficiently. Which is why the VG is "under" the bottom of the wing but the air through the VG flows over the wing.

https://www.youtube.com/watch?v=OKdr4EsrByE

Now how is this relevant to a car? I can't actively pitch my car up or roll it to help turning, but the wind changes and I have to yaw (steer) the car to counteract the sideways forces which is never constant. And VGs on the trailing edge of a body seem to help "pressure recovery" as seen in the airtab wind tunnel civic data.

I've played around with several dozens of airtabs for past 5 years and even had 6 airtabs along the top of the rear window nearly eliminating any gusts when the weather app said up to 30mph.
Here's a little side note. For every airtab placed anywhere on the top or the side of 2007 Acura TL, it decreased my highway MPG pretty linearly by approximately ~0.5%. I tested from 2 to 12 airtabs using OBD2 fuel consumption data on a roundtrip of 7 miles (2 rural exits on the interstate little over 3 miles away) averaging around 21MPG.

Anyway, I concluded that airtabs having 1" height is the thickness of laminar flow at the end of 80ft box, not on a 15ft car. I needed something with less height as these are going to be placed across the hood, along the transverse frontal edges of stagnation bubble in the transition from hood to windshield. Airtabs could work in a pinch, but they're not as efficient (ECOmodder right?) being too tall for the airflow.

So what fits my bill? DIY VGs for general aviation planes.
These are 3/8" high and rigid.
https://www.blackmaxbrakes.com/airwa...tex-generators
These look about the same but flexible.
https://www.stolspeed.com/
Both are less than $1 / piece, less than $1.375(2.75/2) / piece for Airtab.

Since I don't want people tripping and falling on my car's hood and suing me for the "dangerous barbed plastic pieces" on my car, I decided to go with the soft one.

Some advantages
- Designed to work around 60mph (40-60kts in testimonies)
- Clear color (most won't bother to notice)
- Semi-flexible (no injury)
- Removable, semi-permanent, paint safe adhesive unlike airtabs' the "Ultimate Epoxy in Sponge form" which never comes off clean.
- (PROBABLY) not going to induce much drag even if I fail to install it at exactly θ = 0 since they're meant to be on an airplane without inducing too much drag at an angle at all times.


After finding out the direction of airflow with tuft test, they will be placed in a obtuse V-shaped arrangement with the pointy side forward, aligned θ = 0 in no crosswind airflow.
MS Paint drawing to help visualize...
https://i.imgur.com/12yCPW6.png


Some visualization of airflow around a sedan in crosswind. From this link
https://hal.archives-ouvertes.fr/hal-01449542/document on page 8 by Rossitto et. al.
https://i.imgur.com/ckZBp1X.png

Whatcha think??

Lockheed Georgia airtab study
 

Because people will ask for it, I went and found the cited airtab study (I think this is the one):

[Anon,] "LOCKHEED GEORGIA LOWSPEED WIND TUNNEL HONDA CIVIC HATCHBACK AIRTAB(R) MODIFICATION RESULTS" http://www.airtab.com/test-results.htm

"Executive Summary:
This report shows conclusively that the Airtab® product reduced aerodynamic drag
forces at the base area (the rear facing surface) of the test vehicle. The test showed a 4%
reduction in horse power required to maintain a steady speed of 55 mph. Only the sides
of the vehicle were fitted to assure the most aerodynamically symmetrical run data. By
adding Airtabs™ to the rear roof line as well, a conservative performance benefit
extrapolation of these findings would be in the 50% range resulting in a further HP
required reduction to approximately 6%. The test also shows that the vehicle drag
coefficient (CD) is reduced at every yaw angle from zero to thirty degrees angle from the
airflow and that this CD reduction increases at greater yaw angles."

https://ecomodder.com/forum/attachme...1&d=1590977808

https://ecomodder.com/forum/attachme...1&d=1590977823

https://ecomodder.com/forum/attachme...1&d=1590977837

https://ecomodder.com/forum/attachme...1&d=1590977849

https://ecomodder.com/forum/attachme...1&d=1590977862

The above test was done on a 1982 model Honda Civic with a Cd of 0.45. I very much doubt the relevance of that testing to any more modern car shape.

Ford tested Airtabs and found increased drag.

I tested AirTabs over a 150km daily round trip and found increased drag.

In my book I cite a paper using VG (not Airtabs) at the upper rear of a hatch-shaped car and they increased drag and lift.

I love using AirTabs to promote flow attachment but I don't think they work in reducing wake drag.

To the OP: good find on the flexible small VG, and I think the basic idea of VG behaviour changing with crosswinds is sound.

I was not vouching for the study. The OP mentioned it. I was curious. I am not willing to assume it means they work, either. Anyway, my understanding it that the Evo design's airtab placement took a lot of windtunnel time. I imagine micro adjustments matter a lot.

Interesting, not sure how much concrete benefit you'll get for the effort though.

My thinking is that generating vortices in a crosswind could possibly make the lateral forces on the car worse.

I now have 40 of the Stolspeed VGs on their way. Very cheap including postage to the other side of the world - AUD$40. Good guy to communicate with too.

https://www.stolspeed.com/uploads/im...key__Clear.jpg

Those look good. 1" height?

I don't think the OP's plans will work, if I understand correctly what he is attempting. He bought 3/8" hieght instead of 1" height and plans to place them on the hood, positioned with yaw in mind. The idea being to increase pressure on the low pressure side under yaw. I agree with serialki11r. It'll create more drag, yaw or no yaw & make crosswinds worse.

I appreciate criticism but only when provided with a reasonable thought process. If one can say it won't work I can also say it will and it is not contributing to anything other than a blobby mess of opinions. All I got is "It ain't gonna work" without any hows. (That's how online forums get multiple threads claiming they get better mpg with hotter coolant but that's another story)

Let me give you an example. I think these will work in similar ways that roof extension spoilers work to reduce lift and drag in hatchback vehicles. Crosswind is never constant, which also promotes more abrupt separation on the trailing edge (in this case, A pillar and front roofline). The VGs when activated by crosswind, "...will prevent abrupt air separation at the trailing edges and therefore, contributes to lesser Wake creation, which in turn reduces the drag of the vehicle", quoted from explanation of hatchback rear spoiler.

This is why I also had an idea to just attach something to promote cleaner separation on A pillar but it already creates giant vortex as is which is out of my thinking capacity at this moment. End example.


I like to look at Toyota stuff since they seem to be going crazy with VGs for the past decade, which is my go-to item for when I can't modify the actual shape. The idea of letting the wind vary VG angle is probably not my original. If you look at any 15+ Toyota/Lexus vehicle from behind you can see the plastic cover under the trunk floor has four 1" x 6"(approx.) 2.5cm x 15cm plastic protrusions that are in line with the length of the vehicle. And I'm not going to bother posting visualizations of airflow under the car in varying crosswind because you should know it before you reply in this thread.

Toyota got the variable angle part down, and yes I'm still not certain if it wll help reduce the pull caused by crosswind and how to measure it if I could at all. Especially having seen that Airtabs do not "fill the wake" or "increase pressure on the trailing side", if I had a Camry or RX400 I'd already be testing the car with the strakes covered vs uncovered on windy interstate.


If those little pieces in line with airflow "creates more drag", I strongly believe it is smaller than margin of error in real world testing which is the only thing I could do so I won't be able to see it. But I will keep an eye out just in case I made wrong assumptions.
And, sure, airliners only have 2 of them but if they affected FE in any significant way, they wouldn't be on LH commercial airliners.

In crosswind, I actually do not know if they will help fight crosswind or not for the Airtabs did not reduce drag anywhere on my car. But like I explained in the OP I'm crossing fingers they would be efficient considering they're designed for vehicles much more sensitive to aero drag than cars.


And no those are 3/8". If one would care to read several posts above, we have been just talking about how 1" airtabs hurt MPG even on the trailing edge. It can be visualized how thin the laminar flow is on the front part of the car, put tufts on a straw perpendicular to the surface and give it some diagonal supports and go fir a drive.

You tend to write a lot which makes it hard to follow what you intend doing and why you intend doing it. So, is this right?

Fit fin-type vortex generators on the hood in front of the windscreen to change A-pillar separation and so reduce susceptibility to crosswinds.

I tried quoting but it won't accept more than 3 :( so I tried to just lay them out. I don't think any of them were replying to you tho... not too sure what I was exactly thinking but yeah.

And yes. Gosh I'm going to suck at writing thesis..es...

I don't see why it wouldn't work. Just adding two of them to my 2020 prius prime in the back on the black plasic piece behind the shark wing at an angle increased my fuel mileage about 4-8 percent in two different speed ranges 60-67 and 78+ but also drastically reduced my crosswind issues. Also I installed them with the high side of the vg facing forward to center of the car rather than backwards away from center.

Not trying to attack you. Just responding to your idea with some feedback.

You'll do fine with your thesis. You'll write drafts. You'll get feedback. Your next draft will improve. Write lots of drafts. Is this project part of the thesis?

How did you do your testing? There are lots of variables, not just placement of VGs, that could contribute to changes in FE. I am not saying I think the VGs did not work for you.

https://www.youtube.com/watch?v=VDmqxGxvSL0
https://www.youtube.com/watch?v=g-Tkpwr5v-0
Here are two videos visualizing vortex from delta shaped VG with the pointy side facing forward and backward.

I just need to find some tests to tell me which is "better".

Ok, with this approach:

Fit fin-type vortex generators on the hood in front of the windscreen to change A-pillar separation and so reduce susceptibility to crosswinds.

I don't think they will work.

1. Car stability is most determined by the lateral centre of pressure location. If the car turns away from wind gusts (most common), you need to move the centre of pressure rearwards. Rear fins are the easiest way of doing this, and in my testing on a Gen 1 Insight, have worked extremely well.

2. Separated flow on the downwind side of the car will, however also cause instability (as you earlier indicated). The yaw angle at which downwind side separation occurs depends on:

- the shape of the front corner of the car
- the shape of the A pillar
- the surface roughness of the side of the car

The only thing that your vortex generators could impact on this list is the behaviour of the A pillar - and the hood location is a long way from the A pillar.

I'd try the VGs on the A pillars and the front corners of the car.

Truthfully, I did it for the crosswinds not fuel economy. The fuel economy was the 20k miles before and thus far the 4k after. But an observation afterwards of a sudden spike in fuel economy immediately following was a bonus on my part and it's been pretty consistent, i've not seen a single decrease in fuel mileages. I've even beaten previously set fuel economy highs on trips with higher average speeds no less. Where I used to get 44 to no more than 47mpg covering a large part of a 72 mile each way journey at 80-82mph now I'm getting 51-56mpg (no battery) and up to 78 mpg (previously 72-75) mpg when I daily charge each way. Which would require me driving 70-75mph before to achieve and now doing at higher speeds. With the Covid 19 traffic, I've had to speed up to avoid basically being run over on the I5 interstate route I take every day. It's pretty crazy that such a simple modification would make so much difference.

All I can say, is try it the way I have it. See where it goes for you. If you have a way to wind tunnel test it, I'd be curious as to the results on that.

I wish I was studying this.. it would be more fun. It's just a hobby.

So, from #1 I understand that VGs will "attract" the center of pressure toward their downstream area.
And from #2 I understand that what I was going to do could easily (when overdone) worsen the crosswind instability by keeping the center of pressure even more off.

I was also considering directly placing them on the A-pillar which would also deal with the buffeting with open window at medium to high speeds. So I start at the bottom of A-pillar and where should I stop? For some reason I feel putting them on the roof between the top of B and C-pillars would be effective.

Speaking of 1G Insight, are you the person who wrote Autospeed blog? That's where I read about the idea to place airtabs under the car.



Feel free to consider this fairy tale but I have experienced surprisingly good mpg as I increased crusing speed a bit which made my rpm higher so the BSFC was actually lowered more than the increased drag so I got better MPG going faster than usual.

I recognize those VGs. I considered them at one point but they were too big and thick for my taste so I passed and then they went out of business so yeah.

I tend to believe VGs near trailing edges of box shapes reducing drag (probably due to reading Airtab marketing material too much) but trying to work with a sedan I feel there are a lot more that I should consider but don't even know.

Well longterm fuel economy returns can be hard to argue with, and I don't especially want to. It's just that there is a lot of hype about those tabs. It seems you may have happened on some very good placing of the tabs. I have read that placing matters a lot and that tiny changes can have notable effect. I think it would be really informative to see pictures of how you have placed them. Wanna post some pics and measurements?

EDIT: Ooops... missed the photo you posted. Still, what are the relative measurements to the edge of the lip, height, and details like that.

When I increased cruising speed before, I would get significantly worse average round trip gas mileage in the 40s, I would have to slow down to get 50s. now it's in the 50s consistently and I'm getting what Toyota says I should get in the City at highway speeds. Also, they are still selling these things on Amazon where I got a 6 pack for 11 bucks. My goal was purely for crosswinds which these actually do a pretty good job at. My theory was the curved rear window down the middle of the prius prime would benefit as the angling I used was similar to the Prius TRD setup.

Pretty sure the spacing is 11 inches apart at the farthest point. I'd have to double check the narrow point though.

I'll get a yard stick to get the measure points right. Suppose there are a million priuses out there that would love to know.

Exactly. Look forward to seeing what you post. You might even consider a dedicated thread for a write-up, since that would get you more attention.

No, I don't think so, but I've never really thought about it in those terms. VGs just energise the boundary layer and so potentially give better flow attachment where separation is occurring.

No idea. Testing is king.

No I don't think so.

Yes I was employed as editor at AutoSpeed for a long time. The very brief mention I made there of putting AirTabs under the car was very anecdotal - and was stated as such. There's far more detail, testing and analysis of VGs in my book.

The Stolspeed VGs arrived. I didn't realise that they were coming from within Australia - no wonder postage was cheap!

https://i.postimg.cc/nrk4ZXsT/IMG-0499.jpg

They are indeed very small - 24.7mm long, 10.3mm wide and 10.9mm high. With their claims of being flexible, I thought they'd be a bit rubbery but the plastic is still quite hard. It 'gives', but only a little. I think they'd cut someone if they fell on them and the VGs contacted bare skin. The upright is only 0.7mm thick.

They come with a sheet of double-sided adhesive that has been template-cut to suit the shape of their base.

In car use my guess is that they'd be suitable only for A pillars, and between the A mirror and pillar. They might also be suitable for the bonnet/hood, as the OP nominated. I might do some testing tomorrow.

I have so far started to reconsider the arrangement on the hood after some 8 hours of tuft testing. A-pillar sounds more viable. And I feel I'm not qualified enough to touch the interactio between A-pillar vortex and the side mirror wake so I'm interested to see some testings from someone with vastly more knowledge than myself.

If the goal is to raise pressure on the leeward side in a crosswind, Maybe you can achieve your goal with a roof-top strake.
https://www.mrn.com/wp-content/uploa...-Las-Vegas.jpg
https://www.mrn.com/wp-content/uploa...-Las-Vegas.jpg
These are designed to kill lift when driving sideways, but they should have the same effect as a Gurney flap/wickerbill at the rear rolling vortexes down the lee side.

Well, don't leave us in suspense - what were the results?!

I did some testing of various vortex generators on the A-pillar of the Insight. I've previously seen some flow separation immediately behind the pillar on the side glass.

Initially I decided to see if I could measure a pressure change on the side glass.

Test:

• Honda Insight
• Magnehelic 0-3 inch of water gauge, converted to Pa
• Reference pressure pitot tube static port 2.2m above ground
• 80 km/h
• Two way average
• Light winds, 15 degrees C
• Sensor puck on side glass as pictured

No VGs: -149 Pa
5 Stolspeed VGs aligned horizontally: -167 Pa
5 Stolspeed VGs angled up from horizontal: -162 Pa

I then fitted rubber wedge VGs.

5 wedges aligned horizontally: -167 Pa
9 wedges aligned horizontally (as shown in pic): -154 Pa

https://i.postimg.cc/kXBvtQpd/IMG-0500.jpg

So basically none of these did very much.

I then decided that I was trialling stuff that was too small, and to go for a monster. I then fitted one Airtab.

https://i.postimg.cc/RZxJVwrd/IMG-0502.jpg

1 Airtab: -184 Pa

So that made a difference, presumably providing faster flow.

But what would tufts show?

No Airtab:

https://i.postimg.cc/9MDyqVLY/DSC-3701.jpg

Airtab:

https://i.postimg.cc/nrZcfG99/DSC-3692.jpg

(The pressure sensing puck was located about where the middle tuft is.) So I wouldn't call that a success.

As I have previously posted, by far best results have come from using an aerofoil guide vane on the A pillar. (But I can't get an extrusion long enough and I don't want to make my own.)

Footnote: I was surprised at how low the measured pressure was at this location. It might be interesting to measure right down the side of the car, and see if the low pressure at this location is as a result of the A pillar. (Has anyone seen pressure measurements down the side of a car? I haven't.)

think
 

I've no knowledge of anyone actually optimizing VG type, number, and location placement outside a laboratory environment.
Also, I believe that if you read the report on the Mitsubishi Lancer Evo VG research, that the TBL was an inch thick at the rear of the EVOs roof.
If you're not running a university supercomputer and Dassault-Exa POWERFLOW CFD software, you can expect nothing from any lesser CFD.
The cheapest wind tunnel you could get into is A2, in Mooresville, North Carolina. Two hour minimum,at a beginning $480/ hour, then $ 500/hour afterwards. And you'll catch hell from a certain member for not spending $4,000/ hour at Gene Haas Racing' moving-floor wind tunnel.
I strongly recommend you look at the other end of the car for mods.

Pressures on side of Insight.

• 80 km/h test speed
• Two way average
• AR1890 digital manometer, inches of water converted to Pascals
• Pitot tube static pressure ref, 2.2 metre from ground
• Custom measuring puck
• Light winds, 14 degrees C
• Front and rear undertrays, rear with diffuser. Edgarwit front air curtains.

https://i.postimg.cc/7hMYYKpy/Side-p...-dig-gauge.jpg

Starting from front, the higher pressure recorded inside the Edgarwit means, I think, that the puck was disrupting flow (narrow gap here).

Generally -100 Pa down side of car at approx half-door height. Surprisingly good flow attachment after front wheel - no change in pressure here. (Would be interesting to test with and without Edgarwit.)

Pressure after edge of door - leakage through door seal? I would have thought I'd be able to hear this, so I don't know.

Upper side readings show really low pressure after A-pillar. I am guessing this is the base of a vortex - tufts here show some odd patterns (eg tufts on glass pointing upwards along pillar). Note that reading was on side glass, not on curve of pillar.

Pages 146 -150 of my book.

It was nothing more than driving around with tufts to see if I can get any serious crosswind, so the results may not sound like worthy of 8 hours.

I thought the V arrangement (opening facing either forward or backward) was not taking advantage of the full number of VGs since in crosswind, one side of the V will be overlapping too much.

From Mr. Edgar's testing, I think that would be sufficient for my purpose to reduce the sideways forces in crosswind since I already tamed it down with some airtabs behind the rear axle, for the time being.

Now I'm looking at my car how I can firmly attach rooftop strake, and tryign to find some information on low speed characteristics of MiG-15/17.

Also about to buy the book.. the lack of scientific knowledge on my part while trying to have some fun with this is too painful.

And about "anyone actually optimizing VG type, number, and location placement outside a laboratory environment", I see too many toyotas with tiny nubs around the mirror, tail light, and D-pillars. But most importantly the plastic cover under where spare tire would/used to be.. if they are not VGs I don't know what else they are so a point in the right direction would be very helpful, thank you.

That is good thinking. Hours of frustrating testing or a mod that doesn't seem to work as expected... more information needed... report back what you learn. Would love to see updayes.

Yeah I just would like to reiterate, I dont care about improving MPG as far as this thread goes.

knowledge
 

Like I said, I've no knowledge of anyone succeeding in the optimization of VGs. The metrics by which 'success' would be quantified doesn't even have the surface scratched between pages 146-150.
'It's not what you know. It's what you think you know that just ain't so.' Samuel Clements.
Any reading of a 'real' scientific investigation will 'explain' the situation.

tiny nubs
 

Those tiny nubs are the outcome of acoustic wind tunnel testing, and simply 'tune' the natural harmonic frequency ( wind noise ) of that surface, and it's octaves, beyond the reach of the human ear.
Noise pollution is an issue which all automakers must address. Tire noise especially.

design
 

One reason there was a lack of widespread adoption had to do with commercial car washes and the damage rotating brushes could inflict on the VGs. Rooftop, satellite antennae must be very robust to survive the rigors of a modern car wash.
The difficulty of hand washing would be a turnoff as well. Class-action lawsuits from those snagging wedding rings on the damned things.

Toyota excrescences
 

What you're observing could be explained simply as a styling gimmick.
A perfect belly pan would be required for the diffuser to work in the first place. Failing that, it wouldn't matter what the diffuser looked like, it would be submerged in turbulence either way.

4-8 %
 

Run the energy balance calculations on that data and compare to empirical data obtained in a laboratory, then see if you draw the same conclusions.

better
 

1) there are around six (6) different types of VGs in existence.
2) of those six, there would be applications for:
* sub-boundary layer thickness VGs
* equa- boundary layer thickness VGs
* and extra- boundary layer thickness VGs
3) 'solutions' require empirical testing, they cannot be mathematically modeled.
Each of all VGs, under at least the three height categories would need to be investigated.
4) Zero-yaw and crosswind condition testing would be mandatory.
5) optimum number of VG testing is mandatory
6) optimum placement of VG testing is mandatory
7) all the testing would require a laboratory environment, with a single variable allowed, the VGs themselves.
Only under these conditions would data have any scientific merit.