Materials for Dimple Surfaces (reduced separation)

[bentring]

Originally Posted by aerohead

*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.

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.

[chillsworld]

Originally Posted by aerohead

*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.
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*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.
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*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.
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*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.
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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.

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...



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)."







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 There are also additional studies, they are just hard to find on the googlesphere.


~C

[aerohead]

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.

[ChazInMT]

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.



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!

[freebeard]

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.

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 want to reduce the drag of a pig?

You'd wind up with a porpoise.

[aerohead]

*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).
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*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).
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*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.

[rumdog]

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.

[ChazInMT]

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.

[chillsworld]

Originally Posted by ChazInMT

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.

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.

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.



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?)

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: ____________________________

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

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.

Anyway, fun stuff all of it! Keep learning!

Couldn't agree more!

[freebeard]

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

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: ____________________________

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

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.

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


exploder ii concept 02 by criarpo d5b2ct2

/That's rhetorical, I can find the thread