Beetle Inspired Aerodynamic Thoughts

[ChazInMT]

Interesting post Patrick. But the Beetle is not flying through the air. It is in Ground Effect which makes a bit of a difference. Compare these 2 simulations.

http://www.youtube.com/watch?v=RqCnwJ0Gfqc

And in ground effect.

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

When the Flow Illustrator site is back up, I'll make one of the ground effect shape in mid air and post it here too.

My point to this is that in ground effect, the air will tend to stay attached on more extreme shapes. Also, as Kamper Bob points out, if the air coming in from the sides has a smooth transition on a 3D shape, it will tend to help the air stay attached. Although I'm not clear on whether Bob thought it would help or hurt flow remaining attached.

But Again I say, this attached flow on an extreme shape is not our friend, it creates a lot of drag.

[KamperBob]

Originally Posted by Patrick

Patrick, good video - thanks! The cylinder is instructive. Attaching freeze frame at 23 seconds capturing turbulent wake. Of course a trip wire can reduce the wake field. And a sphere behaves a little different than a cylinder.

Cheers
KB

[KamperBob]

Originally Posted by KamperBob

Attaching freeze frame at 23 seconds capturing turbulent wake.

Doh! Attachment didn't work. Take 2...

[Patrick]

Originally Posted by ChazInMT

But Again I say, this attached flow on an extreme shape is not our friend, it creates a lot of drag.

I have to disagree with you once again. Maybe this treatise on golf balls will convince you:

"The air hits the front of the ball, creating a high pressure area, and splits around to the sides. The air however is going too fast to make the turn around the back of the ball. It separates from the surface, leaving a low pressure wake like the one a boat leaves in the water. This combination of high pressure on the front of the ball with low pressure on the back is the main source of a ball's drag. This may seem hopeless, but it's not. Maybe you can't control the teenager, but you can put better tires on his or her car. The solution? Dimples. When the surface of the ball is covered with dimples, a thin layer of air next to the ball (aerodynamicists call it the boundary layer) becomes turbulent. Rather than flowing in smooth, continuous layers (a laminar boundary layer), it has a microscopic pattern of fluctuations and randomized flow. Here's the good part: a turbulent boundary layer has better tires. It can better follow the curvature of the ball's profile. It travels farther around the ball before separating, which creates a much smaller wake, and much less drag. In fact, a dimpled golf ball has only about half the drag of a smooth one."

http://www.furthereducationlessontra...rodynamics.pdf

[Patrick]

Originally Posted by 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).

I didn't mean to suggest that the car should be shaped like that. I was just showing that as the angle increases the area of separation does too, which increases drag. The second shot of the airfoil (when it's at a high angle of attack) is similar to the angle of the rear portion of the beetle and the flow is almost completely separated.

[C3H8]

Originally Posted by Patrick

I didn't mean to suggest that the car should be shaped like that. I was just showing that as the angle increases the area of separation does too, which increases drag. The second shot of the airfoil (when it's at a high angle of attack) is similar to the angle of the rear portion of the beetle and the flow is almost completely separated.

i understand, but i think that using an airplane wing shape as an example is not so good. one of the main ways a wing gets lift is through the difference in speed between the air passing above the wing, and the air passing below it. on a beetle, or any car really, i would guess that the air passing below the car is minimal, compared to the air going over the top. that is why i think an airplane wing is a bad example.

although airplane wing shape is still probably better then a brick as far as cars go.

there are some smart cookies on this board. why don't you guys apply you're knowledge to designing a combustion chamber, or a fuel efficient cam for the metro engine? seems to be to be a better use of these talents.

[ChazInMT]

So Patrick, Please explain then how the wing shape you put up will react in Ground Effect, that is my whole point here. For that matter, put something up showing how a golf ball will react in Ground Effect. It has been shown time and again that Golf Ball aerodynamics are for Golf Balls only. If you want to say that Ground Effect and Free Air have the same effect on a given shape, then say that. If there is something about my observation that you think is incorrect, then say that. If you want to start a thread on objects moving through free air, then please do so, and quit trying to somehow figure out a way to bend my thread in order to argue with me for the sake of arguing.

[bnmorgan]

Yes, but dimpled cars don't work otherwise the hailstorm would have caused me to stop every 50 miles and let some gas out of my festiva

Originally Posted by Patrick

I have to disagree with you once again. Maybe this treatise on golf balls will convince you:

"The air hits the front of the ball, creating a high pressure area, and splits around to the sides. The air however is going too fast to make the turn around the back of the ball. It separates from the surface, leaving a low pressure wake like the one a boat leaves in the water. This combination of high pressure on the front of the ball with low pressure on the back is the main source of a ball's drag. This may seem hopeless, but it's not. Maybe you can't control the teenager, but you can put better tires on his or her car. The solution? Dimples. When the surface of the ball is covered with dimples, a thin layer of air next to the ball (aerodynamicists call it the boundary layer) becomes turbulent. Rather than flowing in smooth, continuous layers (a laminar boundary layer), it has a microscopic pattern of fluctuations and randomized flow. Here's the good part: a turbulent boundary layer has better tires. It can better follow the curvature of the ball's profile. It travels farther around the ball before separating, which creates a much smaller wake, and much less drag. In fact, a dimpled golf ball has only about half the drag of a smooth one."

http://www.furthereducationlessontra...rodynamics.pdf

[KamperBob]

Originally Posted by ChazInMT

Also, as Kamper Bob points out, if the air coming in from the sides has a smooth transition on a 3D shape, it will tend to help the air stay attached. Although I'm not clear on whether Bob thought it would help or hurt flow remaining attached.

Chaz, I don't know. Who really does? The vast majority of wind tunnel smoke trails held up as proof show only the flow down the vertical plane of symmetry. A 2D slice through the center of a 3D field tells us nothing about the rest of the field. That leaves intuition, speculation and good old fashioned opinions. (shrug) Even with PIV and CFD how to convey the complex data is still hard stuff. Tuft testing seems like one of the most pragmatic tools. Gee, what if some grad student made his/her paper just about one classic aero problem (say, a 2002 4WD Tundra xCab - wink) but using all the tools and comparing the results for similarities and differences. Wouldn't that be cool?

Cheers
KB

[aerohead]

I recommend that you read through the entire sticky on Seminars.If you'll do that,you'll have touched on aero at a Master's Degree level and won't have to speculate about anything.
You're thinking and that's what we need,but without exposure to basic fluid mechanics you're never get beyond a theoretical cul d'sac.
Flow is not attached on the back of the Beetle,new or otherwise.If you'll dig into the sticky you'll discover why.
Eat a fish for a day or learn to fish and eat for a lifetime.