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

[wyatt]

Originally Posted by ChazInMT

Wyatt, I know you mean well

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

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?

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".

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.

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!

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

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.

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.

"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.

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.

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.

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 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 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.

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.

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

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.

[aerohead]

Originally Posted by 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.

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.