Up & Down Aerodynamics, Another Way To Look It. Perhaps?

[aerohead]

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.

[ChazInMT]

Originally Posted by aerohead

The face of an analog watch or clock is enough to measure your distances.I tried to use this......

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 ....Very Prophetic of you.

[aerohead]

Originally Posted by ChazInMT

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 ....Very Prophetic of you.

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.

[ChazInMT]

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.

[dcb]

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

Originally Posted by botsapper

...Armchair aerodynamicists, ecomodders, do not have (yet) easily available 3d flow aerodynamic performance tools. Maybe soon.

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

[dcb]

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).

[wyatt]

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.

[Sven7]

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?

[ChazInMT]

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.

[euromodder]

Originally Posted by ChazInMT

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?

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