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Mythbusters - Golf ball dimpled car MPG
Topic Started: Nov 3 2009, 08:55 AM (6,620 Views)
rmcelwee
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Just wondering if anyone else saw this and what they thought about it. If you didn't see it, the Mythbusters got an 11% increase in fuel efficiency by putting dimples (looked to be softball sized) on a car's exterior.

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Cobb
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First off, they did not measure the 0-65 consumption and half way through the experitment they changed the way they measure fuel consumption.

For the clean and dirty fuel runs they use a sight glass. For the slick clay coat and clay dimpled they weighted the fuel cell.

PS, the removed clay dimples were carried inside the car in a large tub.
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arollinstone
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On a Mission from God

I remember something like this being tried by a nascar team years ago - it worked so well they didnt alow it. I suppose it didnt make the cars look to good either.
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ki4syr
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I don't see how dimples can improve mileage. You'd think that a smooth surface would have less restriction of air, correct? Hince, port-n-polishing heads and intakes, airplanes etc...
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nerys
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Grr

a smooth surface IS cleaner and better for fuel economy than a dimpled surface. This is aerodynamics and fluid dynamics at work. I will try to explain.

You have several kinds of drag that slow your car down. The dimples are BAD for gas mileage. Period. you get NO GAIN from the aerodynamics of dimples.

BUT the results they got are accurate and here is why.

you have OTHER drag effecting the car to a GREATER extent than the parasitic or surface drag from the dimples.

in rocketry we call it "base drag"

do you know how you measure 2 kinds of drags on our cars. FRONTAL area and Surface Drag.

well there is an opposite to FRONT AREA and that is base drag IE the "back" of your car. this is why the solar cars come down to a WEDGE shape in the rear. to virtually eliminate base drag.

the dimples allow you to accept a penalty in parasitic drag that is SMALLER than the benefit or GAIN in efficiency via the decrease in BASE drag.

Think of if this way. WHY do we add a Kamback to a car? well when the air comes off the roof of the car its not able to make the transition so it "brakes" laminar flow and you get massive "base drag" I mean when you add the kamback your increasing mass but the GAINS from the reduce drag are greater than the losses from the extra mass.

the dimples do the same thing in a different way that the kamback does.

it reduces base drag. by disturbing the air in this particular way it becomes more energetic and is able to transition a LARGER ANGLE than it could before effectively making the car's rear end SMALLER in base drag.

the INCREASE in efficiency from this base drag reduction is LARGER than the penalty from the parasitic drag of the dimples. IE net positive gain in efficiency.

I think they could go even further. first the entire FRONT 75% of the car does not need dimples. that alone would eliminate the parasitic drag for 75% of the cars surface (the added drag of the dimples) you only need the dimples on the rear half of the roof and trunk and rear quarter panels.

Why did they not test 0-65 in this equation. SIMPLE the only practical way they could add the dimples was to add 850 pounds to the car in clay.

done on the manufacturing level you would not have that 850 pound penalty.

also its hard to "calculate" that since you can never accelerate the same way twice. its a lot easier to just go 65mph and keep it there smaller experimental error.

while they DID change the way they measured BOTH HALVES of each test using the same method so its still valid and there second method is far far more accurate and consistent.

I am wondering if a dimpled kamback would be EVEN MORE effective! and then add some dimples to the rear quarter panels to further reduce the base drag dimensions. and it can be done a lot lighter than clay. I am thinking a molded poly carbonate kamback with the dimples molded in.

SO your right dimples INCREASE drag and REDUCE fuel economy. but as a side effect they REDUCE BASE DRAG and this reduction is much larger than the "increase" in parasitic drag the dimples cause. SO you get a net gain. Better aerodynamics.
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billy508
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billy508

Seems to me it may be a little like comparing apples to oranges as a golf ball spins between 2000 and 8000 rpms. Be a hell of a ride in your Geo if it was spinning that fast. :banana :banana :banana
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nerys
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Grr

The spin is irrelevant. its a sphere. all lift or drag is neutral and even. (well not quite but now we are getting into parasitic drag on the boundary layers of air around the shape which CAN be utilized to generate lift (every make a bic pen tube generate lift and FLY like an airplane?) but this effect is extremely minimal on a sphere and is unrelated to the dimples and its drag reducing properties.

now the only reason a golf ball is dimpled ALL OVER is because of that spin. since you can not know which face will be facing the wind you have to assume ALL of it will.

since a car does NOT spin and you DO know which end faces the wind you can strategically dimple. no need to dimple the entire car.

For the bic pen try this. take an old style white bic pen that you can remove the pen and end cap from so you just have a STRAW so to speak.

now place this on a smooth table and place your first two fingers spread apart on top of the tube and FLICK down hard with your fingers SHOOTING the pen forward and applying spin to it.

it will actually fly like an airplane. you can even make it LOOP if you do it hard enough. this is because of the reverse spin you applies to the tube. the TOP of the tube is going "back" and the drag on the air currents speeds up the air while the portion of the tube on the bottom is traveling FORWARD relative to the flight path dragging the wind layers slowing them down.

Bernoulli takes over from there. anyone who says Pressure difference is not where most of wing lift comes from are quickly quieted with this demonstration since a cylinder can not have incidence to the path of travel since its a cylinder.
Edited by nerys, Nov 3 2009, 04:21 PM.
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EA6BMECH
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So the dimples act like a vortex removal (if I'm stating that correctly)?? or straighten out the air flow somewhat?
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Cobb
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The dimples allow the air to move around the object better. It helps to close the rear envlope so the hole the car pushes through the air doesnt slow it down as much.

I think a demple roof or deck lid is all you need. I doubt the ones on the sides or front of the car has any effect.
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nerys
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Grr

no just the opposite they screw it up more - in this higher energy state it can "stick" to a larger transition than it normally could. Watch the mythbusters episode if you can (download it if you missed it) I think its episode 715??

they used a water die tank that REALLY demonstrates the effect quiet visibly.

If you understand how a KAMBACK works then you understand how dimples work. they do exactly the same thing but without the kamback. they reduce your base drag.
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Bad Bent
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Facetious Educated Donkey

I found a pretty good article that explained the golf ball dimples at: www.aerospaceweb.org.

"In the case of a golf ball, increasing the speed is not an option since a golfer can only swing the club so fast, and this velocity is insufficient to exceed the transition Reynolds number. That leaves tripping the boundary layer as the only realistic alternative to reducing the drag on a golf ball. The purpose of the dimples is to do just that--to create a rough surface that promotes an early transition to a turbulent boundary layer. This turbulence helps the flow remain attached to the surface of the ball and reduces the size of the separated wake so as to reduce the drag it generates in flight. When the drag is reduced, the ball flies farther."

"The reason we do not see dimples on other shapes, like wings, is that these particular forms of boundary layer trips only work well on a blunt body like a sphere or a cylinder. The most dominant form of drag on these kinds of shapes is caused by pressure, as we have seen throughout this discussion. More streamlined shapes like the airfoils used on wings are dominated by a different kind of drag called skin friction drag. These streamlined bodies, like that pictured above, have a teardrop shape that creates a much more gradual adverse pressure gradient. This less severe gradient promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.
However, there do exist other types of devices commonly used on wings that create a similar effect to the dimples used on golf balls. Though these wing devices also create turbulence in order to delay flow separation, the purpose is not to decrease drag but to increase lift. One of the most popular of these devices is the vortex generator."
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redpepe


hi all,

fascinating discussion. i have no links to the research but i want to mention a 'myth' i heard a long time ago which the dimpling prompted. it went something like this .... a shark has a specially adapted layer of skin which is 'bumpy' and flexible on a very small scale. it was claimed that those conditions allowed flow dynamics that were advantageous for speed. i don't know if it was similar to a cumulative reduction in 'base drag' or some other phenomena.

coincidentally, i've often wondered about my cow 'dimpled' hood [folks often guess that i've gone through a hail storm] and any overall mileage changes on my metro. seems unlikely but ....
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nerys
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Grr

no a dimpled hood would REDUCE your fuel economy since at that position on your car you do not NEED turbulent boundary layers to keep the air attached to the car. its at the BACK of he car where you get the wake separation. this is where you would need the dimples.
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greggholmes


Well, i can tell you that the smoother the surface is the "stickier" it is. that's how the tesla turbine/pump works

"This effect was exploited in the Tesla turbine, patented by Nikola Tesla in 1913. It is referred to as a bladeless turbine because it uses the boundary layer effect and not a fluid impinging upon the blades as in a conventional turbine. Boundary layer turbines are also known as cohesion-type turbine, bladeless turbine, and Prandtl layer turbine (after Ludwig Prandtl)."

tesla turbine

"The disks must be maximally smooth to minimize surface and shear losses. "
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Coche Blanco
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I do not think that the test was accurate.
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