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Old 01-01-2012, 07:48 PM   #1 (permalink)
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Clean air vs dirty air.

Driving along the freeway recently I was passed by a Toyota Hilux towing a small boat with an outboard motor attached to the transom of the boat.

The bottom of the boat was well clear of the road and the outboard propellor was spinning around fairly rapidly.

The combination went past me a pulled in ahead of me and behind another vehicle in the same lane.

After a few seconds the propellor slowed to a complete stop.

I was wondering about the effects of (aerodynamically speaking) clean air vs dirty air and the flow under vehicle being disturbed by following another car.

Peter

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Old 01-01-2012, 09:38 PM   #2 (permalink)
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I don't know about official testing, but I know that when I follow people on the interstate instead of passing, I get better gas mileage. When I am passing people, my rpm's go up and my fuel consumption goes up. I wonder if it creates a vortex because of the continued disruption in air. Sort of a aerodynamic tunnel?
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Old 01-01-2012, 10:22 PM   #3 (permalink)
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...let their wake "pull" you along, not the other way around.
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Old 01-01-2012, 10:25 PM   #4 (permalink)
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It would be interesting to see how much of the prop stoppage came from him following that other vehicle, or you following him... or both.
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Old 01-01-2012, 10:41 PM   #5 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
It would be interesting to see how much of the prop stoppage came from him following that other vehicle, or you following him... or both.
Wait, does that mean that your aerodynamics could effect the vehicle in front of you, and the wake of the lead vehicle could pull you along helping your fuel economy? Oye, my head hurts. Is there any research on this, it would be interesting to set up a wind tunnel test using various size vehicles, ie..... semi shape, car shape, truck, bus, another car, all in a line and see how the air flows.
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Old 01-01-2012, 11:32 PM   #6 (permalink)
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I have no proof, no math.
But seeing the problem of rear drag as being caused by sucking a big vacuum behind the vehicle - if something were filling in that vacuum from behind, the "lead" car's fuel economy should improve. It doesn't have to create that vacuum all the time, so it gets a free ride, relatively speaking.

Of course, I think that trying to take advantage of this would be even more hazardous than tailgating. Please don't do this!! A square-front bus, following a close distance behind you, would likely do a good job of filling in your rear vacuum. But I wouldn't feel comfortable driving that way. I'd rather have the bus a hundred feet back.
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Old 01-02-2012, 01:14 AM   #7 (permalink)
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It would be interesting to see how much of the prop stoppage came from him following that other vehicle, or you following him... or both.
Frank,
It would indeed.

So far as I could tell the vehicle and boat seemed unaffected by my vehicle since the prop was still stationary for some distance after passing me.

I am guessing since the towing vehicle (Toyota Hi Lux) is fairly high off the ground the air was passing virtually uninterrupted beneath it and then along the underside of the boat (a small - 14 foot or so aluminium runabout on a very basic trailer with a simple beam axle) where it then found the waiting propellor.

Pulling in behind the other car in the same lane would put it in a region of disrupted air so I am guessing the "streamlining" of the boat hull was not able to get things suitably straight to get the prop rotating again.

I have seen and heard of F1 cars having stability problems when overtaking another car especially at high speed due to aero interaction.

It is difficult to find much information on the aero interaction of vehicles although I do recall some research on the inputs into a parked vehicle from a passing bus done in the late '70's.

Peter.

Last edited by Peter7307; 01-02-2012 at 01:28 AM..
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Old 01-02-2012, 11:10 AM   #8 (permalink)
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Quote:
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I have seen and heard of F1 cars having stability problems when overtaking another car especially at high speed due to aero interaction.
F1s don't lose stability when drafting, they lose stopping power. They want to draft down the straight because they can gain on the lead car which has to punch through all that air, leaving very little for the drafter to fight through. This becomes a problem when they have to brake for the corner because the front wing presses the front end down, letting them brake harder- but if there's a car right in front of you then he gets all of that downforce for himself. The lead car can brake normally and the drafting car has to brake early or lock up his brakes and likely overshoot the turn.
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Old 01-03-2012, 07:19 PM   #9 (permalink)
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Nascar

Quote:
Originally Posted by Peter7307 View Post
Driving along the freeway recently I was passed by a Toyota Hilux towing a small boat with an outboard motor attached to the transom of the boat.

The bottom of the boat was well clear of the road and the outboard propellor was spinning around fairly rapidly.

The combination went past me a pulled in ahead of me and behind another vehicle in the same lane.

After a few seconds the propellor slowed to a complete stop.

I was wondering about the effects of (aerodynamically speaking) clean air vs dirty air and the flow under vehicle being disturbed by following another car.

Peter
If you have access to Hucho's book,he has a section on stock car racing.
In a NASCAR two-car draft,the lead car will see a 30% drag reduction,and the trailing car will see a 37% drag reduction.
If the boat behind the Toyota has a small enough frontal area such that it is 'hiding' behind the frontal area of the pickup,the boat can see a zero,even a negative pressure at it's front,with only it's base pressure from the turbulent wake behind the boat/trailer.
Getting behind a really 'dirty' vehicle could have you driving fully embedded within it's wake,effectively taking all the air off you,i.e. the dead prop.
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Old 01-03-2012, 08:48 PM   #10 (permalink)
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Hucho's section on caravan driving shows some complex interactions that depend not only on relative distance but shape of vehicles involved. Results are not all as intuitive as you might think. Interesting...

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