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Old 03-06-2012, 11:14 AM   #11 (permalink)
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Originally Posted by MetroMPG View Post
Still skeptical.
To be sure! Per the slide presentation, these "pull the air vortices that are around the side of the car in close to help stabilize the car at highway speeds."

1. In significant cross winds of the sort that can destabilize a front wheel drive vehicle that already has a reasonable distribution of area in side view, airflow at the rear end of the car is completely unpredictable. On the lee side of the car, large areas can become separated and turbulent with largish (10 degree) angles of attack, even very far forward of the taillights. Real vortex generators (big things, not near microscopic bumps closely aligned with the airflow) placed much closer to the longitudinal middle of the car could have a (wind tunnel) measurable effect, for a particular cross wind condition. Even this would be a stretch, though, for real word effect. In a gust producing a 15 degree angle of attack, the entire lee side of a car can be separated.

2. Wind tunnel smoke does not operate at anything close to the very fine granularity to show this mechanism of "pulling in vortices."

3. Even if these "vortex generators" were positioned in an advantageous location, they would need to be much larger and at a more dramatic angle from the airflow.

4. Real vortex generators look like this: Big, positioned where they might do something.

5. At car scale, trailing edge "stuff the wake" VGs look like this:

... and even then have little-to-no measurable effect. In this test, they appeared to have a negative effect, but road tests using built-in fuel flow measurement gauges are inadequate to say anything. Given differences in temperature, barometric pressure and especially wind direction & speed, consumptions in the range of 2.8 to 3.1 are all the same. (It's like trying to measure ball bearing diameter with a yardstick.)

6. Here are some big VGs positioned to actually do something, as shown by tufting. Per the tufting, they appear to eliminate a bubble. But they have no measurable effect on fuel efficiency.

These bumps on the Prius are for styling, so the customer can tell their friends: "See, they've even sweated all the details right down to these vortex generators."

Car pics above from: http://autospeed.com/cms/title_Blowi...1/article.html

The "Vortex Generators" on the top of my POC, positioned where they actually could be predicted to have some effect, have no effect at all. They represent something like 1/3600 of the frontal area, and add as much drag as I do when I sneeze.

Huge styled wheels in several unaerodynamic styles, no rear fender skirts, but "Vortex Generators". Please.

Beautifully engineered little car though. To bad that the real engineering gets lost in the noise of the media presentation.

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Old 03-06-2012, 02:09 PM   #12 (permalink)
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Mitsubishi measured the thickness of the boundary layer at the trailing edge of the roof of the Lancer Evo at 30 mm (source). Consequently, they sized their VG's to that height - "the optimum height of the VG would be nearly equal to the boundary layer thickness."

I can't tell for sure by looking at the Prius C, but I don't think those bumps are anywhere near that height from the surrounding surface. Yet we can be pretty sure that owing to side mirror turbulence, the boundary layer around that part of the car is not going to be very thin.

So, still skeptical.
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Old 03-06-2012, 02:56 PM   #13 (permalink)
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A video of the Prius C in the wind tunnel would be great ! Seems to me that Toyota has made a commendable effort, aero-wise, on this car.
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Old 03-06-2012, 03:49 PM   #14 (permalink)
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semi-hijack, but I have a first gen prius like in the picture above, but with a fuel consumption display. However, it also has a small spoiler/wing. I have noticed flow separation in the same place as that Prius though. No tuft testing, just blowing snow and rain.

Anyone think the spoiler would interfere with any improvements? Would improvements in an A-B-A test at 70-80mph be measurable, or within the margin of error for the Prius mileage computation?
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Old 07-09-2012, 04:35 PM   #15 (permalink)
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How would putting them on the trailing underside or 2/3 of the way back on the underside of the body work? Wouldn't this help to stabilize the underside airflow as it comes up to the tail and cleanly released? Humm...

Originally Posted by Ken Fry View Post
... with very strong input from marketing.

The angle may be getting almost close to VG, because the flow there is downward, (i.e., roughly parallel to the roof line) so their angle of attack might be 5-10 degrees.

Mercedes used to advertise their tailights that cleaned themselves aerodynamically. But then they dropped the design with the next styling change. Hmmm. "We were only kidding... made a great story though, didn't it?"

I have "vortex generators" on the top of my proof concept prototype -- they look like little wings. They are actually the supports for the canopy when it is flipped open. They are streamlined mainly for fun, and as a conversation starter.

The other issue is that vortex generators are intended to energize flow over the surface of a vehicle (typically one that flies). Putting them at the very back of a vehicle makes no sense at all. The energy you have put into creating these supposed vorticies would be entirely lost.

Fins on late 50's cars had no real function at all, as proven by their elimination in the early 60's. But the manufacturer's (especially Chrysler) advertised them as stabilizing devices, rather than as the space-age-looking styling gimmicks that they were. They were positively huge as compared to these tiny bumps on the Prius taillights.

(Chevy: 1956: rounded fin sprouting; 1957: real fin; 58: no fin at all. 59: fin flopped over on it side; 60: flopped over fin with a corner. 61 fins gone for good, never to return.)
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Old 07-09-2012, 05:05 PM   #16 (permalink)
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Vortex generators do work and are a proven technology. It just takes an engineering team of aerodynamicists to size and position them on a car to make sure there is a positive change.
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Old 07-10-2012, 01:25 AM   #17 (permalink)
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Sometimes dumb luck works. I put some vg's on the spoiler on my smart cabrio and noticed a definite improvement in stability and about a .5 increase in mpg. I'm not sure why, but I'm not complaining. I put them on to begin with to help keep the rear window clean, made no difference on that but the stability improvement was noticeable immediately.

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