Mythbusters tests "golf ball" dimples

[MetroMPG]

I give up.

Those with more patience to continue discussing with ACEV, please continue.

[gone-ot]

a paraphrasing of sort: "...you can lead the jackass to water but you can't quench his thirst for stupidy..."

[ConnClark]

I figured I would chime in on this


Arguments about things warming up are mitigated to some degree by the fact the car was allowed to cool down while they added the dimples.

Cruise control was used on both runs.

The mythbusters results of a 11% difference Would tend to rule out most variations between the 2 runs. (not to mention the fact that the clean and the undimpled clay car got almost exactly the same mileage)

Also of note is the fact that the windows received no dimples. this would mean that air flow over the windows would be almost like it was stock. the dimples over the top of the car would have ecited the boundary layer air over the top of the car thus getting more of the air flowing over the top of the car to flow down the rear window. This combined with the relatively stock flow over the windows would have reduced the C pillar vortex strength and thus reduced induced drag.

As far as suggestions that the dimples up front would only contribute to drag this isn't entirely true. The dimples up front would keep the boundary layer thinner all along the car and enhanced the performance of the dimples towards the rear of the car. It is not totally uncommon to use two or more rows of vortex generators to get this effect. I'm currently trying this out in my simulations and it is quite effective.

[gone7]

@ConnClark,
Thanks for those very interesting thoughts. You've pointed out some interesting items. They deserve some serious thought.

Could you please explain C pillar vortex strength and induced drag concepts? Also, isn't all drag "induced", because there is none inherent in a stationary shape?

@MetroMPG,
You were never "required" to continue in the discussion. However, you were asked to explain things simply. I understand if you can't, but don't imply denigration to others who cannot understand what you are struggling to get across.

@redyaris,
If we completely remove the spherical part of the discussion, how do you see the application of your ideas to a varying surface shape of an automobile?

Thanks.

[ConnClark]

for an explanation of induced drag see here
Induced Drag Coefficient

for c pillar vortex see page 3 here

http://www.grandmarq.net/blaze/Blaze...of%20Autos.pdf

[aerohead]

Originally Posted by ACEV

@aerohead #46,
Could you please put that into simple language so we all can understand? Your good points are very difficult to visualize.

Basically, what I have learned about a round shape going thru the air is that the fairly aerodynamic front is offset by they very unaerodynamic back side, therefore negating any beneficial affect. A Borg sphere in space has no such effect upon it since it is in a vacuum. One must assume that the ideas you put forward are having to do within atmospheric situations.

Also, once we have the test done, ie; by MythBusters, then the observable facts have been presented for all to see. The idea presented by Grant-53 must have his definition basis in observation. At this point we do know that dimples in a car body surface do increase fuel economy since we have all observed it.

If we feel that there is something missing in the given test, then it should be pointed out. If we feel that there are other things in the test making the fuel economy better, then they should be pointed out.

Short of that, the average person should be able to rely upon the fact of the fuel mileage improvement as demonstrated. If we overthink everything, we will never move forward.

ACEV,if you'll do a search at GOOGLE IMAGES for 'golf ball flow separation' you'll find some pretty nice illustrations.
*as you see for the smooth ball,the separation is occurring ahead of the widest part of the ball's body and the wake is enormous.This is what happens with a laminar boundary layer.
*the next image illustrates the flow with a turbulent boundary layer (triggered by the roughness of the dimples),and you can see that the flow remains attached downstream a bit from the widest part of the ball.By the point where the new wake forms,the air velocity has slowed down some,and from Daniel Bournoulli's research,we know that the static pressure has grown,and this new,smaller wake is at a higher pressure such that the difference in pressure,from front to rear is less (reduced pressure drag) and the overall profile drag is lower,allowing the ball to range further down the fairway.
*On a car,because of it's length,compared to velocity,it will attain this turbulent boundary layer(without any roughness) when it hits about 20 miles per hour,and the flow will remain attached the maximum amount in the aft-body region.
*since the Taurus has kind of a 'fast' roofline,it suffers from flow separation before the air makes it all the way back.
*The dimples appear to behave as turbulators (vortex-generators are a form of turbulator) which,while themselves add additional drag,they are able to transfer enough additional kinetic energy into the boundary layer such that it can remain attached in an otherwise hostile pressure gradient,postponing separation and leading to a smaller wake of higher base pressure behind the car,netting an overall drag reduction (exactly like Mitsubishi's vortex-generators do on their Lancer notch-back).
*if we take MythBusters at their word,then yes,the dimples reduce drag,but not for the same reason they help a golf ball.They might have attached VGs to the Taurus and achieved similar results for a lot less trouble.

[oil pan 4]

Originally Posted by ConnClark

for an explanation of induced drag see here
Induced Drag Coefficient

for c pillar vortex see page 3 here

http://www.grandmarq.net/blaze/Blaze...of%20Autos.pdf

I'm going to add that grandmarq pdf to a bunch of aerodynamic wiki articles.
Its a good one.

[niky]

Originally Posted by ACEV

Further, A-B-A testing is questionable because of such little difference between that and A-B. Why not A-B-A-B..... to infinity? Would that help? At what point do we say that the improvement is real? Or even good enough?

A-B testing is flawed simply because you can get hiccups either way. That's why world speed records require back-to-back runs in opposite directions. That's why when we do acceleration tests, we do sets of three or four.

Originally Posted by ConnClark

Arguments about things warming up are mitigated to some degree by the fact the car was allowed to cool down while they added the dimples.

Cruise control was used on both runs.

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.

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.

-

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.

[Christ]

Will my bullets be more stable if I dimple them?

[Otto]

Originally Posted by Christ

Will my bullets be more stable if I dimple them?

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.