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Old 11-26-2020, 04:12 PM   #1 (permalink)
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Cancelling trailing vortices, is it possible?

I just watched this video:



And it made me wonder if we could adapt a counter rotation vortex onto one of our cars.

How can I identify where and how strong vortices are? I had an idea, haven't tried it out yet though. Could I use a small diameter rod (~1\8"?) to hold a tuft off the surface of the car an inch or two?

What do you guys think? Would it work? Does the slower speed of our car vs the f1 example mean it wouldn't be worth it?


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Old 11-26-2020, 05:20 PM   #2 (permalink)
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I just skimmed the video (F1 car aero is basically irrelevant to road cars) and it appeared that he was talking only about reducing vortex strength of very high downforce wings - not really relevant to road cars.

Rob Palin (ex Tesla lead aerodynamcist) told me about reducing the pressure difference as the air separates to reduce vortex strength, and in fact I did some experiments on my Insight sides along these lines - but they didn't work in reducing drag.

I've tried measuring vortices on the road by using tufts on short rods, but it didn't work. However, Dick Barnard (the aerodynamicist I wrote my big book with) talked about showing his university students the trailing vortices off models in the wind tunnel by using a long rod and a single tuft.

Since the only relevance of vortices to road car drag / lift is the pressure exerted by the vortices on the body panels, I am hoping that the high-speed logger that I have on loan, working with 16 surface pressure pucks, will be able to show the action of vortices eg on the A-pillar / side glass, and on the inclined rear pillars. I'd expect these pressures to rapidly oscillate as the vortices are shed - something a Magnehelic gauge / digital manometer wouldn't show.

I've previously thought of using a paddle type rotating anemometer on a long pole to show rotational speed and direction of trailing vortices. I have bought the anemometer head but not done anything further at this stage.

The whole area of trailing vortices on different body shape cars is very complex. Aerodynamics of Road Vehicles (5th edition) has a lot more on it than earlier editions of the book, but to be honest I've not really absorbed everything in this book yet.
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Old 11-26-2020, 05:35 PM   #3 (permalink)
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Thanks (today) for the video. The front end part was interesting; the active wing, not so much.
Quote:
a small diameter rod (~1\8"?) to hold a tuft
A golf tee carved into an airfoil section with a magnet embedded in the head? One could move a flock of them around to different parts of the car.

edit:
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Quote:
However, Dick Barnard .... showing his university students the trailing vortices off models in the wind tunnel by using a long rod and a single tuft.
I've imagined a stepper motor in the front bumper with a telescopic arm with a streamer (or smoke generator) the length of the vehicle.

All could be controlled by e. g., a Raspberry Pi with GIPO.
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Old 11-26-2020, 05:37 PM   #4 (permalink)
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Quote:
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A golf tee carved into an airfoil section with a magnet embedded in the head? One could move a flock of them around to different parts of the car.
Not on an aluminum car!
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Old 11-26-2020, 05:49 PM   #5 (permalink)
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Originally Posted by JulianEdgar View Post
Aerodynamics of Road Vehicles (5th edition) has a lot more on it than earlier editions of the book, but to be honest I've not really absorbed everything in this book yet.
I just ordered this edition, nice to hear it has more information on vortices.

Scibor-Rylski makes an interesting assertion that I hadn't read anywhere else: deliberately introducing a counter-rotating vortex to reduce the strength of already-present vortices can reduce the vortex-induced lift by simple vector superposition (summing vector components of the velocity in each axis reduces or cancels the velocity along them, in other words), but drag increases according to the summed scalar strength (no cancellation) of each vortex. I don't know if that's true or not, but it may be possible to find out on the road if you can design a device to induce rotation outward (counterclockwise looking from the rear of the car) and then throttle-stop test.

I've wondered if this shape, in fact, is designed to do just that and reduce rear lift:

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Old 11-26-2020, 06:10 PM   #6 (permalink)
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Remember also that the direction of rotating vortices depends on the overall lift/downforce of the body. That is, when viewed from behind, bodies that develop lift have clockwise vortices on left, and anticlockwise on right, whereas downforce bodies have vortices that rotate in the opposite directions.

It's easiest (for me at least) to think that this is so because, on a lifting body, there is a lower pressure on top, and so air passes from the sides to the top of the car. (Very clear on A pillars for example.) Now if there is this airflow direction, but the car is moving along, the 'flat' flow becomes a spiral ie a vortex.

On that basis, you'd expect vortex formation to be lowest on a body with zero lift/downforce, and I understand that this is the case. But then Rob Palin told me that in the wind tunnel, you can see vortices coming off every rear edge of the car, so as I say, it gets much more complex. When I told him I had borrowed the 16 channel high speed logger, he wrote:

Measuring pressures on-car is always interesting, and gets even more so (although increasingly obfuscating) as you start to dig into transients, both 'intrinsic', from periodic shedding & couplings around the car, and 'extrinsic', from ambient turbulence, and interactions/admittances/resonances between the two. Something I've definitely witnessed is the coherence & amplitude of such transient phenomena becoming much larger as the car's shape gets more slippery, and a lot of the 'white noise' from bluff body / chaotic turbulence fades away. It makes the analysis easier, but what shows up can raise the eyebrows!

Re the 5th edition, I find it a much harder book than earlier editions - there's just so much in it on every area of car aero.
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Old 11-26-2020, 06:31 PM   #7 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
I just skimmed the video .....
Would the basic idea not transfer to the vortexes made by a road car? Is it because there is (I'm assuming) a lower difference in pressures of the road car than the wings on the F1? Which probably means the effect on the F1 is much greater than a road car and so isn't high on the fix-it list on road car aero.

Care to share (briefly or detailed) those experiments? My first thought when posting was maybe for the a pillar, a wing tip placed in front of the windshield to try and lower the effects of the a-pillar vortex as it seemed easier to alter.

Do you think your failure with the rod and tuft could be attributed to the less-uniform wind on the road, the scale of the car (vs a model), or just a difference in execution?

Will be interested in the results.

I have realized that when talking mods you (you, in general, not you) tend to think of how they would apply to your own car, so of course I am thinking of the Mercury which seems to be a rather out dated shape.

I will have to add that to the list. Doesn't seem to be too costly either.

Quote:
Originally Posted by freebeard View Post
Thanks (today) for the video. The front end part was interesting; the active wing, not so much....
My 1/8" rod was a coat hanger (good luck getting metal coat hangers though)

I still think about the smoke generator as I think it would be easy(er) to see some of the effects. The only attempted reference I have though is Mr. Edgar. I feel like I would need a large volume of smoke.

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Originally Posted by JulianEdgar View Post
Not on an aluminum car!
Not everyone has that luxury


Quote:
Originally Posted by Vman455 View Post
I just ordered this edition, nice to hear it has more information on vortices.

Scibor-Rylski makes an interesting assertion that I hadn't read anywhere else: deliberately introducing a counter-rotating vortex to reduce the strength of already-present vortices can reduce the vortex-induced lift by simple vector superposition (summing vector components of the velocity in each axis reduces or cancels the velocity along them, in other words), but drag increases according to the summed scalar strength (no cancellation) of each vortex. I don't know if that's true or not, but it may be possible to find out on the road if you can design a device to induce rotation outward (counterclockwise looking from the rear of the car) and then throttle-stop test.

I've wondered if this shape, in fact, is designed to do just that and reduce rear lift:
Do you mean Scibor-Rylski's Road Vehicle Aerodynamics? I have started to try to go through this book so if you've got a page I might jump ahead to check it out.

That is an interesting shape and I can see how you can come to that conclusion.
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Old 11-26-2020, 07:15 PM   #8 (permalink)
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I found this in the Whiteaker neighborhood.



Compare:


https://images.newscientist.com/wp-c...r9-800x533.jpg

And that to an F1 front wing. Maybe layers of serrations that curl up at the tip? Each one a little boxed cavity/anti-reversion valve?
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Old 11-26-2020, 07:29 PM   #9 (permalink)
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I don't understand what you are saying freebeard.
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My herd includes:
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Old 11-26-2020, 08:02 PM   #10 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
Remember also that the direction of rotating vortices depends on the overall lift/downforce of the body. That is, when viewed from behind, bodies that develop lift have clockwise vortices on left, and anticlockwise on right, whereas downforce bodies have vortices that rotate in the opposite directions.
Right you are--I should have specified, I was thinking of a fastback or notchback (or somewhere between the two, like the Civic I posted) developing a clockwise-rotating vortex on the driver's side (US)/counterclockwise on the passenger side.

Quote:
Originally Posted by M_a_t_t View Post
Do you mean Scibor-Rylski's Road Vehicle Aerodynamics? I have started to try to go through this book so if you've got a page I might jump ahead to check it out.
In the 2nd edition (1984), pp. 56-62, with some more discussion in the next section on fastbacks pp. 62-65.

The copy I bought turned out to be a library copy from City University, where Scibor-Rylski taught, with 14 due dates spanning 1996-2005. That makes me wonder who read through this particular copy before I got it!

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