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.

http://i53.tinypic.com/1z6e548.jpg

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.

http://i54.tinypic.com/2aalwcm.jpg

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.

http://i55.tinypic.com/1zcn250.jpg

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.

Ummm, I think you gots the big arrow at the top of the windshield backwards--that's where pressure is least.

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!

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?

I'm questioning the direction of the arrow in the diagram.

http://i184.photobucket.com/albums/x...lt-2aalwcm.jpg

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.

Kach, here is a more detailed wedge.

http://i51.tinypic.com/1ze9z83.jpg

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.

Here are some revised drawings of the Scion & VeeDub.

http://i54.tinypic.com/2rroq3r.jpg

http://i54.tinypic.com/148n2it.jpg

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

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

Exa PowerVIZ

Nice links, in another forum a guy was using Corvin and explained a little about what I was looking at.

Cfd
The second video in this link shows what's going on outside of the slipstream. It actually looks alarming to me for some reason.

Exa PowerVIZ

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

up and down
 

The face of an analog watch or clock is enough to measure your distances.I tried to use this as an example some time back.
*The air 'strikes' the clock at 9:00 o'clock,flows up over the 12:00 o'clock position and separates at 4-seconds after.
*This is what every golf ball does.
* If you superimpose a watch dial in place of the clockface for the 'leading' part it creates essentially the 'Template'.
* the curvature of a clock from 12:00,to 4-seconds after,describes a separation-free flow and contains Mair's maximum 22-degree tangent angle.
* If you take a string and measure the distance of the 'nose' and 'tail',it reveals that the air over the tail requires twice the distance and twice the time to recover from the displacement created by the displacement of the nose.
* When this aft-body section is compared to that of the 'Template' you see a remarkable similarity.

Hey Phil, thanks for responding. I know it is sort of an odd Idea, but what do you think about my line of reasoning here? I'm trying to get the idea across that:

1) The air wants to stay put, the actual air does not move much in relation to where it starts after the car passes through it, much like an ocean wave, where the water molecules end up in the same place from where they started.

2) In trying to fill in behind the car, if the air can maintain contact with the skin of the car, it essentially gives back the energy it took to move it out of place.

So in essence, we end up kind riding on our own wave we create as we drive through the air, this is the actual force behind creating low drag, we reclaim the energy.

Am I onto something or should I put away the crack pipe? Does it make sense what I'm saying?

I have the template down cold, thanks for that. Our discussions in the thread "Cinderella" were the turning point in my understanding of a lot about aerodynamics, and I really, really appreciate your patience and willingness to teach.

flow separation: Tiger Woods meets Cinderella,or 5-seconds to a perfect roofline

Much Respect, Charlie

BTW, In hind sight......Tiger Woods probably was meeting with Cinderella :eek: ....Very Prophetic of you. :D

reasoning
 

Charlie,your reasoning is as I think of it.
Somewhere I used the illustration of a mechanical ignition distributor or camshaft,in which,at a particular rpm, the points or valves would 'float'.
The air column 'spring' above the car would not be able to push the air back down if the 'cam profile' of the car's aft-body were too aggressive.
The float would lead to turbulence.

OK Phil, you say turbulence, I say returned energy wasted. I think the turbulence is a result of the air being "Hung out to dry" and not maintaining contact. But the real reason it takes more power when you exceed the template curve is that the air column spring is no longer pushing the car, returning the energy it took to raise it up.

I think that this would explain why sometimes flow is attached when the template is exceeded too, if you look back at Cinderella I had numerous examples of attached flow despite an exceeded template. If the flow is attached, but not really pushing on the car, what good is it? So the template is valid for getting the optimum Cd, there are times maybe when a car shape is faster than the template and flow remains attached, but it isn't giving its energy back to the car. Ha. Making more sense everyday.

Chaz (and phil) thanks for expressing this so clearly. I am reminded a bit of the discussions regarding potential energy return from closing valve springs :)

FWIW, I finally broke down and installed ubuntu on a virtualbox (stuck w/windows as host, don't ask) and installed and ran the first openFOAM and parafoam tutorials :) I will have to see how serious people are about helping to sort it out, or if it is just talk. Too much speculation in this arena...
OpenFOAM&#174 - The Open Source Computational Fluid Dynamics (CFD) Toolbox

openfoam will need it's own thread (forum?) but one thing that is interesting is that once you have a 3d model (mesh) you can analyse the forces on the tiny individual panels, and it should be possible to just look at the forces for, say, the tail section and see if they result in a forward vector (assuming openfoam is up to the task of detecting this phenomenon).

The drawings of the Scion and VW are still wrong. The area right over the A-pillar is the lowest pressure, thus should be pointing up. Assuming a flat roof (station wagon, van, Scion) behind that the pressure increases until at some point, the pressure is zero, after which the pressure starts to go up (yes, we can get some down-force on the back end of our station wagons!). The streamlining template exploits this fact, and starts at the location where pressure is zero (relatively) and starts to taper, doing it's best to maintain zero pressure. If there is to be a positive pressure maintained, we would wind up with a longer than needed tail (think of this pressure as friction [skin drag], not a force pushing you forward). If a negative pressure is maintained, we wind up with a shorter tail, but higher Cd and lift on the tail. If length is not a constraint, we would want to go for the Aerodynamic Streamlining Template every time (or something close anyhow). When we start clipping the tail end off, a lower drag option may be to take that shorter length, but maintain the full boat tail. This may not be true if we cut off 10% of the tail, but is true (in my experience) for short tails, like on a Prius, Insight, or the SedanKamm I built for my car.

I see what you're saying, Chaz. Wyatt I think you two are on the same page, but different paragraphs. We're saying after the zero pressure point (the "floating valves" scenario), the air will lose its upward momentum and want to go back down, creating very slight pressure for the rear slope of the car, and pushing it forward ever so slightly. Yes, a T77A will have more return than a Prius or Insight, but they're both doing better than the Scion.

DCB, does one have to build the model in that program or can it be imported?

Wyatt, I know you mean well, but I cannot understand what you're trying to say. You talk about zero pressure, which would imply a vacuum, and negative pressure like it is somehow less than a vacuum. Are there relationships maybe you're trying to convey?

Then you make it sound as if you will get just as good of a result by not following the template curve and just making whatever looks like it may work for an aft body. You throw around angles like flat surfaces will suffice without any real need to transition into them. These are not backed up by any logical reasoning.

Yes, putting something in to fill the space will help. But will it optimize flow? No.

I only say following the rules of the template will optimize what we want to achieve. In this thread I am trying to explain how I see it actually working. I have never been one to take things at face value, I want to understand how and why it works like it does.

I guess one of my main points, and lines of reason, is that to move an airdrop up 3-7 feet in the air (13ft for slowmover), it takes energy, actual work. If some of this energy is not returned to the vehicle, than our fuel mileage would always be crap. What mechanism could there be for returning the energy? This is how I came up with this idea of pressure pushing us along. The answers I came up with support why the Aero Template works.

Skin drag, is btw, essentially a non-issue for cars, it really applies to laminar flow systems, which car bodies are not, and only would make up a small difference in the overall drag created by going faster. Also since a car with or without aeromods will have about the same skin area, the drag would be equal. It has been brought up before, and has always been seen as a non-factor.

I still say that the density of the air, and air pressure are 2 very different things when a moving vehicle is discussed. In all the equations I see, the velocity is squared over density which is not. So twice the speed change and twice the density will yield much lower pressures. We more than double velocities all the time, so pressure drop will always win. I don't even really think the air density increases by more than 40% even under the most extreme condition, so it isn't thought of as much of a factor, yet I say it is the density change across the length of the car that I think is the mechanism for returning the energy.

If you want to make some point, please explain it logically, tell us why you think the way you do. If you can't explain it, then I have a hard time lending any credibility to your theories. You tell me I'm wrong about the pressure, but offer no real reasoned alternative theory of your own to tell me how it does work. Read the paragraph you wrote above again and try to make bullet points out of what you are trying to say, and then explain each one. I know I have done this in the past, and then I never post anything because I realize, I don't have it really figured out well enough to offer a good alternative. Several times I wanted to tell Aerohead he was nuts, well you know, like bad nuts....we all know he's nuts....but I mean like, nuts and stupid, not the nuts and brilliant I now know he is.

You say several things which imply that the template is only good for a full boat tail, this as far as I know has never been put out, and in fact, truncating the template shape will yield the best result for a shorter body length. Aerohead quantified these returns in this thread here, I refer to it often. It is a saved favorite bookmark.

Aero Drag Reduction Quantified

So, the template is a tool to be used to get the best performance out of a given modification. Will other things work, Certainly. Are they optimum, certainly not.

It's not just a boat pushing the ocean aside.
The ocean also pushes back at the boat, and closes the wake.

I really do, I am not trying to be rude, just trying to explain my perception on pressure drag.

Not a vacuum, zero pressure relatively (ie. not creating lift, not creating down-force). I suppose this could also be said as "local ambient pressure".

Not at all what I meant. I think I may have tried to cover too much ground, and like a toddler, since I understand, you understand, right? Ok, I will try again...
The streamlining template is the ideal. If you want to go longer, fine, but you will incur additional skin drag, not to mention down-force (perceived weight) additional weight (from additional materials). I for one, would never pass the 15 degree mark, since I have only seen this work in wind tunnels, and never in real world driving. This would make my ideal shape longer and heavier, but from what I have seen and experienced, I feel it is more likely to work better in the long run.
The streamlining template is the ideal. If you want to go shorter, fine, but you will incur additional base drag, but you will save weight. You can accomplish shortening two ways. You can lop a bit off the end (10 or 20% or however much) and do quite well. However at some point you will see that taking a steeper angle (thus increasing the slope's pressure drag, but reducing base pressure drag) will win out. This means you have to do some testing to get the optimal shortened shape. On the other hand, this means you get to do some aero testing!

I stand by what I said above, optimized flow depends on the length you plan on taking your shape to, thus the streamlining template is the ideal, unless you don't plan on following it all the way out.

"Pressure pushing us along" would be thrust. The aerodynamic streamlining template exploits zero pressure as I tried to explain above. It generates no lift, and no downforce (at least none aft of the high point of the roof). With no downforce on the sloping aft body, how will we generate thrust? We won't! If we go longer than the ideal, we will (ideally) generate some down-force and thus a negligable amount of thrust, also incurring penalties of mass and skin drag. If we go steeper than the ideal template, we will generate some lift, and thus additional pressure drag on the sloping area. The steeper we go, the more pressure drag, as we literally try to pull a vacuum in the air behind us. It's not that we are magically tapping some hidden source of energy by following the AST (getting tired of typing that out), rather, we are eliminating a very low pressure area in our wake that is literally trying to pull us backward.

Non factor? perhaps, but it's there. I think the additional mass might be worse, but I guess we would get to do some testing to find out! The same skin area? My Suzuki Swift without any aeromods vs my Swift with a boat tail following the AST will have very different skin areas. Granted, if you follow the AST, that additional skin area will see no drag (ideally), so the "percieved skin area" would be (should be) the same.

If you go to "Tools" and "Aerodynamic and Rolling Resistance" you can pull down and see how air density changes with temperature. 1.341 kg/m^3 at -10C to 1.164 kg/m^3 at 30C. -10C is 14F, 30C is 86F, so these don't cover the range of temperatures most people would see in a year (I have seen -60F in Wisconsin, and +114 in Alabama, others have probably seen higher highs, and lower lows). Just providing info.

If what I have elaborated above does not help you understand what I am trying to say, please let me know. I can make bullet points, pictures, free body diagrams, etc. but would prefer to not spend hours in paint drawing pictures if I don't have to.

Thanks for the link. I actually tired to find this information since I knew I had seen it. What I am talking about is likely the "very truncated" template. Something in the 10, 20 or 30% area. In these realms (and perhaps further out) we could see better Cd numbers with a steeper back, with increased slope drag but decreased base drag.

pressure
 

Chaz,I think Wyatt is following proper conventions from fluid dynamics and boundary layer theory as propounded by Schlichting.
Air density will be fixed by temp/un-corrected baro pressure/and elevation.The density doesn't actually change.Only pressure.And typically,yes,the lowest presure(negative when measure against the forward stagnation pressure)does occur near the windshield header,or 12-o-clock position on a golf ball ( max. camber point on Template ).
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Some wind tunnel investigations involved simple models of no curvature whatsoever,and drag minimums were ascertained at a given 'angle.'I don't recommend there use but they'd be way out ahead of nothing.
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All streamlines are equipotential.When the vehicle deforms there position,they will adjust their interior velocity to accomodate a change in 'length' so as to conserve the 'volume' of flow.The change in velocity mandates a change in pressure to balance the volumetric constant.
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The trick of streamlining is to reduce or eliminate separation.If a vehicle has a decent forebody,then the drag will be governed by the rear shape.As far as I know,if you're looking for low drag,build to the Template as far as you dare,and make your chop.Anything else,technically will have higher drag.Separation is impossible until the truncation.Plug and play.You want lower drag,just go back further.
If you stay with the Template,for any given aft-body percentage,you should get the lowest drag.You can see this in the fuselage chart in Template Part-E.
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Without everyone taking Fluids it may be hard to get a common 'language' to describe the physics of airflow.Lets keep working on it.