Hexagonal Dimples as a way to improve aerodynamics
 

Yes, I did a search and you're probably thinking, wtf are hexagonal dimples. Think golf ball.

I know that unless you have an unlimited budget you can't modify your car exclusively with these dimples and i seem to remember the backing of some refrigerators have a sheet of aluminum that has similar dimpling. I want to conduct some experiments by blocking my grills or improving aerodynamics in some way with dimpled material, does anyone have a cheap source for this kind of material?

Oh yes, sorry, I have to explain my reasoning. Well (taken from here)

Cylinders and spheres are very convenient and strong shapes when designing bicycle frames and components, but it turns out they are terribly un-aerodynamic. I know they look all smooth and round, how could the air not just flow right on by them? Part of what makes an object aerodynamic is the object’s ability to keep the air attached to the surface as long as possible. As soon as you have what is called flow separation from an object, drag increases. This is why deep rims and disk wheels are more aerodynamic, because the air stays on surface longer, and the flow separation takes place much further along the flow of that object. The same with those thick downtubes that almost all tri bikes have now. More surface area on certain parts of the frame mean less flow separation and improved aerodynamics.

It turns out that with a cylinder or sphere, that flow separation takes place very early as the air travels over the object. But, please dimples on that sphere, like a golf ball, and something changes. Those dimples increase turbulance, which normally you would want to eliminate in aerodynamic design. But this turbulence, or “dirty air” on a sphere has the effect that it actually speeds up the airflow and gives it more forward momentum. As a result, flow separation takes place much later in the flow over the sphere. Even though there is increased turbulence, the trade off is that the increased speeds in airflow has a net benefit on the aerodynamics of the sphere, and the air stays attached to the surface much longer.

It is also important to note this is why we don’t put dimples on just any shape or object for which we are trying to improve aerodynamics. If dimples improve aerodynamics on a ball, why don’t we dimple the wings of an airplane? Or make the bike frame itself dimpled? The reason is that those dimples increase turbulance, and on a wing or aerodynamically tapered bike frame, the flow separation is already fairly good. The net result of increasing turbulence on a shape that already has late flow separation is decreased aerodynamics, while the net aerodynamic result of turbulence on a sphere is positive, because the turbulence contributes towards delayed flow separation.

What would be the effects of aerodynamic dimpling on a car body? If a golf ball has a high CD on the trailing end then can dimpling decrease the cd of steep rear ends? Dimpling sports wear helped Lance Armstrong and the Japanese Women's Swim team, it helps reduce heat from high speed aircraft.. so, what exactly stops testing on cars? Again, anyone have an idea on cheap materials i can use to test with?

Hi,

I think this is the same principle as zigzag tape -- which is a linear version of dimples.

you could always leave your car outside during a strong hailstorm, or drop marbles or ball bearings from high up on your car....

or, go to home depot, go to the cabinet area, get the little vinyl bumpers for drawers, stick them on the car.... check out my "getting my Si slippery" thread to see how I stuck them on mine, just monday even

What's the Reynolds number for a dimpled golf ball, swimmer in water, cyclist?
What's the Reynolds number for a car?

If they're close - then these features may directly apply for the same reasons. The last paragraph is most telling ;)

The effect would be not even remotely close to the benefits of adding a Kamm back or boat tail.

Thanks for the idea, I especially like the idea of sticking those on my mirrors instead of a full mirror delete (I like the aesthetic nature of them ok?).

Before applying them, did you do the string test to ensure optimal placement?

I stayed up until 1am last night doing research and I think the only benefit I can see from dimpling would be from testing.

I would suspect the rear section could gain the most and maybe the trailing edge of the mirrors, sides of the front bumper and a body pan could benefit from a lower drag coefficient, especially those areas which have a greater curve.

To test Reynolds we need to use the following formula:

http://www.aerospaceweb.org/question...arity/eq03.gif

Where:
r = atmospheric density
V¥ = velocity
l = reference length (in the case of a sphere, this variable is defined as the diameter)
m = viscosity (or friction) (the funny symbol for those not familiar with it)

I found another page which explains what golf ball dimpling does. Essentially the dimpling creates a turbulent air flow over the surface of the ball (like applying a layer of oil to a ball bearing) this creates a similar effect to an airplane wing which decreases trailing wind disturbance. If this holds true, what are the effects of 1/4 of a golf ball shaped area (the right and left front bumper area in some cars). This is clearly raising more questions than answers so the research continues.

dimples
 

for the golf ball,the dimples help establish a premature transition to a turbulent boundary layer and the attendent lower drag coefficient which accompanies the turbulent boundary layer. The Reynold's Number environment in which automobiles inhabit guarantees a turbulent boundary layer and lower and stable drag coefficient at and above about 20-mph.Dimpling the leading edges of a car will not lower its drag.---------------------------" Dimpling"or adding turbulators near trailing edges can and may reduce drag in some cases however are never used in the forebody.

:thumbup:

Yes, I agree, doing a homer simpson on your car is not a good idea, especially around those flat areas. I remember one luxury car manufacturer has a dimpled bodypan that they said increases their fuel efficiency for the same model car by a certain % but that could have been a pitch. I like the idea of adding turbolators to the trailing edges and my front bumper = EEW so I might install some dimpling on the outside edges after i do the front grill delete. I like the idea of adding those silicone bumpies on the mirrors though. I lack the money to invest in a $200+ mirror delete (the cost is in the cameras).

For the rear area I read on a Smart Four-Two forum that aside from them complaining that their not so 'smart' cars aerodynamics were inhibiting his cars potential he went to a canadian semi-truck site and invested in some AirTabs. He installed 8 at the cost of $2.75 each and went from 34mpg to 42mpg... maybe another vortex generator option? Sorry, lost the link =/

Another site with various color options is here

Search around here for airtabs, Darin (metrompg) did some aba testing and didn't get any of the miraculous results people tend to talk about...

is there a decent solution for the rear roof? I searched various forums and haven't again found anything conclusive, I'll read that now when i find it.


Later:

Ok, I checked out that forum post with the airtabs, and there is a problem with it. A 2006 Corolla has a CD of .296, my car has the same CD as a new Volkswagen Beetle, .36. Now I have to find an article with either car tested.

How does dimpling help on a golf ball if the ball is spinning constantly ?

As far as dimples on automobiles, I notice that Lexus did a famous job of dimpling the LS430 underside, and to my surprise VW is still using dimpling on the undersides of their cars . There must be something to it, otherwise they wouldn't bother ( Lexus used the dimples as a PR stunt, and this may have been the actual reasoning behing the dimples, but why then is VW using them if they don't really work ? )

The spinning of the ball has some effect but the important thing is that the dimples cause a surface layer of air which directs other air away from the surface, the greater the speed, the greater the effect.

I'll be back in a little bit, going to head out to Home Despot to look at aluminum sheeting to see if any of it is dimpled, I'll be back with the results of my search.


Update: No Aluminum sheeting aside from some dimpled 4" studs that are used as wall supports in buildings that would have to be flattened out to be usable. I considered a rain gutter but with weight = well more weight. Oh well, now to go teach class..

FYI, two of the most knowledgeable people on the subject of aerodynamics on this forum have already posted in this thread regarding dimpling and automotive aerodynamics.

I don't want to discourage anyone from learning through experience something that has already been established by others, but dimples on a car = dead end.

Where did you read that your Civic has the same Cd as a New Beetle?

On Motortrend it says my cars CD is 0.38 (surprise to me)

and good old untrustworthy Wikipedia says it is 0.36. Type in 2001 Honda Civic when you search the page.

I was wrong in stating that it has the same CD as a beetle, but one guy had put a giant wing on the back of his bug and it reduced it to 0.36 (the article is amiss, sorry). The actual beetle CD is 0.48 according to Wikipedia

Ouch. Aren't most trucks and vans 0.45 to 0.49?

Dimpled underpans would have another non-aero reason for being dimpled...
NVH reduction - Noise, Vibration, and Harshness.

Especially for metal parts, dimpling, ribbing, and other forms of bracing or mass-damping would keep an otherwise perfectly flat panel from vibrating at higher frequencies.

High frequency vibration are more likely to bother occupants inside the car, than lower ones. As an overall structure, a more rigid car has better handling, to boot.

That's just my take on that idea. It may be good PR, may possibly have some benefit for delaying flow separation, but my best guess is it's a compromise of all three.

nope, I unscientifically stuck them on there. I will be trying to zigzag tape later.

one thing about metrompg's test of the airtabs, I think he used them incorrectly, the air bubble was ONLY in the center of the car, one or two of these tabs could have disrupted that bubble without the drag from a whole back end of them, the string tests show that and I think should have been used to place the airtabs. if you have a drag reduction from the center 2 tabs, but the other 6 tabs are adding drag, well then just keep the center 2. I think its at least worth a shot, so I may be trying with some homebrew airtabs and such on the wifes jetta3. string test first to determine bubble size and shape. then airtab placement

Dont Get Air Tabs!!

Get "Vortekz" generators; airtabs have a MUCH larger frontal area- remember their made for HUUUGEE trucks and not small metros?!

The large frontal area simply negates any good side effect that they might have on airflow. Vortekz on the other hand add very minimal frontal area... and thats why I have them on now actually

Same as before, just on one side (the low pressure side) :thumbup:

Other things to note.....
The seam on a baseball, create a similar effect as golf ball dimples. There's a reason why baseballs are changed frequently and attempts to rough them up is banned ;)

The slightly raised seam on a cricket ball - it too :thumbup:

Tennis balls... they're fuzzy... Another aero changing feature

The texture of a football

Panels on a soccer ball

Jai Alai ball - arguably the fastet/most lethal ball...

etc. etc. etc.

That's interesting and all.... But only applicable for similar Reynolds numbers :thumbup:

:thumbup:

Look at where dimples are being applied for aero - super tight curvature objects that just happen to spin pretty damn fast. A flat, or even mostly flat panel is not going to be directly applicable.


Now, some cattle chew....
http://supersquadra.net/wp-content/u...7/100_1578.jpg

Zipp offers dimpled wheel sets for cyclists... again, fast spinning, tight radii, small length..... And yes, their claim is aero - not strength :thumbup:

bodypan
 

I think I saw that ad.I believe the "dimpling" has to do with the "sandwiching"Lexus does to the sheetmetal to bond sound-deadener material,and also move the resonant frequency of the panel outside the range of vibrations (or their harmonic counterparts),so the panel won't hum.------------ The dimples form inverted pools of stagnant air,which the surrounding flow field merely passes over with virtually no aerodynamic penalty.Clever!

golfball
 

Evidently,the spinning is of little consequence, as the revolving "face" always presents the dimples to the "oncoming" flow.At a clubhead departure velocity of 110-mph,Reynold's Number goes supercritical immediately,and drag coefficient drops from 0.47 to 0.10.----------------------- As mentioned elsewhere,the floor of the Lexus is a sandwich of 2-layers of sheet steel,encapsulating a membrane of acoustic deadener.The presence of the dimples further alters the resonant frequency of the floor,along with any harmonics which might occur within the range of the engine/driveline vibration.

Mythbusters!
 

Time to resurrect this thread...

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

Amazing. If there is any fault in their methodology it is that they dumped all the dimples in the back seat, thereby raising the nose of the car relative to the undimpled pose. But that ain't much, and I would be surprised if it would account for an 11% difference in FE.

I am thoroughly impressed - another example of why aerodynamic interactions are phenomena (that is, you can't use intuition to predict!). My prejudices were wrong, and I'm quite intrigued :)

I haven't watched the episode (yet) - but how did they scale up the dimples? By relative size to a golf ball? Or relative velocity? Or just a guess?

Even if they pitched up a little, that would have been working against them... and 11% difference sounds, potentially, outside the scope of reasonable error :thumbup:


-----
Yes, impressed enough to un-silence myself :p I graduated school this past spring, have been really busy, visited 30 cities on the all you can jet pass, and am now doing contact work/working super hard for a start-up in San Francisco to meet fast approaching deadlines....

that was one of the most beautiful things I have ever seen...

That's my opinion as well, but by golly the dimples on the roof may well be simulating a crude Kammback. 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.

Watch the date stamps, this thread is a back-from-the-dead special. :)

Dimples make sheet metal more rigid as well, more strength less weight under the right conditions.

Dave