## How about dimples?

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My idea is pretty simple actually. It would be about putting dimples in the nose cone, sidepods and maybe also in the engine cover, with the objective of reducing air resistance, thus improving the air flow around the car. Just like golf balls. Their dimples improve their aerodynamics, by reducing turbulences and drag, making them fly much farther than non-dimpled balls. Or like shark skin, whose scales reduce the drag in the water making them swim much faster with less energy (I now F1 cars don´t go underwater but the principles are appliable too). Now I don´t know how it could affect the downforce generated by those parts of the car, but it could go well in the nose cone whose function is to direct the air flow (under, over or around the car).
Now about the size and depth of those "identations", I have no idea, but I would say about the size and spacing of those in a golf ball.
What do you think about this?
Mitsuhirato

Joined: 24 Apr 2008

Welcome by all means, Mitsuhirato. Is that name related in any way with Tintin's "The Blue Lotus"?

Well, no. Shark skin, might be, dimples, I don't think so. Brace yourselves for one of my famous "short" posts. Say thanks, I'm in a hurry.

Why are dimples used? It's not because they diminish drag, but because they create turbulent flow. This flow has more energy, so the separation it experiences behind the ball is smaller. This is the idea:

Now, let's go back:

If you tried to imagine the pressure at the trailing edge, the middle and the leading edge of a sphere (measuring the angle theta in the figure) and you assumed air to be frictionless, you get something like this:

At 0 degrees (the front) there is high pressure, at 90 degrees (top of the ball), very low, at 180 degrees (back of the ball) the pressure is equal and opposite to the one you get in front. So, a sphere should not have any drag! This is know as the D'Alembert paradox, because he stated it for the first time.

Fast forward one hundred years: thanks to George Cayle and his invention of aerodynamics, it was understood that the air that impacts on the sphere drags around it until it has lost all its energy. At that point it separates from the sphere, like this:

You can see that at the 180 degrees position (the back of the sphere), the pressure is negative. This is what creates the drag for the whole sphere. Dimples, as shown in the first picture of this post, make "less negative" the pressure around the ball, so in the end you have lower drag. Check this graph:

Now, back to your question: why aren't dimples used? After all, many designers play golf... Well, it should be clear that this "dimple trick" works when you have flow separation. Unfortunately, not all kind of shapes have the same problem. Just look at this:

The black and white squares, to the right of the image, show the proportions of skin drag and pressure drag: the pressure drag is insignificant for a wing. Ergo, no dimples.

You use vortex generators, that's true. Here you have them:

Their purpose is not to decrease drag, it's to delay separation to avoid stalling at low speeds. The shape (they're not dimples, but small protrusions) makes them to provide you with the same effect of dimples (less separation on the trailing edge) with less drag.

In racing, serrated edges, with a similar purpose are widely used (well...):

Serrated gurney flap

Serrated windscreen (with holes)

So, sorry, man. Back to the drawing board. Comments?

I might have made several mistakes, I know that I don't have the discipline of proffessionals here, I have amateur knowledge, so any corrections will be a ray of sun for my dangerous (incomplete) knowledge.
Ciro
Ciro Pabón

Joined: 10 May 2005

Not sure if I'm correct off the top of my head. I remember reading on RE last year that dimples works better when the parts that has them rotates. I'm will dig out the issue on the weekend and varify (or someone could do that faster...).

It is also noted that fish like coelacanth has scales that reduces drag using the same princle, however the scale is stationary relative to the fish's body. Any biomechanic specialist here?
elise787

Joined: 2 Feb 2008

The forum gets this question about once a month..

Remember that the golf ball is spinning through the air, too.
zac510

Joined: 24 Jan 2006
Location: London

The last time somebody asked was a year ago. I checked: it's the sixth thread on the issue. AFAIK, is the first time somebody tries to explain it on the thread, and also AFAIK, nobody explained clearly why are they NOT used.

I did not went into lift by rotation, but I think it would be good to put it here too. I really don't know how dimples influence on that. I mean, what's the difference between a spinning dimpled ball and a spinning smooth ball? Does spinning really creates lift only in dimpled balls? I don't think so, but I don't know. Thanks in advance if someone has the time (really)

I wonder sometimes if we need a wiki for certain things: regenerative systems, alternative engines, perpetual motion machines, you name it.
Ciro
Ciro Pabón

Joined: 10 May 2005

I think I recall that a spinning ball generates lift of its own (magnus effect). I take it delaying separation in this situation is beneficial because it increases the lift generated. So I'm not sure if it's a 'spinning balls work better with dimples' or 'spinning balls work better if you delay separation - hence dimples'?
Any golf experts?

B
bazanaius

Joined: 8 Feb 2008

I think the OP should think about where he wants to put dimples and ask himself what is wrong with the at part of the bodywork that means it needs dimples or 'reduced air resistance' as he put it..
Does a nose cone not already have laminar flow all over it.. How can you possibly improve on that?
Then the answer becomes a bit more obvious.

A golf ball on the other hand does have a drag penalty from which an improvement would be beneficial. The golf ball is always hit in a consistent and repeatable manner too (unless it's me hitting it) so a non-moveable aerodynamic device can work.

I have heard of Renault experimenting with dimples on their driveshaft but am not sure if they still do it. There is an example where there is a drag penalty and an improvement could be made.

Just to contradict myself, if there were separation from a sidepod or nosecone in yaw then perhaps dimples in the region would be beneficial.
zac510

Joined: 24 Jan 2006
Location: London

ss_collins

Joined: 31 Oct 2006

If only the wheel fairings on the F1 cars were allowed to move eh? Although claiming dimples all over your wheels for 'brake cooling' purposes would probably not wash...
B
bazanaius

Joined: 8 Feb 2008

I think that the rough surface improves the magnus effect.
So on a dimplet rotating ball Magus effect works better.

Maybe I try to explain this more detailed but my aero-book is
at my workplace right now.
mep

Joined: 11 Oct 2003
Location: Germany

Excellent post, Ciro (and others). I think I have a small point to add:

Several years ago NASCAR outlawed textured vinyl-covered roofs on NASCAR sedans because of concerns they would give an unfair aero advantage. (Textured vinyl-covered roofs were all the rage then and are still seen occasionally).

I have to believe that the dimples would definitely NOT provide better racing, otherwise Max would outlaw them.
Enzo Ferrari was a great man. But he was not a good man. -- Phil Hill
donskar

Joined: 3 Feb 2007
Location: Texas, USA

Excellent response(s)Ciro...

Not sure about NASCAR but I do remember reading in Mark Donohue's book, Unfair Advantage, that Penske used vinyl roofs at one time on their Trans Am cars. Not for aerodynmaics though. It seems Penske had discovered acid dipping of the unibody to reduce weight and got a bit overzealous. The acid was eating through the relatively thin sheetmetal of the roof leaving holes!!! So, they decided to get stylish and install the vinyl roof to cover the damage. What a hoot!!!

BTW... Mark Donohue's book should be on every gearhead's reading list.

Have fun with it...
countersteer

Joined: 28 Apr 2007
Location: Spring Hill, TN

Probably just adding to what has been said above..

The gold ball's advantage with dimples is not so much in reducing and surface drag , because they actually increase surface drag, but in flow separation on the trailing side of the ball. The flow of air around the back of the golf ball hugs closer and further around it than without - so when it finally separates the drag 'wake' is far smaller. (the 'lift' part is something separate - google "magnus lift")

Putting dimples on the nose of an F1 car wouldn't achieve quite the same because the flow is not 'separating' from the bodywork until much much later (ideally at the rear wing).

A golf ball needs the dimples over it's entire surface because it spins and can't be hit so the same side is always at the 'front' (it would also lose its lift if it did too). An F1 car, however, remains in the same orientation (relative to the air flow) and can have fixed, low-drag rear edges on most parts (suspension struts etc) - in similar fashion to the trailing edge of an aircraft wing - so doesn't really need dimples.

R
Rob W

Joined: 18 Aug 2006

For simple (layman) descriptions try the following:

Symscape, Computer-Aided Engineering for all
syguy

Joined: 22 Feb 2007
Location: USA

Very interesting readings, here, the old thread Ciro mentioned:

http://www.f1technical.net/forum/viewtopic.php?f=6&t=881&st=0&sk=t&sd=a&start=20
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti

Joined: 10 Jul 2007
Location: Argentina

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