Anyone have 800 Lbs of clay handy? New mythbusters episode

[Matt Herring]

Originally Posted by Jabber

VIDEO: Mythbusters test golf ball-like dimpling effect on fuel economy (*Spoiler Alert!*) — Autoblog Green

They actually got a 15% increase in fuel economy by "dimpling" the car.

Their methodology is hardly a scientific experiment...far from it.

Buy this and come back in a year with your results...it's not going to help you get better FE...sorry.

MPG-Plus™, Drag Reducing Technology, Improve MPG, Vehicle Wraps, FastSkinz™

Better off taking your money and putting it on the #3 horse in the 4th race. Next they will be telling us that "shark skin" car coverings decrease drag because sharks are "really fast" in the water!

[carz89]

Originally Posted by Matt Herring

Next they will be telling us that "shark skin" car coverings decrease drag because sharks are "really fast" in the water!

Just imagine how fast sharks would be if they only had dimples!

[qbee42]

Originally Posted by hyo silver

Sounds plausible, but why does it work? I remember posing this question to my high school physics teacher, and he said it works for golf balls, but not cars. The dimples, he said, actually increase drag, but because of the spin of the ball, more lift is generated, and it flies farther. On things that are only somewhat streamlined, the dimples would help by bringing the airflow closer to the surface. On really smooth surfaces where the boundary layer flow is already laminar, dimples would only cause turbulence. When I asked about pressurising the inside of the body, and letting air out through tiny holes in the base of the dimples, to make little 'ball bearings of air', he said I was making his head hurt.

Maybe I should watch the show, eh?

Golf balls are round. By their very nature, round objects generate a lot of drag. There is no way to get anything close to laminar flow over a round surface. The dimples induce turbulence, which helps reduce flow separation drag. You see something similar with the kammback rear end of the Prius. The flow is going to separate there, so the kammback induces turbulence to fill in the void, thereby reducing drag. As other posters have also noted, golf balls spin at a very high rate. The dimples drag the air around with them, producing lift. It's a twofer: lower drag and increased lift, all for one low price.

As for your idea about pumping air out through little holes, it's not a bad idea but you are doing it backward. The process is called boundary layer control, and involves sucking air through thousands of tiny holes. It works by sucking in the turbulent layer of air adjacent to the skin of the aircraft, allowing for a longer laminar flow. Several full scale jet aircraft have successfully used this technique.

Now we can look at micro dimples, sometimes called shark skin. This is an idea that has been exploited for years by the Navy and the designers of high performance racing yachts. Marine animals beat us to it by a long shot, which is why it is often called shark skin. The idea is to cover a surface with tiny bumps or scales. The tiny bumps or scales induce a micro layer of turbulence which rides along the surface of the object. The turbulent layer is so thin that it doesn't easily perturb the adjacent laminar layer, but instead buffers it. You can think of it as a lubricating layer helping to speed the laminar air on its way.

Tom

[ALS]

Thus the new body suits Olympic swimmers are using.

[radioprius1]

Originally Posted by qbee42

Golf balls are round. By their very nature, round objects generate a lot of drag. There is no way to get anything close to laminar flow over a round surface. The dimples induce turbulence, which helps reduce flow separation drag. You see something similar with the kammback rear end of the Prius. The flow is going to separate there, so the kammback induces turbulence to fill in the void, thereby reducing drag. As other posters have also noted, golf balls spin at a very high rate. The dimples drag the air around with them, producing lift. It's a twofer: lower drag and increased lift, all for one low price.

As for your idea about pumping air out through little holes, it's not a bad idea but you are doing it backward. The process is called boundary layer control, and involves sucking air through thousands of tiny holes. It works by sucking in the turbulent layer of air adjacent to the skin of the aircraft, allowing for a longer laminar flow. Several full scale jet aircraft have successfully used this technique.

Now we can look at micro dimples, sometimes called shark skin. This is an idea that has been exploited for years by the Navy and the designers of high performance racing yachts. Marine animals beat us to it by a long shot, which is why it is often called shark skin. The idea is to cover a surface with tiny bumps or scales. The tiny bumps or scales induce a micro layer of turbulence which rides along the surface of the object. The turbulent layer is so thin that it doesn't easily perturb the adjacent laminar layer, but instead buffers it. You can think of it as a lubricating layer helping to speed the laminar air on its way.

Tom

Thank you for this explanation. Do you think there is any use for this type of stuff for vehicles? That is, are the MythBusters on to something?

[jburns]

Originally Posted by carz89

Just imagine how fast sharks would be if they only had dimples!

I tried to put dimples on a shark once. He didn't like it.

Best wishes.

Stumpy

[dogfriend]

What about adding shark fins? That would look cool.

[qbee42]

Originally Posted by radioprius1

Thank you for this explanation. Do you think there is any use for this type of stuff for vehicles? That is, are the MythBusters on to something?

Quite possibly. There are a few complicating issues. The first is that most cars are not that aerodynamic in the first place. The special surface is only going to help a streamlined surface. If you are pushing a brick through the air, it isn't going to help much. The two other issues are cost and aesthetics. Consumer products such as automobiles are very sensitive to costs. Likewise, aesthetics play a big part in car sales. If a car looks funny, it's not likely to sell well. Look at all the criticism of the Prius and people thinking it looks dorky. It looks like it does to get good aerodynamics.

I suspect coatings may become used on cars, but only as efficiency begins to take top priority. It will be a while.

Tom

[dogfriend]

Originally Posted by qbee42

. If you are pushing a brick through the air, it isn't going to help much.

You don't think it will work on my GF's motorhome?


Hey, that gives me a great idea - I will come out with a shark skin kit (shelf liner with contact adhesive) for RVs. I need something to hawk on Ebay if I don't get a job soon.

[qbee42]

I think shark fins were tried in the 1950s.

Tom