Hi

Yesterday i saw part of a mythbuster on Discovery. They tried the fuel economy on a clean, a dirty and a golfball holed car.
They wanted to see if a dirty car had better fuel milage due to the "golfball effect"
apparently the clean and dirty car had a fuel milage at 11km/l at 105km/h, but then they tried to make a 450kg clay shell for the car. First they ran it with a smooth finish, after that they ran it with a golfball finish. the smooth was 11km/l, the golfball finish was 12,6km/l.
I would never have thought that the golfball finish had such an effect. It makes me thinking if the "golfball effect" actually could be used, either on the roads or in motorsports, eg. on a DTM car??.


The Golfball car can be seen at the end of this ½ minut clip.
http://www.youtube.com/watch?v=R0xc8DppsRM...PL&index=14

Bjørn

Team Lotus, or perhaps Club Team Lotus, had a newsletter, and one issue, I seem to remember, had a letter from a fan, suggesting exactly that. It caused some merryment in the drawing office.

BMW used a dimpled finish on their wing mirrors at one point (not the bit you look at, dumkopf). Maybe it was just another of Crazy Chris' styling innovations, I suspect the idea was to encourage re-attachment for windnoise.

Strange enough... I was looking into this today and found some very useful information

http://www.zipp.com/_media/pdfs/technology...onary_speed.pdf
from
http://www.zipp.com/technologies/aerodynam...erodynamics.php


http://www.racecar-engineering.com/allarti...c-surfaces.html

Dr Kenji Takeda also has some interesting ideas with dermal denticles on sharks.


It looks like the drag is reduced because of the rotation of the ball (or bike wheel) so having it on the body of a car wouldn't help but maybe on a spinning hub or axle.


I may seriously consider looking into to these ideas for my graduate research if they seem promising.

I remember seeing that episode, and being amazed. I read up on the earlier thread on this forum, and the consensus seemed to be that it was good in a straight line, but bad in the corners, and the holes got covered up with dust fairly quickly. Then again, I'm not sure that an approach as aggressive as what Mythbusters used was ever tried, the holes they used were huge.

... let the games begin (again)

Just watched an interesting episode of Mythbusters, where they test a car very clean and get 26.4 USmpg, and with it covered in dirt & rough crud it gets 24 USmpg. Not unsurprising, but then they start doing some tests with golf balls on the range and the usual balls flew about 35% further than the smooth golf balls - again not very surprising.
But then they get the same car and cover it with clay, then test it again (26 USmpg) and finally they cut golf ball-like dimples of about 80mm diameter I'd guess into the clay. Much to my surprise the car got 29 USmpg odd, or about 11% better fuel economy.
They didn't gather a huge amount of data for each of the test runs, but what they got was supposed to be pretty consistent.

I was wondering if anyone here has done any similar testing? The Mythbusters pair seemed to just take a punt at the size & depth of the dimples, no doubt there's room for further improvement with different shapes & spacing, etc.
My interest, of course, is not improving the fuel economy of my racing car but using less power to do the same speed - Though if I ever get around to building an economy petrol car I'd still look at the information.

I imagine the size & spacing of the dimples would vary with speed and to a lesser degree temperature .... ?

*edit, here's a pic ->

I'm trying, driving in Chinese traffic is getting me closer every day and to think in about a year I will be getting 11% better economy!

Whoops, I didn't notice the other thread on the same subject - Ta to the mods for combining them.

Speedo makes swimsuits using this principle:

http://www.speedo.co.uk/en_uk/aqualab_tech...fsii/index.html

I'm sure I read about F1 teams experimenting with various "skins" for cars, but don't know what came of the research.

Here's what I understand so far.

A rough surface on a body will increase the boundary layer of the flow along it when the flow it turbulent. For a body with little pressure drag this increase does not reduce the pressure drag enough to make a difference. BUT with non-streamlined objects this will 'smooth' out the flow greatly reducing the pressure drag (I have seen experiments that a rough surface has 1/5 the drag force of an identical smooth surfaced body).

A big point is that this only happens when the flow is turbulent.

So for streamlined bodies this would not reduce drag but for non-streamlined bodies (like a mirror) it will as long as the fluid flow is turbulent.

I hope that made some sense. It's just from reading random things online and what I already know.

A friend of mine at work watched that episode. He says that they took all of the removed dimple material and piled it in the back of the car. That way the weight of the car wouldn't change between the smooth clay surface and the dimpled clay surface. However, all those dimples on the hood (bonnet) and front fenders ended up behind the driver. My friend says that the change in weight distribution caused a very obvious change in rake angle. It could very well be that just throwing some weight in the trunk (boot) would have the same effect, and the dimples were actually worthless.

So, the big question is: Which is more important, dimples or angle of attack?

Good old Mythbusters - they always manage to leave out a bit of scientific method somewhere.

And sometimes science itself. My favorite is when they calculate the distance the object needs to fall before reaching terminal velocity, using the basic physics 101 equations that don't incorporate drag. Apparently the irony of calculating terminal velocity distance from an equation that implies no terminal velocity is lost on them. That said, scientific method or not, they often manage to show something interesting.

As for the rake theory, I don't really see how that is plausible. If a car can gain 3+ mpg just from a rake difference created by a box of clay on the back seat, then it seems like an even more miraculous discovery, since it's a lot easier to change the rake than to make a car look like a golf ball. And how far away is the position of the box of clay from the center of mass of the dimples taken out anyway? The clay was all over the car, not just on the hood.

Fish scales keep water from actually touching the fish and hence reduce friction. But water cannot be compressed, air can...so the characterisitcs would be quite different.

Seeing a car with scales all over it would look kida fishy, don't you think. Yes, I actually wrote that...a cheap, short reach for a little humor...I wonder what Pamela Andersone would look like in a fish suit...

I agree - the amount of weight they added to the rear seat was not a great deal and if anything it was close to the middle of the wheelbase but more off to one side. Not significant.

My normal response would be "I wonder what she would look like out of the fish suit" but then we've all been there haven't we?

Hell, everyone has been there

I wouldn't touch her even with someone else's dick.

I would touch her but she would have to agree not to talk...that would spoil everything.

I remember something I read on this being tried on a WW2 Spitfire (dimples in the aircraft skin that is, not fantatising about Pammy). It did make a small increase to the top speed of the aircraft but was so labour intensive (no cnc in the '40s) that it was abandoned. As usual, probably more info on this via Google.

The Spitfire test involved adding split peas to the head of flush rivets to see the difference it made. The aircraft was tested and the peas gradually removed to see where flush riviting had little or no effect on performance. Strangely enough, all the ones I have looked at seem to be completely flush rivited.
So they were actually testing the reverse effect.

One of the more attractive features of Pammy is the relative absence of dimples (also known as "hail damage")




CNC? Labour was cheap in those days and a craftsman armed with a ball-peen hammer would surely be able to modify a whole Spitfire in a few hours?

An Bf 109 could do it considerably quicker than that.

If it could catch it...

With or without dimples?

You can't beat a dimple...

Try a shrinking dolly.

The P-51 Mustang was one of the first aircraft to use a laminar flow wing.

The problem was that any small imperfections reducedd the advantage in drag of the wing.

Also, pilots found with their Mosquitos that polishing the surface made a useful gain in speed.

http://www.aviation-history.com/theory/lam-flow.htm

http://yarchive.net/mil/laminar_flow.html