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Old 09-01-2011, 01:51 AM   #1 (permalink)
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Up & Down Aerodynamics, Another Way To Look It. Perhaps?

I have an “Aero” idea, coming at it from a perspective of the air itself and what it is trying to do. My main idea is that the air molecules we drive through on, let's for ease of understanding say, a calm day, pretty much just want to be where they are. Not unlike a bunch of guys standing around. The particular air molecule we are interested in is 16” off the ground and it is the one which is going to go over the top of the car, dead center, we shall name this guy Alan. Let’s have the car going 45mph or so, as Alan rises up the hood, and windscreen of the car, he is being forced up against a bunch of other guys, that aren’t too particularly thrilled to move up either, and as such, are pushing back on Alan. At the moment Alan is on top of the car, he is seeing the greatest push back from the guys above him. If he at this point is just dropped, in the case of a 0% aft body template, than he would just drop back down, trying to get to the point he was before the car came along, and he would not be able to push on the back side of the car at all, all the energy that went into raising Alan up high, would just be wasted as he drops into the abyss created by the passing car. A Scion Xd has this characteristic.



If the car is shaped right, then Alan will be able to slide down along the back of the car and push it forward a bit as he goes, picture a wheel on the end of a spring pushing down on a sloped surface. The energy put into raising Alan up would now be used to help push the car forward.



Since Alan has mass, and also is rubbing up against a bunch of other guys creating friction, he is not able to move too fast in the downward direction, so his ability to accelerate, and, maintain pressure on the surface below him has limits. As long as the shape below Alan is gradual, he will help to push the car forward as he tries to get back the his 16” high place in the world all the while being pushed by the guys above him. Cue the VW XL1.



So, simplistically looking at it, all Alan does is move up and down as the car goes by. To him he ends up where he started. So here is where the leap of logic takes place…..we look at flow as being a horizontal motion, which, relative to the car, it is. But what does Alan the air molecule see? Up & down, that’s it. So looking at it from this perspective, attached flow according to Alan, is being able to get back to where you started by riding down the back surface of the car moving under you while keeping pressure on its surface. If the shape changes too fast, Alan loses contact with the surface, and can no longer push on it, so his “spring energy” is not returned to it.

I liken this to when you squeeze a fish and it shoots out of you hand. If pressure is maintained as it is builds in at the proper pace, it will help to propel the car forward.

I hope this make sense. It sort of unifies what we are trying to accomplish by looking at the movement of air not from the perspective of the car, but by the air itself. In roofing, you make a better roof by trying to think like the water drop. I think in aerodynamics, it would help us to think in terms of the air molecules movement instead of the ocean of air rushing over the skins of our cars. Where does Alan want to go? What forces can he impart? What forces are acting upon him?

So to conclude, when thought of from the perspective of stationary “Air Drops”, Alan et all, the air primarily moves up and down. (Obviously it swirls about a bit and moves forward some, but really I doubt any air molecule is displaced by more than 50-70 feet) The learning point here is to look at this from the real perspective of the air as we drive through it, not a wind tunnel with the air passing across the car. The shape of our cars determines whether the air helps to push the car along as it moves through this stationary air, or, lets the air loose contact and therefore, waste the energy put into it to move it out of the way. Aerodynamics in our case, optimizing the drag coefficient, is trying to reclaim the energy we put into the air when we push it out of the way to move through it.

I could go on about the implications of this idea, for quite some time. I thought this to be a good intro.

So, Duh? Or am I missing something? Is this a basic fact known for centuries that I've come to on my own? I never seem to be the first one to think of something. I've never seen air discussed from this perspective, we're always debating what it looks like as it flows in primarily the horizontal direction.

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Old 09-01-2011, 02:15 AM   #2 (permalink)
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Ummm, I think you gots the big arrow at the top of the windshield backwards--that's where pressure is least.
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Old 09-01-2011, 08:56 AM   #3 (permalink)
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ChazInMT - I too have contemplated along this line of thinking a few times in the past. It seems like somehow we ecomodders and the car engineers need to think more about how our vehicles are blasting through a somewhat stationary wall of air(depending on circumstances of course) and how the air splits up and gets around our vehicle, instead of focusing primarily how a stream of moving air slips around our vehicles.

I don't recall seeing any other threads with this type of idea before. I really appreciate you taking so much time to work on your initial post with the idea, pics, and the explanation.

I'm thinking there could and should be some nice dicussions to follow.

Thanks!
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Old 09-01-2011, 09:44 AM   #4 (permalink)
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Otto, Thanks. Der, frickin Bernoulli. It still seems, actual pressure aside, there is something to what I'm on to here. The reason the pressure is so low here (Where I say it's highest) is because the air is moving fastest, so it's sort of a dynamically driven low....actually it isn't sort of...It Be a Dynamic Low.

So what is Alan feeling as the area between the top of the windshield and the front of the sun roof dash underneath him? What force is acting on our "Air Drop" friend? Keep in mind, we are thinking in terms of sitting by the side of the road and we're watching a neutrally buoyant sphere our Air Drop Alan as it passes over the top of a car that goes by, how does it move? What force does it feel?

Last edited by ChazInMT; 09-01-2011 at 09:50 AM..
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Old 09-01-2011, 11:45 AM   #5 (permalink)
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I'm questioning the direction of the arrow in the diagram.



Not saying the first one posted is wrong, just want an explanation of why it is not the way I've revised it.

Also the negative pressure or vacuum of the Scion Xd end/behind is what "PULLS" Alan down, he is not pushed, right?

See the rolling vortex of air behind the Chevy Volt recently posted.
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Old 09-01-2011, 12:24 PM   #6 (permalink)
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Kach, here is a more detailed wedge.



As far as Pushed or Pulled, again, semantics I suppose. I'm thinking that the air molecules all bunch up as the car passes underneath and forces them to move up. So your question got me to realize instead of "Pressure" maybe we should be talking in terms of "Density", the number of air molecules or Alans that exist in a given volume. When I think of it this way, then the denser air above will want to go into the less dense air being dragged around in the back of the Xd. So is Alan being shoved down by all the guys above, or pulled down by the few that are being sucked down the road? Either way, Alans goin down.
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Old 09-01-2011, 12:29 PM   #7 (permalink)
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Here are some revised drawings of the Scion & VeeDub.



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Old 09-01-2011, 01:19 PM   #8 (permalink)
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Quote:
Originally Posted by ChazInMT View Post
Kach, here is a more detailed wedge.

As far as Pushed or Pulled, again, semantics I suppose.
Yes, thank you for altering the diagram, the bearings help me see that for every action there is an equal reaction. You were taking reaction, I was talking action.................we are now on the same page.

Think about the parasitic drag on boat hulls, air does much of the same, it may not push as you want/think.

http://en.wikipedia.org/wiki/Parasitic_drag
Quote:
Parasitic drag (also called skin friction drag) is drag caused by moving a solid object through a fluid medium (in the case of aerodynamics, more specifically, a gaseous medium). Parasitic drag is made up of many components, the most prominent being form drag. Skin friction and interference drag are also major components of parasitic drag.
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Last edited by kach22i; 09-01-2011 at 01:32 PM..
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Old 09-01-2011, 02:45 PM   #9 (permalink)
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Just like to add to the cursory discussions on simple concepts of lift, drag, pressure & velocity. Unfortunately our discussion images/even video flow analysis are usually done in 2D, but further evaluation really requires 3d visualization. Vehicle design and actual airflow dynamics are realized in three dimensions. If you only choose a 2d cut plane of any shape, the flow will actually give you different values if it is done in 3d analysis. Armchair aerodynamicists, ecomodders, do not have (yet) easily available 3d flow aerodynamic performance tools. Maybe soon.

http://ww1.prweb.com/prfiles/2009/01...s15cropped.png

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Old 09-01-2011, 04:11 PM   #10 (permalink)
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Nice links, in another forum a guy was using Corvin and explained a little about what I was looking at.

Cfd
Quote:
Originally Posted by Idesign View Post
In response to George, there is in fact airflow calculated over the entire car, we plant "seeds" so we can pinpoint the effect of specific areas of the car, the flow lines you see are a result of that, it is user defined what you see. If we want to know where the flow goes after it hits the mirror for example, we plant a seed on the mirror and it traces back. Conversely, if we want to know where the air is coming from that is entering the intercooler we plant a seed back there and trace it forward. The body is also broken into discrete panels so that force measurements can be taken and a contribution of down force and drag for each component, doors, hood, Mickey Mouse antenna ball, etc. can be assessed.
The second video in this link shows what's going on outside of the slipstream. It actually looks alarming to me for some reason.

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Here is what I'm used to seeing: Click link to see Chevy Volt.
http://ecomodder.com/forum/showthrea...s-11183-4.html

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George
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1977 Porsche 911s Targa
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Chin Spoiler:
http://forums.pelicanparts.com/off-t...effective.html

Rear Spoiler Pick Up Truck
http://forums.pelicanparts.com/off-t...xperiment.html

Roof Wing
http://ecomodder.com/forum/showthrea...1-a-19525.html

Last edited by kach22i; 09-01-2011 at 04:22 PM..
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