Quote:
Originally Posted by MetroMPG
Still skeptical.
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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.