Mythbusters tests "golf ball" dimples

[Christ]

Originally Posted by Otto

Once talked to a guy who said he'd been a military sniper, and that he roughened his bullets, giving them better speed retention.

That's what the man said. I have no idea whether valid, or not.

It was kind of a facetious question, and the answer is no, despite what some think. Bullets are traveling at speeds in excess of 1,000 mph at times, their boundary layer at that speed is basically the thickness of the bullet. The only real fixes are boat tailed projectiles and finning (think arrows and missiles), each serving a different purpose.

[gone-ot]

Originally Posted by Christ

It was kind of a facetious question, and the answer is no, despite what some think. Bullets are traveling at speeds in excess of 1,000 mph at times, their boundary layer at that speed is basically the thickness of the bullet. The only real fixes are boat tailed projectiles and finning (think arrows and missiles), each serving a different purpose.

Christ, also, since the bullet is exceeding the speed of sound, isn't it essentially traveling "within" a air-cavitation area over most of it's (bullet) length?

[ConnClark]

Originally Posted by niky

A-B testing is flawed simply because you can get hiccups either way.

Its used in science quite a bit. Go read some Nasa papers.

In this case the tests were run more like an A B C test.

A no clay
B with clay
C with dimples

A cooled car is different from a cold car. The residual heat in the radiator and transmission is still there hours later. The huge time interval between runs is also a big factor when you consider the effects of external air pressure and temperature on the way an EFI engine runs and rolling resistance.

Those factors aren't going to get you an 11% change. The fact that the A and B tests were so similar shows a high level of consistency in their test methods. A 5% change would be debatable but an 11% change can't be ignored.

This is why I feel they should have done the test with fiberglass panels or inserts, quick-swapped in just minutes between the runs. That would enable you to do three back-to-back runs over the course of the morning.

But then your introducing error because you have to create to sets of molds. and you can't be sure they are the same except for just the dimples. Pop on and Pop off panels will vibrate thus creating drag.

-

The fact that there was a result is interesting. The fact that there was no verification of the result by re-testing is frustrating. You can have a lot of errors and miss a lot of things working to a deadline like the Mythbusters do. And more than once they've had to revisit a "Myth" and change their verdict because of some flaw in their methodology.

Keep posting on their fan page then and get them to do it. I however haven't seen one decent argument that will nullify an 11% gain. In reality the gain may be lower but its not going to be negative

[Christ]

Originally Posted by Old Tele man

Christ, also, since the bullet is exceeding the speed of sound, isn't it essentially traveling "within" a air-cavitation area over most of it's (bullet) length?

'biundary layer the thickness of itself' yeah, basically. The step before super cavitation, really.

[gone-ot]

...and, yes, I was sorta leaning toward "super cavitation" as occurs with those Russian 'super velocity' torpedos that basically "fly" inside a vacuum cavity created within the water space behind the shock wave(s).

[redyaris]

Missing Data / controls
for
Mythbuster tests "golf ball" dimples

Air speed
Air temperature
Viscosity of the air
Air Density
Humidity
Wind speed
Wind Direction
Slop of the road start to finish

Follow up wind tunnel tests to messure change in Cd between clay cars with and without dimpes

[Christ]

Originally Posted by Old Tele man

...and, yes, I was sorta leaning toward "super cavitation" as occurs with those Russian 'super velocity' torpedos that basically "fly" inside a vacuum cavity created within the water space behind the shock wave(s).

I had a thread about it here about 2 years ago. Probably about the time you came inboard, I think.

[niky]

Originally Posted by ConnClark

Its used in science quite a bit. Go read some Nasa papers.

In this case the tests were run more like an A B C test.

A no clay
B with clay
C with dimples




Those factors aren't going to get you an 11% change. The fact that the A and B tests were so similar shows a high level of consistency in their test methods. A 5% change would be debatable but an 11% change can't be ignored.

Which is why the tests should be repeated. Heck, I'd be completely satisfied if they took off the clay, stuffed it in the back of the car and drove around and measured one last "A" after performing ABC.

Originally Posted by ConnClark

But then your introducing error because you have to create to sets of molds. and you can't be sure they are the same except for just the dimples. Pop on and Pop off panels will vibrate thus creating drag.

If both sets of panels are pop-on, then that's good enough. It's not like having clay layered over the top of your paint, exaggerating shut lines, panel gaps and edging around windows doesn't introduce error, right?

I was thinking a Fiero would be the perfect candidate, but the Taurus probably has better starting aero.

Originally Posted by ConnClark

Keep posting on their fan page then and get them to do it. I however haven't seen one decent argument that will nullify an 11% gain. In reality the gain may be lower but its not going to be negative

Aside from what has already been mentioned, consider... the car got better and better economy as the testing went on. If the clay introduced turbulence that caused worse consumption, it could have been masked by different testing conditions between "A" and "B-C".

Do the fiberglass molds. Then test the steel car back-to-back with the glass-bodied car. Economy should be within measurement error. If not, there's something wrong with your molds. Then pop on the dimpled panels. Measure. Pop off, measure. Pop on, measure. Pretty simple. Doesn't involve nearly a ton of clay.

[Xist]

This came up again. The Discovery page is still live, but this is the direct link: netstorage.discovery.com/DMC/2009/DSC/video/126108606888012580444601197_629GolfCar.flv
At least this was entertaining fiction, unlock Fifth Gear's video.

[aerohead]

Originally Posted by gone7

See response at the dimples thread. But basically, what works is the proof, and theories are not proof of anything and never have been.

Boundary layer theory is what a student is required to study,which is derived from the results of laboratory experiment and empirical results.There is nothing theoretical about it.
What is known,is that automobiles operate in a fully turbulent boundary layer flow regime and separation is already minimized.
The dimples could have only acted as a crude vortex generators which COULD have helped aft-body flow remain better attached over the backlight and onto the boot.
But if you're going to add VGs,there are many designs which would be better performing than dimples.Dimples are chosen for golf balls because they cannot be knocked off and they always present the same 'face' to the air,regardless of the ball's orientation to the atmosphere.
Here,in the upper two images are smooth spheres,at top,with sub-critical Reynolds number and large wake and drag,underneath,with super-critical Reynolds number,smaller wake,and smaller drag.
With spheres as large as a car,the flow would be super-critical by 20-mph.The sphere could be as smooth as glass and it would still be super-critical.

With small,smooth spheres, at low speed,you can force the lower drag by introducing artificial roughness.Here,the US NAVY has glued grains of sand to the leading edge of a smooth bowling ball,and in side-by-side comparison,you can see the modified attached flow,smaller wake (less drag).

The Ford Taurus would have enough critical roughness for it's size,that it would be super-critical by 20-mph.Dimpling the car wouldn't improve upon that.
However,the dimpling COULD behave as a very crude VG,energizing the boundary layer ahead of the backlight header,and this WOULD help for flow attachment,whereas without the dimpling,would suffer worse separation and higher drag.