Mythbusters tests "golf ball" dimples
 

Admin note: 2 threads got started on this topic. Another discussion is here: http://ecomodder.com/forum/showthrea...oct-10658.html

---

Hi,

Yet another interesting test of an aerodynamic effect by Mythbusters (with video):

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

http://www.blogcdn.com/www.autoblog....dimple_opt.jpg

Hard to dispute their method and testing.

Makes me want to go out and get a Volkswagen Golf ... or *make* one ;)

No, it's actually quite easy to dispute it. Go read the other thread on this same topic.

By the other thread, you mean the one about the perforated vinyl film?

Apples and oranges, Chris. :)

Adam and Jamie ran each test 5 times through for each variable, took measurements, using a reproducible method. They did take care to try and control other factors to isolate the effect they were looking for:

- Separate fueling systems for the measured interval
- Measuring/weighing the alternate fuel supply to determine consumption
- Only measuring the above at a constant, uniform speed each run
- Measuring across the same length of road for all runs

What was elided from the show, that may show any major or minor variations:
- Actual run data not published for all to see (sure looked like they recorded it, though)
- It's not known what the weather/wind speed and direction were, and if that fluctated, which could perturb testing runs

Atop that, they ran through models to see if there was an effect appreciable enough to bother testing full scale.

What are you actually trying to poke holes in, with what they presented?

I think it's just human nature, law of conservation. It's easier to criticise than create.

if they sprayed the car with aluminum mud (? just saying)
I wonder what would happen..
the point is cars are on rubber wheels, they use every inkling of their bodies for dispersal of energy created. all powers from coil, nuclears (never ever say never)from combustion, moleculars, hertz from the generators, and wiper motors and heaters, and...well that is my point.

a freshly high grade urethaned automobile respecting the cars system of origin will do as good as it can.

and for the golf ball thing..moving air as fast as possible to back to where it came from (a car plowing through the air) is as smart as it gets. That needs smooth. to our eyesight anyway. in reality, it is simply "micronically bumpy".
I fell for the golf ball dimple thing with a pc fan, the only benefit was demanding more draw because the fan was too light for the air (cheap product). I ended up reverting to the classic large paddled design that has been there since my first computers. It is all a crock.

including skirtings on the bottom of cars. :eek:

concerns for aero? you need a drivetrain. you need better bearings, higher grade grease, and better dispersal of energy created. aero is a silly product after conquering the other things. I sincerely mean it...

I'm not sure how you can look at the results people like basjoos have had and say that. Unless I misunderstand you.

So much for the kook-free thread... might as well let the HHO-philes pile on in now. :P

Exactly what I was going to say.

Ecomodders want more performance...from dimples. The golf industry knows when you have a good thing & wants more distance & less drag... just add more dimples!!!
Power Distance Golf Balls - The Freak - Best Golf Ball for Distance from Top-Flite Golf

..but, was the "final" Scientific Method criteria (ie: Repeatability) been done?

Given the proper speed, a smooth sphere can go farther than a dimpled sphere. It all depends on the Reynolds number. The ball that would go farthest for an average lady does not have the same dimple design as the one that would go farthest for a pro long distance driver. The ball that goes farthest would be the one with dimples adapting themselves to the surface airspeed.

In theory dimples could work on some part of bluff bodies, but not to the extend they measured. Especially since they dimpled everything. It makes no sense whatsoever to dimple surfaces where the flow is laminar to start with.

I just don't like the way they tested it.

Well said. I agree 100%.

If dimples are to be used, they should be placed where the body shape drops away from the airstream too quickly, to energize the air in that location only.

You don't need to energize the air in places where it is still attached.

And any device that creates a small amount of turbulence would work. There's nothing special about dimples. Look at the numerous airplanes that have vortex generators on the wings to improve the stall characteristics.

Jim.

Looking at the results, they were doing something right.
I wrote in another thread how that Audi has dimpled plates on the undersides of some of their cars ( ironically, the dimples seem to be the same diameter as those use in the Mythbusters test ).
VW also had some of their cars with the dimples as well, and who can forget the well publicized dimpled underside of the Lexus LS-430 ?

I doubt any of us want to have huge dimples in the sides of our cars, but the undersides of our cars seem like a perfect place to have dimples.

True, a smooth ball could have a lesser force barrier but that critical speed is attained @ 300+ mph, but a dimpled golf ball passes its critical Reynolds number and with a smaller drag tail @ 55 - 300mph. This is within the practical ranges of a stroked golf ball, struck by a 40"+/- golf club, & within a 4 degrees from tee level, initial launch speed of 160+ mph, consistent backspin and resultant Magnus force lift. A dimpled golf ball creates beneficial lift & stays aloft longer, thus travels longer. The configuration would take all of the advantage of the smaller drag resistance @ the most of its flight. Tests indicate greater lift on spinning dimpled golf balls but even non spinning dimpled golf balls still have lower drag tails compared to spinning/non spinning smooth balls.

Zipp dimpled bicycle wheel covers have shown very good Reynolds numbers compared to smooth ones. It only begs for ecomodders to now use & favor 'dimpled' wheel covers over smooth ones. Another one on the 'must' list.
http://www.zipp.com/_media/pdfs/tech..._the_edges.pdf

...similar "smooth vs. rough/dimpled" analogies exist in the animal kingdom too, with the skins (actually cartlage-scales) of dolphins and sharks.

...smooth surfaces aren't always the "least" restrictive in the ocean either!

Most water skis I've seen have a "pebbled" running surface. And most of the wake boards I've seen have dimples in the running surface.
FWIW.

No, I meant the other thread about this very topic.

http://www.parkcirclediscgolf.com/74datsun210.jpg
:thumbup:

...ah, but the "aero"-hairdo dude definitely don't look very aerodynamic.

dimples
 

I'm glad they did the test but I would take them to task for it.The only reason dimples are on golfballs is to hasten the transition to turbulent boundary layer an it's attendant lower Cd (profile drag/pressure drag).
Automobiles,given their size, transition to turbulent boundary layer and constant Cd around 20-mph,so it's not an issue for cars.And the "roughness" of "smooth" paint is all that's necessary for the transition.
I get a kick out of the show but sometimes I wonder if those folks ever pick up books and look at them.

To follow up on that point, I found this nicely written piece from airliners.net user QantasA332:

Basically, there are three primary types of drag acting on an aircraft: induced drag, skin friction drag, and pressure drag. It is pressure drag that is the main factor involved in the dimple design's existence. Pressure drag is primarily the result of a moving body's wake. Depending on how soon the airflow separates as it passes over an object - that is, how far along the object the flow travels before no longer following the contour of the object - the size of the wake will be larger or smaller. A larger wake equates to more pressure drag (put simply, there is a larger region of stagnant air behind the body meaning the airflow pushing on the front of the body has less impeding its production of drag) and vice versa.

Now, imagine a sphere. Because its height/diameter is large in comparison with its length, it is what's known as a "bluff body." Bluff bodies such as a sphere have disproportionately large wakes, and as a result they have disproportionately high pressure drag. (This is compared to both their own skin friction drag and a not-bluff solid's pressure drag). Obviously, then, overall drag on a sphere (or other bluff body) can be dramatically reduced if pressure drag is reduced. That is, pressure drag is what you want to specifically target and minimize.

Enter dimples. Dimples turbulate the airflow over an object, thus increasing the flow's kinetic energy. This acts to delay flow separation, which then leads to a smaller wake, which in turn leads to less pressure drag. And this solves the bluff body problem! Because bluff bodies have such high pressure drag compared to their skin friction drag, what little extra of the latter drag is created by dimples is more than offset by the drastic reduction of the former drag. So a golf ball - the classic example of a bluff body - will travel farther with dimples than without, and that is of course why they have come to carry these dimples.

Now, to finally answer your question: 'normal' aircraft are very simply not bluff bodies. Dimples would create more skin friction drag than they would reduce pressure drag, defeating their purpose.

So looking at a car, only where you have flow separation, would these dimples or any kind of vortex generator help. On most sedans, the back glass is prone to flow separation, and so you may see some reduction in drag there if implemented right. The rest of the car should have attached flow and thus the dimples would only add skin friction drag.

IMHO that Mythbuster car dimple thing was about the looniest thing I've ever seen. It might make a great Golf Add Car though.

First, golf ball dimples work because at the "Reynolds Number" a golf ball lives at the air is effectively much thicker than say said automobile and builds up a huge boundary layer (compared to it's overall size) and the dimples act like mini tabulators and scrub the thick boundary layer reducing the "apparent" form size of the ball. As a result the air flow boundary layer is reduced allowing the air to more closely conform to the ball. The turbulent surface drag results in being less than the former apparent form drag. You can get the same effect by placing a small wire ring about 25% of the way back from the leading edge to trip the flow. Older Lear jets used that technique just ahead of the ailerons to re-energize and attach the flow from the thick boundary layer.

I think a Ball Peen hammer would work just as well. Or park your car outside in Denver during summer hail season. Or several seasons.

Seriously, the more the air has to bend and go around things the more energy that is imparted to the vehicle from the flow. This results in drag. IRL a car is a universe of high and low Reynolds numbers. For us mortals the nuances of it all are superfluous. :D

SO can anyone think of a way to test the dimpling ourselves so we can have our own data? Besides just piling clay on the car. Which obviously is a no go for the do it yourselfer.

Sorry, I posted this on the "Haxagonal dimples" thread before I discovered the Mythbusters thread, so here goes again:

The dimples on the roof are probably beneficial (IMO). It looks to me like the rear window area is about the worst angle it could be for drag (if I remember my Hucho, 30 degrees is worst, 15 best...YMMV) and the dimples near the back of the roof are probably detaching airflow over the rear window. A little spoiler up there would do it better I'd think, but anything would help.

Mythbusters cynics & skeptics, especially on this site, could easily compel the creators/producers of the show to retest this episode. The show is famous for recreating followup shows from contentious 'busted/plausible' findings. An easy way to reproduce 'bigger explosions' tests. The now famous 'dimples' car should be retested w/ ecomodders' test suggestions. They may have to recreate new 'control' car with the requisite clay layer. From tuft testing, car companies' full size wind tunnel test facilities, different load/constant speeds tests, hypermiling driving techniques, time of day/weather conditions, etc., could be used for several batteries of testing. Any other suggestions?...

Thanks, majestic.

OK, so based on this understanding you could cover the rear glass with dimples and the separation would be delayed, with a reduction in net c/d. Nice.

However -
1) You need to be able to see through the glass.
So we need researchers to develop 'glass' with an index of refraction identical to air (zero??) so the bubbles won't affect your view.

2) The worst offenders in the rear wake department generally are the vehicles with a non-sloping roof: vans, wagons or hatches with a near-vertical rear panel. It looks like dimples won't help them much. You could put the dimples along the rear edge, like vortex generators, but you're not delaying the separation to occur at a point with a smaller cross section to get a smaller wake.

Does that seem right?

...not necessarily, simply implement rearward facing wide-angle camera with dash display.

No, Sir. Since flow is already detached at the glass, there is nothing there to create the vortices to keep attached flow, so they'd be most pointless there. Instead, the dimples would have to be placed at the rearmost point where flow is still attached, which would be the upper rear window frame on any vehicle.

1) Dimples don't necessarily have to be concave. They can be convex, and in either event, there are window shades that are perfectly legal in most states that you can only see through on one side. A lot of money was spent to figure out that it's pretty easy to see through a black mesh, but much more difficult to see through any other color or shade. Make sure the dimples/pimples are solid, and black on the side that sticks to the window, the driver only loses as much visibility through the rear window as those stupid full-window vinyl coverings.

This one is in a separate post so the information doesn't get confused.

Yes, anything with a flat back is going to be worse than something that is smaller and has even a virtually tapered rear end, such as a sedan. If the roof isn't tapered down as it leads rearward, it's going to create a larger wake than one that does taper downward.

In theory, something that would allow flow to stay attached around those rearmost edges would also reduce wake area, but not necessarily reduce drag, as I understand it.

In essence, while you're technically still decreasing the area that the wake occupies, you're increasing drag versus clean separation, so the decreased wake will probably be canceled out by the increased drag.

If one were to use a "duckbill" type spoiler, which is mid-mounted just below the windshield, and it didn't quite stick out far enough to make the 12* virtual slope, one could probably use dimples to train the flow downward, like a vortex generator. That's all they really are.

finally saw the episode
 

That was pretty darned entertaining!

FYI, you can view the video here: http://www.megavideo.com/?v=PMB935S8

When I saw the first test (dirt vs clean), I was sorely disappointed by their fuel measurement technique: note the fuel level in the sloshing tube by eye and mark with magnetic arrows!

I was relieved to see them ditch that method for the smooth vs. dimpled tests and opt for weighing the fuel cell each run.

Mine are mostly about the testing methods, either things I saw and took issue with, or things they didn't show which leave big questions.

1) They need to do A-B-A. Return the car to its original state to see if the observed effect disappears. Admittedly not easy to put the dimple material back in and smooth it again, but they should have tried.

2) It seemed to me they were relying on Jamie to hold the car's speed. That's a no-no! Need to remove the human foot from the most sensitive input in the experiment - the gas pedal. They should have used cruise control or some other speed control.

3) It's fair to assume weather conditions (temp, particularly) would have changed between the two tests - it would have taken some time to carve out those 1082 dimples. They didn't take that into account. (We don't even know for sure if they did it on the same day due to the magic of editing.)

4) We don't know whether the car was equally warmed up for all tests. (Equal engine coolant temp doesn't count.) We're talking full drivetrain temps, including tires.

Their results are also really big (% improvement), which ratchets up the skepticism a bit more. They claimed 26 mpg (US) for smooth vs. 29.65 for dimpled. A 14% improvement! Yikes.

Has anyone double checked their calcs?

smooth clay:

10056 g to 9553 g gasoline
or 503 g burned
26 mpg (US) their figures

dimpled clay:

9450 g to 8972 g gasoline
or 478 g burned
29.65 mpg (US) their figures

Gasoline weighs 6.073 lbs per US gallon (says wikipedia). 1 pound = 453.59237 grams, so 1 gallon = 2754.7 grams.

smooth: 5 mi / 503 grams = 5 mi / 0.182597 gal = 27.38 mpg (US)

dimpled: 5 mi / 478 grams = 5 mi / = 0.173522 gal = 28.81 mpg (US)

That's a 5.2% improvement by my calcs.

So their calcs seem fishy too. Unless I just screwed it up. (Plausible!)

...hm-m-m-m, wonder what the temperature of the gasoline was at each test intervals

...cold at the start, cool by pre-noon, warm by noonish, hot by end of day?

...temperature will affect weight by volume readings.

I pick up on this also but didn't want to write anything because I thought I might have heard them wrong.:thumbup:

I'm going to save any further comments of my own for the other thread on the same subject because it seems to have had more people's analysis in it... I didn't check it before I replied here.

Too late to merge the threads now - it'd make a mess.

Other thread:

http://ecomodder.com/forum/showthrea...oct-10658.html

Attached air flow is exactly what one needs to remove.

One can't argue with results, unless they want to replicate the tests.

Using clay is an excellent method because it can be removed. And let us not forget the extra weight the clay added.

The proof is in the pudding. No theoritical laws mean anything if the process works anyway. Observation is the only true sientific method.

By the way, this is not about windows. They cannot be helped and that is why they are removed in fast cars.

MetroMPG has interesting points, but the test should be done with fuel injection and a air-flow sensor to meet the exactness we need to see.

One can also do the test both ways, different times of day, etc.. But I am not convinced that such a large difference is needing any more detailed tweaks. It pretty much stands alone.

results
 

Their 'results' violate 'science.' If Jaime is the 'engineer' he should have remembered from Fluid Mechanics about boundary layers and Reynolds number effects as pertaining to the scale of automobiles.
Dimples on golf balls are boundary layer devices used to force a transition to turbulent boundary layer and alter the ball's separation point.
Automobiles are already in full turbulent boundary layer above 20 mph.Dimples would be redundant.
The dimples could act as turbulators,energizing the boundary layer to achieve attached flow in areas of high angle of attack (rear of the Taurus),but a drag reduction cannot be associated with pure Rn.

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

If the methodology is questionable, the results are questionable. It's that simple.

The question is not methodology. The question is in duplication of the results. One can never say for sure with theoretical science; only with practical and applied science. One may, however, say for sure so long as they have run the same experiment for themselves.

I do agree that it is too bad that the threads cannot be merged.

ACEV, I first came to Ecomodder.com 3 years ago wondering how to make dimples on my truck without adding 600lbs of clay. I was wondering why it wasn't done everywhere, then I got edjumacated. Golf ball dimples don't have the desired effect on cars.

But, don't take my word for it.

Here's an article from a few years back written with Gary Eaker, a former GM & Hendrick Motorsports aerodynamicist.

You're looking for:


Hot Rod Magazine - Car Aerodynamics Wind Camp Tech Theory

Mr. Eaker has a crazy amount of experience with aerodynamics, ACEV, you basically are saying to him that he is completely full of crap. Are you really that small minded? Aerohead as well is saying dimples are crap.

So you're saying because a pop-science TV entertainment program tested it and found an encouraging result, that settles it, dimples work.

ACEV put your money where your mouth is and test it again, you make the claim that dimples work, you prove beyond a doubt that they do. It isn't up to us to prove they don't, that ain't how science works. You say they enhance significantly, it's up to you to show how significant. Mythbusters does not count as your proof.

It's like you come in here and say you've been to the Moon in your VW Beetle, when we say, "Great, show us some pictures or something." You reply, "You show me pictures that prove I wasn't on the Moon." See what I mean? Quit that. You look really silly. Start seeming intelligent please.