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Old 03-28-2008, 11:27 PM   #1 (permalink)
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Prius wake turbulence

Drove for miles today on the freeway in the rain, behind and beside a Prius so I could study its wake turbulence made visible by the spray and mist.

Prius has noticeably less wake turbulence than other vehicles, especially coming out of the front wheel wells. Why is this? What geometry does Prius have to suppress or avoid the turbulence blowing out and interfering with the slipstream?

And, the boat-like fairings behind the rear wheels seem to suppress wake turbulence, but are probably not the last word.

So, can any Prius owner here post pics and text which address or explain this issue? Maybe we can copycat.

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Old 04-19-2008, 03:49 PM   #2 (permalink)
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Prius wake turbulence

I think the Prius has minimum airflow into the engine bay, has minimized air going under the car,as well as a relatively "clean" underside,all of which reduce the static pressure under the car,which typically is responsible for the outflow blast you witness in other vehicles.Generous radii at the leading edges of the car's nose allow the air to hug the sides with an energetic flow,which help fold the wheel-arch outflow rearwards.Additionally,the roofline allows for some static-pressure regain at the rear of the car,unobtainable with other roof designs,which allows pressure there to reach closer to the forward stagnation pressure of the car.Higher pressure,weaker wake.
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Old 04-20-2008, 09:41 AM   #3 (permalink)
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Hi All,

The Prius, like other cars these days have the wheel well liners, so the engine bay flow is probably not the issue.

The Prius does have the stock wind deflectors in front of all the wheels. Which limits direct flow of air that hits the tires, and gets drawn by the Magnus effect up into the well, and expelled on the down wind side of the wheel well.

Still, on the rear wheels especially, one can see the out-flow dirt streakes from 2/3 s the way up the well on down.

Edit:
Ooops, I got this Magnus effect thing wrong. Its just plain pressure flow up into the wheel well. The Tires are running in the wrong direction for the Magnus effect.

Last edited by donee; 04-20-2008 at 10:32 AM.. Reason: technical inaccuracy
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Old 04-20-2008, 09:46 AM   #4 (permalink)
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Hi Again,

There is some wake turbulence back there, though. I know this because I had a ninja motorcyle bike rider draft off my left rear by about a foot while I was running on level terrain in cruise control at 62 mph. And that improved my mileage, rather than decreasing it. The motorcyle and rider were apparently giving my car a psuedo boat tail effect, and probably smoothing out the side body air flow off the back of the car. This was good for about a 2 mpg improvement.
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Old 04-20-2008, 10:30 AM   #5 (permalink)
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Quote:
Originally Posted by donee View Post
Hi Again,

There is some wake turbulence back there, though. I know this because I had a ninja motorcyle bike rider draft off my left rear by about a foot while I was running on level terrain in cruise control at 62 mph. And that improved my mileage, rather than decreasing it. The motorcyle and rider were apparently giving my car a psuedo boat tail effect, and probably smoothing out the side body air flow off the back of the car. This was good for about a 2 mpg improvement.
The drag of both vehicles is reduced when drafting this way. Whereas it would seem reasonable that the following vehicle would benefit, the lead vehicle does, too.

You are paying for the wake you make, even though it may be well behind you. Less wake = less fuel consumption, apparently.
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Old 04-20-2008, 11:52 AM   #6 (permalink)
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Quote:
Originally Posted by donee View Post
Hi Again,

There is some wake turbulence back there, though. I know this because I had a ninja motorcyle bike rider draft off my left rear by about a foot while I was running on level terrain in cruise control at 62 mph. And that improved my mileage, rather than decreasing it. The motorcyle and rider were apparently giving my car a psuedo boat tail effect, and probably smoothing out the side body air flow off the back of the car. This was good for about a 2 mpg improvement.
I agree with exactly what otto said in response

I love it when I hear someone say that semi truck drivers can feel people behind them because it puts more aero drag on their rig.... And by love it, I mean it makes me laugh and disturbed that disinformation is being shared...
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Old 04-20-2008, 12:15 PM   #7 (permalink)
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Quote:
Originally Posted by trebuchet03 View Post
I agree with exactly what otto said in response

I love it when I hear someone say that semi truck drivers can feel people behind them because it puts more aero drag on their rig.... And by love it, I mean it makes me laugh and disturbed that disinformation is being shared...
The thing they hate is having someone completely hidden in their blind spot. And I doubt that one passenger car drafting would significantly affect the huge truck's FE.
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Old 04-20-2008, 12:30 PM   #8 (permalink)
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Originally Posted by OfficeLinebacker View Post
The thing they hate is having someone completely hidden in their blind spot.
Do you think people care? Drive on the highway and tell me no one is drafting a semi In any case, that wasn't the point I've seen people try and claim the opposite of established aero. The claim: truck drivers have some sort of malfunctioning ESP making them think they're pulling more load from a mysterous tailgating car

Quote:
And I doubt that one passenger car drafting would significantly affect the huge truck's FE.
Likely true - but that doesn't change that I laugh at the conjecture I, for one, love speculation - just not about well established phenomena.
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Old 04-20-2008, 12:56 PM   #9 (permalink)
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Quote:
Originally Posted by trebuchet03 View Post
I agree with exactly what otto said in response ...

Hey Tre, here's a related but somewhat roundabout question:

Dr. Alex Strojnik was one of the great thinkers and designers of laminar flow light planes, designer of the Strojnik S-2 and author of a trilogy of great books on efficient design. His S-2 (Google for it) motorized sailplane had tandem fixed wheels, about a foot diameter each, and closely spaced. He found out that two wheels in tandem had less drag than one alone, as the second acted as a wake fairing for the front one. NASCAR drivers, Tour de France bicyclists, et al have long known the benefits of drafting, which are well established.

So, here's the question for you in particular, being an HPV guy: Why not have two tandem smaller wheels in the front of your HPV streamliner, instead of one larger one? These could much more easily be faired, would fit better under the nose, with considerably less wetted and frontal area. This would also make for less bulbous nose, as you'd not need to include the big front wheel in the same faired volume as your pedal pathways.

Rolling resistance is always an issue, as it adds to total drag the rider must overcome. Has anybody tested the difference between rolling resistance of smaller, solid wheels vs. larger inflated ones? At what point would the lower air drag of smaller tandem wheels be a net advantage over the presumably lower rolling resistance of a bigger wheel?

Sorry about the hijack, my excuse is that it all relates to the drafting-as-drag-reduction thread.
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Old 04-20-2008, 07:17 PM   #10 (permalink)
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Otto,

Small wheels have a higher rolling resistance - at least traditionally... So that's not a good thing... Plus, the "big boys" have fully faired front drive wheels - only a tiny bit is actually exposed. Something like this




I mean, it's probably worth a look There very well could be a point where it would pay off.

As far as fitting under the fairing... Your leg path tends to be the limiting factor, not so much the wheels... At least, traditionally.


Also, as always to point it out... Sailplanes are generally are bigger - and something tells me faster than a practical application (perhaps closer to a world speed record velocity though).

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