Not that I agree with a all of the Mythbuster's methods or results, but in this case, if I am not mistaken, they used the same exact car for each test. Not three different cars of the same model. If they did in fact use the same car, same track, etc. then all other variables are taken away except temperature and fuel density issues between each test. This gives a little more credibility to their results, but I would like to see them do the same test on a couple other models of car, maybe a Honda, Toyota, or just for kicks, a mid 70's Lincoln or Cadillac.

The mythbusters car was a later model Ford Taurus 2001 or so. the one with the blunt rear as opposed to the sharper smother rear of the earlier models. They put a layer of clay on it, tested the millage with the clay, then put dimples that were about 3 or 4 inches round. I suspect the dimples made the air flow round the blunt rear which improved the millage. As I recall the fuel milage smooth was worse than my former 1999 Taurus that I had, and about the same or slightly worse with the dimples.

The results of your tests are the opposite of what one would expect. The CD of a sphere tends to go down as the Reynold's number increases, because the boundary layer becomes turbulent earlier at higher Reynolds numbers. This would reduce the impact of dimples at higher Reynolds' numbers, since the dimples have no effect once the boundary layer is turbulent.

A turbulent boundary layer IS turbulent flow. When dimples are used to reduce drag, they do so by causing turbulent flow in the boundary layer, which in turn delays flow separation. The flow within a turbulent boundary layer is not 'smooth'.

banktoturn

Actually the results obtained are what are expected. The CDof a sphere is much higher for higher Reynold's numbers. the flow becomes turbulent due to flow seperation which increases CD. The flow around a dimpled sphere is still smooth inspite of the turbulent boundary layer.At high reynold's numbers the flow is around dimpled ball is smoother than for a similar sphere.

missil3,

I recommend that you take a minute to do a Google search for CD values of a sphere. You will see that the CD clearly decreases with increasing Reynolds numbers, unless some mechanism is used to force transition. This result is well understood to be a result of the turbulent boundary layer keeping the flow attached.

The word 'smooth' is imprecise in this context. A turbulent boundary layer is not 'smooth', although the flow may appear to be smooth, since you can't see the turbulent fluctuations that occur at very small length scales.

It would be interesting to see the graphs you mention.

banktoturn

Everyone.
Send me your planes.

I will dimple them for you..

Sorry , I notice I made a mistake Imust have said the drag force is higher for higher reynold's number and not the Cd . The drag force on sphere is higher than drag force on golf ball for high reynolds number . Iam guessing the Cd goes down as it is inversely proportional to the velocity.

I have attached my graphs. This is similar to what others who did the same experiment obtained as well.

You get different types of dimpled golf balls - while the weight and diam is controlled, the dimples are not.

The number, size, depth and patterns of dimples on golf balls varies - from 250 dimples to 450 dimples per ball. That is 4 categories of variations just to start with.

Jason Zuback hits a ball at 204 mph (328 kph) then dimples work, not at slow speeds.

Which design golf ball dimple is here under discussion?

Just wondering if a double shot Talisker on dimpled ice will taste better........

missil3,

How are you generating this data, and where did you get your "literature" values for CD? What you say is found in the literature is not correct. Try this site:

http://www.princeton.edu/~asmits/Bicycle_web/blunt.html

It gives a nice explanation, and shows data resulting from well-accepted drag measurements for a sphere at a range of Reynolds numbers.

banktoturn

Actually, dimples will tend to have more effect at lower speeds than higher speeds.

Still wondering about the 18 year old Talisker on dimpled ice though, should I drink it slower too........? Cheers

Dimples, Holes, whatever.

USAF drilled thousands of leaking airholes into a jets wing surfaces to see if NOT smooth was better. It was much better. They bled air off the engine. ?? 20% reduction in drag ??
Problem was keeping .01 diameter holes open. Reason they dropped it.
All correctly sized ROUGH finishes do help. 6 Hydroplanes steps boosted WW II PTB speed from 41 to 52 knots.

Rich

If you look closer at the graph you will actually see that the graph show in figure 2 of the link you just gave matches the literature graph in the plot I attached. Match up the reynold's number values. The graph doesn't include data for all reynold's numbers at the figure in the link. Also your link doesn't compare the data given for a dimpled sphere which is what is being discussed here.

And all the data was obtained using high precision instruments at the University of Illinois.
And Dimples reduce drag only for higher reynold's numbers. For low reynold's number the drag due to flow seperation is not significant.

PS:This is my last post in this thread.

Maybe it is a question of efficiency in certain speed ranges.

Howard Hughes had an obsession with breaking the world airspeed record in the 1930's and he had his team developing fast planes. Finlly he had them to make the rivets of the H1 Racer flush with the surface and polish it to a mirror finish.

In the 1940's they were desperate to improve the performance of fighter planes and the opposite experiment was carried out in Britain on a Spitfire where they went and actually glued peas in patterns on the plane to see what difference in speed it would make. The plane was slower.

So this whole issue is not new and certainly, if there were any merit in dimples or bumps, somebody would have included it in their designs for cars and planes long time ago.

Judging by the bad share performance, all the major companies in both industries are in dire financial straights and is looking at anything to give them the cutting edge....even if it is only to mke "green friendly " products. Just a thought.

Hughes H1 Racer polished to a mirror finish.

Bundubasher,

It's more a question of the shape. Airplanes can be shaped in such a way that they don't suffer from flow separation under most conditions. This essentially means that they aren't "blunt" on the back, as a golf ball is. Dimples, or other methods for forcing flow to turn turbulent, will generally only reduce drag if they can prevent flow from separating when it otherwise would have. Since well-designed airplanes don't have that problem, forcing turbulent transition is not helpful, at least for drag reduction. Such devices are sometimes used for other reasons on aircraft.

banktoturn

Yip, I agree, good point.

One of the things about these forums is you get lots of posts from people who THINK they know the answer , this post right at the front has it correct
Ian

So if I leave my 1 ton dually out in a mid summer hail storm I can expect 12% improvement in mileage from all the dents. So I go from 20 mpg (diesel) to 22.4 mpg. Come on now. I can get 21 if I really try and keep the speed not to exceed 55, programmer on SW1, calm wind, 60 deg day, no traffic, 75 psi in all the tires, but 22.5 with a 7500 pound truck no way. Besides it will take at least a year and a half to make up the $4000+ reduction in value due to all the "dimples".

Actually it was 12% at one speed at a constant rate. The millage from acceleration and climbing uphill would be the same, so there would be much less than a 12% improvement on epa millage.

dimples on balls -
pretty iffy stuff
Now goosebumps on chicks - another matter .

Dimples?
Don' need no steenken' dimples!
http://www.ehartwell.com/afj/Jimsphere

I'm late in the discussion, but there is something important in my opinion, the dimples in a golf ball will have a different effect than that on the car, I love mythbusters, but when it comes to aerodimamics they miss a lot of things, but anyway, the audience do not need to know aerodinamics to enjoy the show, so, there will be always some TV things on it to make it interesting and not 100% true.

The dimples in the golf balls and the car are comparable only on the surface, but after the boundary layer separates, an area of "loose air" is created after the body (golf ball, car,etc), and this are creates a suction effect that joins the separated air mass after the body leaves it's space, so the golf ball will have a certain "suction" holding the ball from the back, if the boundary layer energy is increased it will cover a greater area of the ball before separation, so this "loose air" or"suction to use a term" will be reduced, so the ball speed will be less affected than that of a plane one, so it's the dimples+the reduction in this suction that act as an overall effect, and the car being partially dimpled ( is not round, and airflow is not changed in the bottom) will never have the same effect as a golf ball, it will increase the range by reducing the drag, but it will never be on the same ratio as a round body where it's whole area is being affected.

That's the case on the submarines, as they are geometrically symmetric, the golf ball effect will reflect as a true comparison and results will be similar.

Just my opinion, regards

Would this be the approiate thread to ask if it would be the same principle for that big open space inside wheel pants?

The big open space in wheel pants is clearance to prevent dirt and mud from jamming the wheel.