Materials for Dimple Surfaces (reduced separation)

[California98Civic]

Originally Posted by 2000mc

...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....

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

[chillsworld]

Originally Posted by 2000mc

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?

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



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."



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

[aerohead]

Originally Posted by chillsworld

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?


The backsides of some of these have bumps, so they aren't holes but pockets.


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

*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.

[chillsworld]

Originally Posted by aerohead

*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



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

[ChazInMT]

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.........

[chillsworld]

Originally Posted by ChazInMT

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.........

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.



"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

[freebeard]

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.

[chillsworld]

Originally Posted by freebeard

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 ... 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

They have been experimenting in China and Germany, not sure why we aren't experimenting here in the States 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

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
http://www.zdrax.de/en/assets/pdf/Sp...en_im_Dach.pdf

~C

[chillsworld]

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

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

[aerohead]

Originally Posted by chillsworld

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



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

*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.