Beetle Inspired Aerodynamic Thoughts

[ChazInMT]

I decided to post this outside of the "New Bug .38 CD why?" thread because I felt it would be bending the thread a tad too much and warrants a discussion of it's own.

Here's what I've got.

I had another epiphanic moment a few mornings back when I woke up and 2 & 2 came together a bit more for me.

Here are my thoughts:
1st- Attached flow: This was my first thought on what is needed in order to have a fuel efficient / Low Cd car. As long as flow remains attached.....Bada Bing...yer Drivin for dollars.
2nd- The Ideal aero template has been held up as the shape for which when we deviate from it, flow is not likely to remain attached, and you'll loose Cd.
3rd- These first 2 things have not added up for me for 5 months now, because I have seen in numerous real life examples that a shape can be drastically "Under" the ideal aero shape and still have attached flow.

My thought was that the air had a propensity to follow a surface which has a smooth transition beyond the Ideal Aero Template because the shape helped it to cling. In studying the Flow Illustrator movies I made of the Beetle, I noticed that as I added roof spoiler length, the air that spilled off the back of the spoiler really dropped sharply to "Fill the Void" behind the car, essentially matching the shape of the rear end of the Beetle. Huh. I thought the air wanted to follow the ideal aero template…all else being equal. This did not add up.

So here is what I’ve come to understand.

Air will follow this shape.



I know….This is tantamount to blasphemy saying flow will remain attached to this shape, but how else do we explain flow remaining attached to the back of a Beetle? This is what I call a Beetle Drop shape.

Here is a flow illustrator I made of it.

http://www.youtube.com/watch?v=_VitTX6b9Jo

This is not saying this shape is efficient, because it certainly isn’t. The air dropping down so quickly behind this Beetle Drop shape is travelling relatively fast, and therefore creating a substantial low pressure area, which I have been calling dynamic drag.

Here is an old idea on an aerodynamic truck cap.

http://www.youtube.com/watch?v=vPVcIZCJrkI

Here is the Beetle with a roof spoiler.

http://www.youtube.com/watch?v=fw2qkR3bFoM

So why is the “Ideal” aero shape so efficient? I think it is because the air still wants to “fill the void” and drop down sharply, but because the shape is still there, it forms a higher pressure on the back side of the shape, thereby reducing the low pressure on the back of the vehicle, in a sense pushing the vehicle instead of pulling it. So it stands to reason that the longer the shape the better the effect.

Here’s a basic illustration of what I’m trying to say.



There are a bunch of shapes I wanted to look at in flow illustrator but the site is all jammed up right now.

I know this is a bit simplistic, but it helps me makes sense of things. It explains all the reasons why the aerodynamic principles we discuss on this site fit in with what we see in the real world.

Basically what my whole point to this is:

The ideal shape takes advantage of airs propensity to want to drop in quickly behind the shape passing through it, by forming a higher pressure area on the back side of the shape.
If the template were shorter, the air would form a lower pressure behind it, if longer, skin drag starts to grow quicker than the shape advantage.

[lunarhighway]

I don't think there's a high pressure above the shape as you drew it, there's actually a low pressure, as air needs to speed up to move around the shape and as such pressure drops, this is why wings work and why racecars need little upside down wings to push them to the ground.

I'm no aero expert, and i hope they'll be able to provide some better explanation on the matter, and correct me if i'm wrong, but there's a few things happeing when a car moves trough air. One this is air getting compressed in front of the car (this is why the radiator is there) as it can't move out of the way fast enough, the other is low pressure being created behind the car. Than there's the air moving around the car.

The ideal shape allowes that all air can move smoothly around the shape and not pile up, and than gradually slow the air down again and bring it back to it's original position. If this shape is scaled up to contain a conventionals car interior, it becomes far to long. but chopping of the tail will still provide an efficient design, because the sharp rear cutoff will ensure that as teh care mores a virtual cone of air stays attached to the back and the airflow comming of the car continues to slide over this cone.

This is esentially what real cars with good aero do as well, and why aero eficcient cars almost always have sharp rear cutoffs, even when they visually don't theres often a hidden straight edge in the taillights or the C pillar.

When you get it wrong and the rear shape is rounded (as with the beetle) there's not much surface for this virtual cone to form on, and also the air comming of the roof will trip into this lower pressure area, creating turbulence.

Therefore a car like a prius seems to have a fairly large rear area where the beetle has a rounded trunk, that looks more organic and aero friendly, but really isn't.

[Dr. Jerryrigger]

with connected flow think of the extremes; a very soft angle, so soft that it goes on for miles. This will cause lots of drag, because there is so much surface drag. The other extreme looks a lot like the beetle. The steep angle makes a dramatic low pressure zone. If it were steeper, to the point that there wasn't connected flow, then it would be a turbulent mess back there, but may have less drag. The low pressure formed causes lift, this is seen with a softer angle, but it's far less dramatic. A angle of 11-12 degrees seems to be best for aerodynamics. (which would make for one long car)

[ChazInMT]

Originally Posted by lunarhighway

I don't think there's a high pressure above the shape as you drew it, there's actually a low pressure,

I specifically wrote "Higher Pressure" I did not write "High Pressure". It's a higher lower pressure instead of a lower low pressure. The pressure is killin me here.

[Dr. Jerryrigger]

I'll try to explain my understanding better (FYI: I have no formal training in this sort of thing).
So to start we have an object of a given size and lets make it a ball. The front of the ball is a fine shape, air moves out of it's way as well if not better than any other shape. The air in front of it is of an increased pressure, which causes resistance, but this in unavoidable for an object of it's size. So lets look at the back. The air is unable to hold the steep curve all the way down (except at slow speeds). So to simplify things lets cut the ball in half horizontally, and place it on the ground. The front still works the same and the back still works the same (not so great). If you gradually extend the back end air flow will (at some point for a given speed) stay "connected". The longer it gets the higher this speed can be (everything changes when the speed of sound reached, so this limits the angles worth looking at) Also an angle that is extremely soft will result in a lot of surface drag, as there will be a lot of surface, but for this discussion, that is not worth thinking about (no car can be 80yards long).

So air gets forced up by the front of our half ball, then it wants to stay in it's new place, but the ball needs to end, so we need to bring it down to it's original place in space. If we do this too abruptly we will make turbulence, if we do it less abruptly we will make a smooth flowing low pressure zone. A low pressure zone makes drag, a from of drag known as lift (rather important for an airplane's wing). This low pressure pulls the object up, but this shouldn't be that dramatic at driving speeds in comparison the the weight of a car. So the softer the slope the slower the air needs to get pulled down, and therefor the low pressure is less dramatic, and therefor the drag is lower.

[Dr. Jerryrigger]

This sloppy image may help more, Where the lines are close is high pressure (unavoidable), and where they are far apart there is low pressure.

If you think about it, the high pressure is going to force air up a lot, so you will need to bring it down slowly to get it all.
and speed make a big difference on ideal shape:

vid of round object moving slow

vid of round object moving faster

[KamperBob]

Chaz, some food for thought. Your simulations are 2D. The Beetle is 3D. Air coming around both sides plays a major role. When air is displaced it compresses. Once the object passes it can expand. It cannot expand immediately so a sphere or cylinder draws a wake. In 3D the longer the roof (think limousine) the more time for top and sides to equalize with each and blend with surroundings. So, less stored energy to be recaptured by the tail. Thinking about the physics and math, it stands to reason that the ideal shape might be related to a sinewave or gaussian bell curve. Those are two of nature's best shapes. Take fish for example.

One more thing. It's possible this guy know something we don't.


Cheers
KB

[Patrick]

http://www.youtube.com/watch?v=Vjk9U...layer_embedded

[Grant-53]

You are on the right track but there is much more to it. The Type I Beetle is a high drag shape (Cd 0.48) with lots of lift. Compare it with the Audi 100 (Cd 0.31) back when SCCA started its Show Room Stock Class 40 years ago. The Prius is down to Cd 0.26 last I heard and the Aptera 3 wheeler is based on Dr. Alberto Morelli's Cd 0.05 "ideal" shape. I recommend "Race Car Aerodynamics: Designing for Speed" by Joseph Katz PhD. The math is high school algebra and there are lots of pictures and graphs. Also check the Wikipedia entry for NACA wing shapes.

[C3H8]

Those drawings of wings are airfoils that are intended to create lift, so that low horsepower airplanes can fly. You do not want a shape like that because you do not want lift in a car, nevermind an eco car.

A better shape would be that of a teardrop, or a tuna fish. (not a tuna fish can though).