Materials for Dimple Surfaces (reduced separation)
 

So I was reading an article about tiny homes (one of my other interests), and saw a picture that instantly made me think of golf ball dimples...

Would the holes in the material, along with the smooth edge of the material lead to a similar golf ball dimple situation?
http://i.imgur.com/Xj6lxYN.jpg

The backsides of some of these have bumps, so they aren't holes but pockets.
http://i.imgur.com/g7FExzK.jpg

It's for drainage and hydrostatic barriers, or some such architectural/engineering technical mumbo-jumbo that I didn't follow. But they are made of plastics, rubbers, and high impact polystyrene, depending on application. Miradrain is one of the companies/brands for this stuff, and that's who's product is in the pictures above.

I haven't been able to find exact dimensions on this stuff, but it appears as if the plastic core of the product is around 10mm thick (for the shiny mettalic one above). So perhaps you could shave off the bumps on the back side bringing it down to like 3mm, and then use something like vinyl to attach it to the surface of the roof, rear pillars, and rear quarters? Or even better, to your belly pan?? Or would these be too small to achieve the same result as golf ball dimples? I know that dimples on the bottoms of the newer VW's, Audi's, and Lexus' have much bigger fist sized dimples on their belly pans.

I look forward to your thoughts.

~C

You've been watching "Mythbusters" too much...but, have fun nevertheless (*)










(*) seen any airplanes with dimples? Didn't think so.

I don't watch mythbusters.

Nope, no airplanes... But I'll counter with this, when was the last time you saw an airplane that wasn't aerodynamic? I don't see airplanes with airdams, rear diffusers, grill blocks, intake warmers, pizza pie wheel covers, low rolling resistance tires, or any other common ecomodder modifications... Why? Because it's a plane :thumbup: But thanks for sharing your immense wisdom in your post.


~C

Does your car have a perfectly round shape like a golf ball?

Does the kammback you made for your volvo magically make your car a perfectly aerodynamic tear shape?

And I thank you as well for your infinitely wise and profound comment, I had never realized my car wasn't a sphere :eek:


~C

I see you really do look forward to the solicited thoughts. :rolleyes:

I've created multiple threads here, and each time people have responded with a genuine interest to expand my understanding if they thought I was going down the wrong path. I have the utmost respect and appreciation for people who actually take the time to help others, or to have an open dialogue about something... I'm sorry that you think I should look forward to people openly mocking my intelligence.


Dimpled or non-uniform surfaces work on spheres, ships hulls, motorcycle helmets, wheels, rims, the sides of tires, and the list goes on and on. The way the dimples work on a sphere, or a hull, or a wheel, or a flat surface, are all different. The commonality is that they cause turbulence, thus preventing the separation of airflow from an object, and thus greatly decreasing the wake of the object as it moves through water or gas. This is the same driving force behind turbulator race wings, vortex generators, and many other aerodynamic modifications used for racing and for drag reduction in general.

The reason you don't see them on a plane, is that planes are streamlined and their main source of drag is skin friction drag, as opposed to the drag caused by blunt or non-streamlined shapes. They do however, use vortex generators, which have similar goals... They impact the manner in which air over comes adverse pressure gradients, which directly relates to separation and boundary layers of flow.

And while my car isn't shaped like a golf ball, none of our cars are shaped like tear drops either... Guess I'll keep my questions and ideas to myself from now on.

~C

I think a lot of the aero mods done are aimed at basically emulating an airplane fuselage with wheels as much as possible

I would agree completely, that was the point of my comment... To say that a modification isn't a good idea because it isn't on a plane, is illogical because none of our modifications are on a plane.


~C

Most of the things I've seen dimples on are objects that spin, or don't really have a front. I haven't seen a dimpled helmet, but I know the last street bike helmet I bought, shoei claimed to of done work to try to neutralize changes in lift and drag depending on the direction you're looking. So even though it clearly has a front, I could see dimpling tring to minimize the effects.

Do you have examples of dimpling used on hulls?

Neither of these image prove the idea works, but it does suggest that people with significant budgets are experimenting (maybe often merely marketing experiments):

http://www.everymantri.com/.a/6a00d8...082ca38970b-pi

http://www.airhull.gr/LH2Uploads/Ite...5/New_2/74.jpg

Dimple bottom drift boat, which is said to be super easy to handle on moving water thanks to the dimple hull design. Not saying this is proof that it works on a car, just providing an example other than a golf ball :D

http://i.imgur.com/t5J4AVW.jpg

From their website:

"Why do we love dimples? Indeed it's no accident that both a major European automaker and a power boat manufacturer have introduced dimpled bottoms or undercarriages on their most recent models. Of course, there's also the tried and true modern golf ball, the pocked surface of which has been the industry standard since Herbert Hoover held office.

As a recognized leader in the research and development of drift boat technology and as an avowed enemy of "drag" (exemplified by the unparalleled performance attained by our breakthrough tunnel hull design), we simply had to investigate the buzz behind dimples. Here's what we found: the laminar flow past a smooth surface produces more pressure and resistance than the turbulent flow past a textured or dimpled surface"

And I have found several reviews that are similar to this one:

"I found that the stopping time didn’t seem any better than a normal bottom, but once I got the boat stopped, it took very little effort to hold in place. In fact in some cases, i actually occasionally had to dig the oars into the current to get her to go downstream. I’m not sure how to describe it, but I can feel the water flowing under and past the hull better in this boat than in previous vessels."

http://i.imgur.com/auTyEIE.jpg

Surf Boards had dimples in the 90's, and here is a pretty common statement I've seen made about them (granted I've only seen it talked about like 6 times). The consensus is that it didn't help speed, or handling, but made rougher water feel smoother... Like it was gliding across the top, same statement the guy made about the drift boats :)

"Phazer surfboards and windsurf boards came our in the early 90's, I had two of them myself. Dimples that were carved into the bottom of the boards at the rear 1/3 of the board in front of the fin(s). Bigger of course, about 1 1/2" wide by about 3" long, a pattern of about 10 or 12 of them, spread out in that area. They didn't seem to hurt the performance of the boards, but as for speeding them up? I didn't see it, or feel it. But they did seem to smooth out the ride a bit in rough water."


There's also the non-uniform surface treatments that try to replicate sharks skin. This paint from Fraunhofer-Gesellschaft in Germany:

"They first developed paint made with nanoparticles that allow it to withstand temperatures ranging from -55 to 70 degrees Celsius, intense UV radiation and the speeds of air travel.

They then determined the best way to apply the paint to airplane exteriors in a way to mimic scales was to use a stencil.

The researchers also tested the paint and application process on a ship in a ship construction testing facility, finding that the paint reduces friction by more than five percent. Over one year of use, they say, the paint would reduce a large container ship's fuel needs by 2,000 tons."

~C

dimples
 

*The issue would be a 'one-size-fits-all' booby-trap.
*I can see how the material could function as a turbulator,but each particular vehicle would have to be analyzed for boundary layer thickness at the proposed application sites.
*Vortex-generators might preform better,and there are at least three different kinds of those,and their sizing,spacing,and location are critical to their best success.
*The crenelations and deformations underneath modern cars bellys are a function of laminate sandwiching manufacturing,principally for sound-deadening.They don't require dimples for a turbulent boundary layer if they're going 20-mph or faster.
*The un-dimpled golf balls small size and limited Reynolds number due to the limited 110-mph club velocity pretty much guarantees a laminar boundary layer,high separation,large wake,and high drag,compared to to the dimpled balls turbulent boundary layer,reduced separation,smaller wake,smaller drag.
*We have to bear in mind that a car is so large,that at regular driving speeds,they are already fully immersed in a TBL.

THANK YOU!

That first part was my main concern with the material. I would think that the boundary layer would be much thicker on the vehicle, and might require large dimples like those on the drift boat or surf board?

Also, point #2 of yours, the vortex/turbulators I have seen are generally at the point of detachment on the rear window, the ends or edge of the rear of the car. And as you said, require specific placement... That's why I had envisioned the non-uniform surface that carried over from the roof and rear pillars, keeping the flow attached rather than making vortexes to smooth unattached air. Like this... Concept car by French designer Dimitri Bez

http://i.imgur.com/RSZD31a.jpg

Would that make sense? I mean I'm apparently noy the only person to think of this, since he already has a car using this idea lol


~C

Hey Chills, there is an autospeed article out somewhere in which they visit a wind tunnel guy. I think the article may have some sorta Camaro getting its drag reduced. In the article the wind tunnel expert had a list of things that worked, and a list of things that didn't.

As for dimples, his quote was something to the effect that, "Unless your car is the shape of a 1 5/8" sphere, dimples will not do you any good."

Ironically I first showed up here 5 years ago with all sorts of lets dimple the car ideas, but it was a great starting point to begin exploring this aerodynamics world here.

Needless to say I have learned to just forget anything that seems like a fad after many far better educated minds than mine have said it's not a viable option.

This dimple thing is one those ideas you should just let it go.........

http://i60.tinypic.com/sq14e9.jpg

I'm not coming here with lots of "lets dimple the car ideas"... I don't have answers, I don't claim to know what will or won't work, I just know that I'd like to learn more about this. So I posted the materials and hoped for some enlightening information for or against the materials the application or the science... There is a lot of science out there that supports dimples... The people that say it won't work, never provide any statistical or factual evidence to support their claim... They make statements like "is your car a sphere?" or "do you see planes with dimples" or "this one guy said it wouldn't work in an article". In 2009 Modeling of train cars (in an actual wind tunnel 1:20 models) showed potential for 20% reduction in skin friction at high speeds when dimples were used. Dimples also allowed air to more easily "jump" the gap between train cars which allows for a decrease in pressure drag as well.

http://i.imgur.com/ESn0pj6.jpg

"A parameter called dimple ratio (DR) was introduced. DR is the ratio between the depth of a half dimple over the print diameter of a dimple (Figure 4). In his work, a car model (Figure 5) was simulated with a DR of 0.05 - 0.5.

His Ahmed body car model is a simplified car model for accurate flow simulation, retaining its standard car features such as curved fore body, straight centre section and angled rear end. It is a typical bluff body commonly used for simulation to study the flow past of a car (Figure 4). Flow was simulated using k-ε turbulence model in ANSYS Fluent software with tetrahedral meshing (Figure 6).

For the model without dimple application, there is insignificant turbulent kinetic energy on the car surface (Figure 7). When compared to the model without a dimple, kinetic turbulent energy is generated within the dimple and at the vicinity of the dimple edge. These results suggest that the flows manage to go further before flow separation takes place. The coefficient of drag, CD, is reduced by 1.9% for the model with DR = 0.4.

The results are encouraging since the simulation is only based on one dimple. Different parameters like dimple position, number of dimples and dimple orientation will be tested in order to fully understand the performance of the dimple application on vehicle aerodynamics." (2013)

There isn't a lot of research out there to learn from, and what is out there, is incomplete in regards to actual application... I guess I'll just wait however long it takes for someone to finish testing :( Thanks for your reply though, and I'll see if I can find that article!



~C

Assuming a thicker boundary layer, wouldn't bumps be more effective than a dimple?

If that is so, one would do preliminary testing with bubblewrap and duct tape.

I have no idea :confused: ... I do know that dimples do not do the same thing as vortex generators, and I would think that bumps might be closer to something like a vortex generator? I'll link a few German articles, that I have yet to translate, as well as a portion of scientific publishing on the topic of non-smooth surfaces (dimples, divets, pits, grooves) at the end of this post.

My understanding, and that's all it is, doens't mean I *know*... Is that the dimples decrease surface friction by manipulating the boundary layer, and also allow the flow to stay attached on shapes/angles where it would normally detach and create a "wake" (like a spinning golf ball, or the gap between train cars). Vortexes focus on cleaning a wake up, which can reduce drag. So in theory, dimpled surfaces have a smaller "wake" thanks to a prolonged attachment of the flow, so with a smaller "wake" you would also decrease the amount of drag. It's like having two things, that accomplish the same thing, but do it completely differently. The advantage, from what I can tell, is that dimples are less likely to increase surface drag, frontal drag, or pressure drag. Vortex generators have more potential for an increase in drag, or so that's what I have gathered from my readings :o

They have been experimenting in China and Germany, not sure why we aren't experimenting here in the States :confused: They have tested models of automobiles, trains, and boats. All of which saw a decrease in surface friction, and better .cd values. But none of them have figured out how to implement the idea... All of them say "further research is necessary" at the end of the paper or articles :mad:

English:

This one focuses on reduced friction on the hood and roof rather than manipulating flow at the rear of the roofline. Has equations and numbers and all manner of goodies!
http://www.zdrax.de/en/assets/pdf/Bi...d_Surfaces.pdf

German:

http://www.zdrax.de/en/assets/pdf/St...belbildung.pdf

http://www.zdrax.de/en/assets/pdf/Ha...r_und_Luft.pdf

This last one discusses the dimpling of high speed trains :D
http://www.zdrax.de/en/assets/pdf/Sp...en_im_Dach.pdf

~C

Well, you long enough and you find something that makes sense... I found a company that makes textured/dimpled vinyl wraps for cars, and their explanation is as such:

"Each dimple causes micro air turbulence (3mm thick product), and this creates a continuous air "pillow" across the entire surface of the car. The result is that the friction is no longer between air and surface, but between air and air. This is also what allows it to seemingly stick to the shape rather than detach from the shape."

I know this concept to be true, because it is used with boats. They inject air under the hull in special pockets, and the friction between water and air is less than it would be if it were water on hull. Similar concept to hover craft which maintains zero hull on water friction. So that's an awesome explanation of how it works, or at least it is in my mind haha And it's from Germany, so the have to know what they are talking about :p:D

Companies site: Effect | ZDRAX Supreme Surface Technologies

EDIT: Of course this would be evidence that different size dimples, and different applications of dimples, do different things (assuming they are correct with their research).

~C

dimple size/VGs/non-uniform surface
 

*There is only an inch or so of laminar boundary layer on a car.The rest is turbulent boundary layer.
*This TBL allows for attached flow as long as the body cross-section does not vary by much (area rule/sectional density).
*Once you have TBL there is no need of dimples,as they'd be superfluous and only increase drag.
*If you have contours on the car which are too 'fast',then a dimple might behave as a crude VG,but they are in no way as efficient as a true VG,as far as feeding momentum into a compromised TBL which is otherwise separating.
*It is the vortices created by VGs which feed momentum into the weak TBL,forestalling separation.
*Dimples cannot produce the quality of vortices as with VGs.They are very inefficient in this regard.
*The only reason golf balls have dimples instead of VGs is because the ball spins,and regardless of the balls orientation in the air,the dimples always present the same 'face' to the boundary layer,allowing predictable flight.
*If the ball could be oriented and struck with the driver or iron without imparting spin,then we could put VGs on them and extend a drive or chip shot.
*In water,the dimples would help the board or hull 'plane',as in a stepped hull.The curvature is too gentle to produce separation.
*I feel like Bez has not read Hucho or anyone else,and is clueless about boundary layer theory,otherwise he wouldn't have done what he's done.
*Same for Mythbusters.They must be an embarrassment to whoever authorized an engineering diploma.
*If any notchback car is going to be modified for boundary layer control,it should be done with VGs.It's just better science.

Not sure why everyone keeps bringing up Mythbusters, at no point have I referenced them :confused: I don't even have cable television :eek: :p


I understand that the TBL shouldn't need to be energized on streamlined surfaces to stay attached, but why are there studies and wind tunnel experiments that show a decrease in surface friction and an improvement of .cd values on already streamlined surfaces (passenger train cars for example, they have no curves or bluff characteristics along their sides). According to your statements, there shouldn't be any improvement upon an already perfect TBL/BL scenario via the use of dimples or any other such "fake vortex generator"?? I guess I'm just confused by the fact that everyone says it won't work, yet I'm the only one providing data/links to anything. So while vortex generators might be "better science", is that true because people simply won't look at new data that didn't exist when Hucho did all of his stuff? Or is it because these new studies are incorrect, or maybe they aren't actually as efficient as they would be if they used "real" vortex generators?

"As the layers of air move over a rough surface, the air particles in the layer closest to the surface collide with the surface. This makes the air particles slow right down (and right at the surface, they completely stop!). These particles then collide with air in layers a bit further out and make them slow down as well. As you move further away from the surface, the speed of the air particles is not affected. This boundary layer is laminar at the beginning of the flow, but it gets thicker as the air moves along the surface and becomes turbulent after a point."

So even on a super slippery, super streamlined surface, eventually friction/drag increases and separation occurs right? Vortex generators are used to mix things up and change the dynamic of this occurrence correct? And more often than not, they focus entirely on a point where separation will naturally occur right? Or on the leading edge of something at an attack angle. You wouldn't just put them down the length of the car, or all over the top of the car for no reason. The research I have been linking to, says it decreases surface friction and thus prevents the friction from increasing and the occurrence of separation (to some extent)... Which is similar but different from what Vortex generators do. I don't claim to know how, which is why I'm providing quotes, links, and statements from other places.

If I would have known it was such a hot button topic, and that I would need to look so much stuff up just to not seem like an idiot who watched some TV episode, I wouldn't have bothered posting in the first place :eek: :rolleyes: Anywho, I guess it's not a big deal, I sure don't have the means to test any of it... So I guess I'll move on to something else :thumbup:

~C

You don't need cable; all the high-lites are on Youtube. I recommend Adam Savages one-day builds and resin-kit painting tutorials.

Once again, I don't get to be part of everyone. :( What I suggested was a low-cost testing technique. I think bubble-rap would be halfway between dimples and an omni-directional vortex generator. They all provide a small-scale, localized disturbance of the overall air flow.

Well, thanks for bringing it up so we could all, for better or worse, exercise our 'thinkers'.

You've clearly come on here with a better understanding of the effects dimples than the average poster on the subject. While the general consensus might be against them that doesn't mean they can't work.

Personally, I'd love to see someone re-run the Mythbusters test in more controlled conditions. Their gains were clearly far too massive to take seriously.

You can buy little stainless mixing bowls for artists paint that measure about 1-1.5" diameter, all you need then is a hole saw and some bonding agent :thumbup: For bigger dimples you could cut up baking trays.

All you need now is a beater and a couple of afternoons.

Here you go:

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

The simple truth is that when you say "dimples" 'everyone' says "Mythbusters".

It's like on Mystery Science Theatre 3000 whenever someone in a movie turns a flashlight on and swings it back and forth, the 'bots say "NBC Mystery Hour". They can't help themselves.

Oh ye of little Google-fu:
Dimpled Car MiniMyth | MythBusters | Discovery
http://ecomodder.com/forum/showthrea...les-10718.html
http://ecomodder.com/forum/showthrea...ars-29311.html (look at #4)
http://ecomodder.com/forum/showthrea...und-20548.html
http://ecomodder.com/forum/showthrea...wrap-6807.html
http://ecomodder.com/forum/showthrea...ank-12691.html
http://ecomodder.com/forum/showthrea...mics-6263.html

Those are unique results from the first of fourteen pages putting "Mythbusters golfball dimples" into the search box.

Edit: Old Tami beat me to it. I loaded the page and others responded before I could.

It's no fun if you're like everyone else, not being one of them is what makes you special man ;) :D And I guess I could do the bubble wrap, I'm just not knowledgeable enough to know if the results would even be relevant for the theory behind dimples/pits/etc?? I'd hate to test it, have it do nothing, and then wrongfully provide empirical data that says dimples don't do anything. Does that make sense? If dimples truly are inefficient vortex generators like Aerohead said, then bumps would be relevant... But if dimples act in a manner which is different than a true vortex generator, then bumps wouldn't be a proper test right? I guess I can make a plan for testing it sometime soon, might hit you up for help with it since it's your idea :p

~C

Oh I know what it is, haven't watched it yet. The first time it was mentioned I googled it... Doesn't change the fact that at no point have I referenced it in my posts, and there for would seem irrelevant for people to judge me and assume that I'm posting because of it :confused: Nor does it seem logical to keep referencing it, when I have linked to studies and articles that DO have controlled environments and aren't some TV show :rolleyes: But thanks for the links, I guess I'll try to watch that episode at work today :D


~C

I wouldn't say I have a better understanding. I just don't have a pre-established aversion to the idea, like it would seem most people here have :snail:

I did the math based on boundary layer thickness and the design of the vortex generators used on the EVO, and the two studeis which listed dimesnions for their dimples/pits... And those *could* be perfect dimples/pits, the ratio of the width to depth is supposed to be important though. Now if only I felt comfortable doing that to my daily driver beater hahaha.

As far as not taking mythbusters 11% seriously, I think most people outside this website don't take the % claims regarding 90% of the common modifications done my members of this site seriously... Everything is relative, and each of us is usually quick to judge something we don't understand haha Not saying I personally think the 11% is accurate, just making an observation.

~C

dimples
 

*Chronologically,it seems like the Mythbusters episode was the catalyst for the concept of dimples and drag reduction entering into the public mind.
*Jamie,or whoever is the 'engineer' on Mythbusters,ought to already understand the difference in the relationship between the Reynolds number of a golf ball and an automobile.It would have been covered in Fluid Mechanics,which he'd have to pass in order to get a degree.
--------------------------------------------------------------------------
*Boundary layer aerodynamics of railroad trains would not be germane to a discussion of automobile boundary layers.The volume of the BL of a railroad train can easily exceed its frontal area.
*If you have another research which addresses dimples and automobiles which is peer-reviewed,we could look at that.
*TBL is TBL.Once it's established it works its wonders.
*The only way a dimple could 'work' is if it was acting as a crude VG.
*The 'new' studies must be viewed within a particular context.We have almost 100-years of VG research already documented.I would be very surprised if any stone has gone unturned.Winning world wars have depended upon such information.
-------------------------------------------------------------------------
*The layer of air against the smoothest surface is at rest.
*If the TBL is in a unfavorable pressure gradient,due to Bernoulli's Theorem,it must decelerate.
*It can't decelerate since it's already at rest.
*The only reason it can stay attached,is if the TBL is transferring momentum from the inviscid flow outside the TBL.
*If the contour is too 'fast' the TBL will separate.
*If there is a surface downstream within a proper profile,there can be reattachment.
*VGs can help insure reattachment.Their vorticity feeds kinetic energy into a feeble TBL,re-energizing it,holding it against the boundary wall as if it were being machine-gunned from above.
-------------------------------------------------------------------------
*On a super-slippery surface there will be about 1-inch of laminar boundary layer,then the rest is TBL.
*Friction drag is a consequence of viscosity,and shear stresses within the fluid as the varying strata laminations,at different velocities shear against one another.
*Attached flow is a function of pressure gradients,which are a function of body geometry.
*If the body is 'streamlined' it cannot produce separation,by definition.
*If it is pseudo-streamlined,like a VW Beetle,or,like a Mitsubishi Lancer,then it will have separation.
*VGs increase surface friction,but reduce overall drag through pressure drag reduction.
--------------------------------------------------------------------------
It's not a sin to look and find things and share.The spirit of the whole thing is admirable and it needs to be applauded.
Some research is done by people who haven't done their homework.And because they don't know what they don't know,they don't even realize that they are making contextual comments when they are making them.Then it becomes a matter of damage control,attempting to rebut what is being passed off as true science.It's too dangerous to pass without a test.

I remember reading once that none of them have degrees in expected fields for what they do. IIRC one of them had a foreign language degree, and any others were honorary.

Being a complete aerodynamics rookie, I have nothing to add to the dimple discussion but it is interesting to read.

Yes, on multiple comments I have discussed actual studies involving automobiles. I'll re-post one here, so as to streamline for you and anyone else...

"Use of bionic inspired surfaces for aerodynamic drag reduction on motor vehicle body panels"
http://www.zdrax.de/en/assets/pdf/Bi...d_Surfaces.pdf

Selected pages from the above PDF, to further ease and streamline this discussion...

http://i.imgur.com/WmRy7Yz.jpg?1

Look at the dates on that page, one study is more than a decade before the Mythbusters episode. The last paragraph states "As mentioned above (the part about bluff objects and rynolds numbers)in order to reduce the frequency of burst, the height or depth of the body surface units should be smaller than the distance between the body surface and the logarithmic law layer. To test this, a large range of law layer values have been used (Law layer is based on Reynolds value)."

http://i.imgur.com/aq1t23J.jpg

http://i.imgur.com/38q42FY.jpg

http://i.imgur.com/6yzGdHC.jpg

I'm not a scientist, nor am I an engineer, nor do I know the validity or credentials of those who did this study and published this paper... But you asked for data, and so now you have data. You can feel free to follow the link if you need more sciency stuff than what I have provided with my selected pages :D There are also additional studies, they are just hard to find on the googlesphere.


~C

Song/zhang/wang/hu
 

Perhaps a CFD expert will come forward.
I'm not sure that FLUENT can accurately model the flow,in light of the unmolested CAD model results.
A full-scale wind tunnel investigation with the logarithmic law layer ,with crosswind examination would be insightful.
So many of the masters thesis presented with CFD as the technique used for the investigation have shown significant deviations when backed up by full-scale tunnel testing.
Only Direct Numerical Simulation on a supercomputer is agreed to have the mesh size and time intervals necessary to develop a solution of high certainty for a 3-D bluff body in ground proximity.
Today,in 2015,BMW's CFD is off by 18%.
Today,in 2015,Mercedes-Benz is within 1%.Supercomputer run time is over 48-hours for a single iteration.
All we can say,is that on that CAD model,at that scale,using that CFD,we'd expect results like that.

Apparently you've rather fixated on the layer of air within a few inches of the car, while not totally insignificant, it is really pretty insignificant compared to the huge volume of air that is being affected by our vehicles as we plow through it.

Keep in mind that ideally, our cars are moving through through calm air....the air is not blowing on our cars. When not ideal we are moving along in 6 lanes of traffic with a 25 MPH crosswind....program that into your CFD calculator.

The ultimate goal of aerodynamic drag reduction is to move through the air and displace the least amount of energy into moving the air out of your way, and falling back to calm again after you've passed, as quickly as possible. The less energy expended on making air move about, the more energy you save.

When you consider the amount of energy it takes for a fan to run and blow a little breeze around your room, then relate this to a car that is dragging a fat column of air behind it for a few hundred yards and moving air out to 15 feet in front of and around the car, you realize we a driving in huge fan blades that are stirring up the air. The most efficient shapes allow the air to move back into place in the easiest way possible while not creating any trailing vortices.

The layer of air next to the car has a small impact on the big picture, but the overall shape of the car is really what matters. You can't wrap a pig in a mink and call it a mink. For the ultimate drag reduction, build a VW XL-1.

http://o.aolcdn.com/dims-shared/dims...swagen-xl1.jpg

You want to reduce the drag of a pig? Good luck with trying to mess with less than 1% of the air that is being displaced by the car and hoping the other 99% will bend to your will.

In so many words Aerohead is trying to tell you computers still really suck at trying to figure out aerodynamics because a car is constantly affecting 140,000 cubic feet of air weighing 12,000 lbs as it drives down the road and it's really hard to say how this air will behave. Not to mention it's all numbers in and numbers out on the computer, the numbers in thing is very subjective and a few missed assumptions on the input can create large variations from reality on the output. GIGO. I don't even pretend to know a lot about it, I again just listen to experts that say these things.

By saying you're going to dimple a car, you are asking the 6 lbs of air at the surface of the car to change the behavior of 12,000 lbs of air. (That's not really 1% is it?)

Here's the Hot Rod magazine article I alluded too. Scroll down to the "Aero Stuff That Doesn't Really Matter" for the Golf-ball thing.
Car Aerodynamics - A2 Wind Tunnel - Wind Camp Tech Theory

Anyway, fun stuff all of it! Keep learning!

While what you are saying it true, it is mitigated by the fact that the energy imparted to, and recovered from, the air is less per unit volume at a distance than at the vehicle's surface. The fall-off is logarithmic, while the volume grows cubically. The article (which is hard to quote) on the first page divides the boundary layer itself into three.

It's going to take me a while to translate that into LOLspeak in my head. :)

You'd wind up with a porpoise.

general thought on dimples
 

*The surface friction coefficient is 0.003 (Hoerner)
*The surface area of an automobile is estimated @ 10-X frontal area (Hoerner)
*The surface friction drag of an automobile is 0.03 (Hoerner).
--------------------------------------------------------------------------
*Song,Zhang,Wang,and Hu's CAD model with smooth skin is Cd 0.3197.
*Without surface friction altogether,the drag is Cd 0.2897 (by subtraction)
*The surface drag component of the total drag is 9.38% (by calculation).
---------------------------------------------------------------------------
*Song et al. gives a non-smooth body drag reduction of 10.31%.
*The non-smooth drag is 9.9% lower than the car with zero skin friction.
*CFD results infer that the V-groove serrations have eliminated surface friction drag entirely,plus have cut into pressure drag as well.

So connecting the dots here...

For the ultimate drag reduction, build a VW XL-1.*
...with dimples!

So it looks to me like the dimples (or grooves) create a subtle turbulence within the layers close to the body. As opposed to VGs (vortex generators) which create an aggressive local and downstream mixing action using a vortex.
Does the non smooth surface (NSS!) also reduce the thickening of the boundary layers?
It is interesting how the turbulence (rear bluff rotating eddie) is reduced by the NSS [figure 8] and it almost looks like a vortex develops on each side... disapating energy as vortex instead of rear eddie. Or what?

I think that other body shapes may not be so lucky. Or just need engineering too.

I mostly want to emphasize that the air close to the skin of a car is not the only air being affected. I think people loose sight of this, (or never thought of it really) that as we drive we are constantly moving huge volumes of air, if we look at just 5 feet out from a car and back 300 feet, we're talking about 40,000 cubic feet and 3,200 lbs. Even that number is mind boggling. The ideal car shape will gently push air out of the way and let it go back to where it was with the least amount of energy put into it. Anything that sets the air swirling about or moving along behind the car will have required energy to make it do so, and this energy is what raises the Cd and thus drag of a shape that departs from ideal.

It is the Shape of the vehicle that affects this, air in the "boundary layer" is merely the tip of the iceberg. If you look at the mass of air in the 4 inch area around a car it is minuscule compared with the total air mass being displaced, it is therefore nonsensical to believe that changing the behavior of this tiny fraction of air will somehow result in any thing other than a very slight change in Cd. The boundary layer has no magic powers anymore than a fraction of a percent of hydrogen can change how the engine uses fuel.

6 lbs against 3,200 is just too much.

I'm not fixated on anything. I have created threads on ducted radiators, off road friendly aero modding, contributed to conversations about down force generation vs drag reduction, etc. etc.


If a reduction in friction drag occurs on a pig, no matter how insignificant, does it not decrease the energy needed to push through said massive amount of air? If said friction reduction leads to a boundary layer that stays attached to the pig longer, there by reducing the pressure drag at the rear of the pig, would it not lead to an even larger reduction in the energy necessary to move through the air? Further more, if there was no validity in this, then why has "Lufthansa Technik AG participated in a multifunctional coating research project since mid-2011, along with its partners, Airbus Operations; the Fraunhofer Institute for Manufacturing Technology and Advanced Materials in Bremen, Germany; and coating manufacturer Mankiewicz in Hamburg."??? On a plane, which is already streamlined, and an object we try to use for inspiration, this texture should decrease fuel consumption by 1% (no results, study on going)... Not much huh? That's 94 million dollars a year in fuel savings across their fleet! I think I would trust people trying to save 94 million dollars, before I would trust auto manufacturers who don't stand to gain anything from said research. Now before we jump on me for comparing aerodynamics and surface friction with dissimilar shapes and vehicles used under vastly different circumstances... I'm simply providing information regarding the decrease in energy that would occur via nothing other than manipulating your "less than 1%" of the air, and I understand that implementation and results would obviously be different.

I get that computers suck, but this just seems kind of silly. So far this has been the gist of the this thread:

Response: Do you see it on planes cause that's proof it doesn't work.
ME: Actually here is an example of planes and trains and boats.
Response: Oh, well in that case they aren't like cars and so it doesn't count. Show me data to support this on cars
ME: I show research focused on cars.
Response: The data isn't good enough because computers suck, and there's no way manipulating 1% can impact anything.
ME: Well manipulating 1% is worth millions in research for some companies, and they are hoping to see a decrease in drag/decrease in consumption.
Response: ____________________________

His statement: "They do not work on cars, regardless of the scale of the dimples, unless your car is a 1.68-inch-diameter sphere spinning through the air with no ground plane." There is no reference to any testing, no reference to any modeling, no reference to any outcome of either. I find the opinion of an "expert" who provides no explanation for his very broad and absolute claim, to be somewhat irrelevant to this conversation. Especially when other companies are spending millions of dollars on objects other than golf balls. Industry experience is not to be ignored, education is to be respected, but an opinion is just an opinion.


Couldn't agree more!

Isn't this the archetypal story arc for every discussion thread on the internet, ever?

How about "You can hope in one hand and poop in the other and see which one fills up first"?

Which thread is that, and more importantly did I repost this:

http://ecomodder.com/forum/member-fr...po-d5b2ct2.jpg
exploder ii concept 02 by criarpo d5b2ct2

/That's rhetorical, I can find the thread