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Old 10-24-2009, 04:53 PM   #21 (permalink)
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dimples

I'm glad they did the test but I would take them to task for it.The only reason dimples are on golfballs is to hasten the transition to turbulent boundary layer an it's attendant lower Cd (profile drag/pressure drag).
Automobiles,given their size, transition to turbulent boundary layer and constant Cd around 20-mph,so it's not an issue for cars.And the "roughness" of "smooth" paint is all that's necessary for the transition.
I get a kick out of the show but sometimes I wonder if those folks ever pick up books and look at them.

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Old 10-26-2009, 12:46 AM   #22 (permalink)
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To follow up on that point, I found this nicely written piece from airliners.net user QantasA332:

Basically, there are three primary types of drag acting on an aircraft: induced drag, skin friction drag, and pressure drag. It is pressure drag that is the main factor involved in the dimple design's existence. Pressure drag is primarily the result of a moving body's wake. Depending on how soon the airflow separates as it passes over an object - that is, how far along the object the flow travels before no longer following the contour of the object - the size of the wake will be larger or smaller. A larger wake equates to more pressure drag (put simply, there is a larger region of stagnant air behind the body meaning the airflow pushing on the front of the body has less impeding its production of drag) and vice versa.

Now, imagine a sphere. Because its height/diameter is large in comparison with its length, it is what's known as a "bluff body." Bluff bodies such as a sphere have disproportionately large wakes, and as a result they have disproportionately high pressure drag. (This is compared to both their own skin friction drag and a not-bluff solid's pressure drag). Obviously, then, overall drag on a sphere (or other bluff body) can be dramatically reduced if pressure drag is reduced. That is, pressure drag is what you want to specifically target and minimize.

Enter dimples. Dimples turbulate the airflow over an object, thus increasing the flow's kinetic energy. This acts to delay flow separation, which then leads to a smaller wake, which in turn leads to less pressure drag. And this solves the bluff body problem! Because bluff bodies have such high pressure drag compared to their skin friction drag, what little extra of the latter drag is created by dimples is more than offset by the drastic reduction of the former drag. So a golf ball - the classic example of a bluff body - will travel farther with dimples than without, and that is of course why they have come to carry these dimples.

Now, to finally answer your question: 'normal' aircraft are very simply not bluff bodies. Dimples would create more skin friction drag than they would reduce pressure drag, defeating their purpose.


So looking at a car, only where you have flow separation, would these dimples or any kind of vortex generator help. On most sedans, the back glass is prone to flow separation, and so you may see some reduction in drag there if implemented right. The rest of the car should have attached flow and thus the dimples would only add skin friction drag.
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Old 10-26-2009, 01:57 AM   #23 (permalink)
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IMHO that Mythbuster car dimple thing was about the looniest thing I've ever seen. It might make a great Golf Add Car though.

First, golf ball dimples work because at the "Reynolds Number" a golf ball lives at the air is effectively much thicker than say said automobile and builds up a huge boundary layer (compared to it's overall size) and the dimples act like mini tabulators and scrub the thick boundary layer reducing the "apparent" form size of the ball. As a result the air flow boundary layer is reduced allowing the air to more closely conform to the ball. The turbulent surface drag results in being less than the former apparent form drag. You can get the same effect by placing a small wire ring about 25% of the way back from the leading edge to trip the flow. Older Lear jets used that technique just ahead of the ailerons to re-energize and attach the flow from the thick boundary layer.

I think a Ball Peen hammer would work just as well. Or park your car outside in Denver during summer hail season. Or several seasons.

Seriously, the more the air has to bend and go around things the more energy that is imparted to the vehicle from the flow. This results in drag. IRL a car is a universe of high and low Reynolds numbers. For us mortals the nuances of it all are superfluous.
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Old 10-26-2009, 04:36 AM   #24 (permalink)
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SO can anyone think of a way to test the dimpling ourselves so we can have our own data? Besides just piling clay on the car. Which obviously is a no go for the do it yourselfer.
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Old 10-26-2009, 01:26 PM   #25 (permalink)
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Sorry, I posted this on the "Haxagonal dimples" thread before I discovered the Mythbusters thread, so here goes again:

The dimples on the roof are probably beneficial (IMO). It looks to me like the rear window area is about the worst angle it could be for drag (if I remember my Hucho, 30 degrees is worst, 15 best...YMMV) and the dimples near the back of the roof are probably detaching airflow over the rear window. A little spoiler up there would do it better I'd think, but anything would help.
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Old 10-26-2009, 02:18 PM   #26 (permalink)
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Mythbusters cynics & skeptics, especially on this site, could easily compel the creators/producers of the show to retest this episode. The show is famous for recreating followup shows from contentious 'busted/plausible' findings. An easy way to reproduce 'bigger explosions' tests. The now famous 'dimples' car should be retested w/ ecomodders' test suggestions. They may have to recreate new 'control' car with the requisite clay layer. From tuft testing, car companies' full size wind tunnel test facilities, different load/constant speeds tests, hypermiling driving techniques, time of day/weather conditions, etc., could be used for several batteries of testing. Any other suggestions?...
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Old 11-01-2009, 07:08 AM   #27 (permalink)
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Quote:
Originally Posted by majestic View Post
... only where you have flow separation, would these dimples or any kind of vortex generator help. On most sedans, the back glass is prone to flow separation, and so you may see some reduction in drag there if implemented right. The rest of the car should have attached flow and thus the dimples would only add skin friction drag.
Thanks, majestic.

OK, so based on this understanding you could cover the rear glass with dimples and the separation would be delayed, with a reduction in net c/d. Nice.

However -
1) You need to be able to see through the glass.
So we need researchers to develop 'glass' with an index of refraction identical to air (zero??) so the bubbles won't affect your view.

2) The worst offenders in the rear wake department generally are the vehicles with a non-sloping roof: vans, wagons or hatches with a near-vertical rear panel. It looks like dimples won't help them much. You could put the dimples along the rear edge, like vortex generators, but you're not delaying the separation to occur at a point with a smaller cross section to get a smaller wake.

Does that seem right?
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Old 11-01-2009, 02:56 PM   #28 (permalink)
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Quote:
Originally Posted by brucepick View Post
However - 1) You need to be able to see through the glass.
...not necessarily, simply implement rearward facing wide-angle camera with dash display.
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Old 11-01-2009, 10:19 PM   #29 (permalink)
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Quote:
Originally Posted by brucepick View Post
Thanks, majestic.

OK, so based on this understanding you could cover the rear glass with dimples and the separation would be delayed,
with a reduction in net c/d. Nice.

However -
1) You need to be able to see through the glass.
So we need researchers to develop 'glass' with an index of refraction identical to air (zero??) so the bubbles won't affect your view.

2) The worst offenders in the rear wake department generally are the vehicles with a non-sloping roof: vans, wagons or hatches with a near-vertical rear panel. It looks like dimples won't help them much. You could put the dimples along the rear edge, like vortex generators, but you're not delaying the separation to occur at a point with a smaller cross section to get a smaller wake.

Does that seem right?
No, Sir. Since flow is already detached at the glass, there is nothing there to create the vortices to keep attached flow, so they'd be most pointless there. Instead, the dimples would have to be placed at the rearmost point where flow is still attached, which would be the upper rear window frame on any vehicle.

1) Dimples don't necessarily have to be concave. They can be convex, and in either event, there are window shades that are perfectly legal in most states that you can only see through on one side. A lot of money was spent to figure out that it's pretty easy to see through a black mesh, but much more difficult to see through any other color or shade. Make sure the dimples/pimples are solid, and black on the side that sticks to the window, the driver only loses as much visibility through the rear window as those stupid full-window vinyl coverings.
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Old 11-01-2009, 10:24 PM   #30 (permalink)
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Quote:
Originally Posted by brucepick View Post

2) The worst offenders in the rear wake department generally are the vehicles with a non-sloping roof: vans, wagons or hatches with a near-vertical rear panel. It looks like dimples won't help them much. You could put the dimples along the rear edge, like vortex generators, but you're not delaying the separation to occur at a point with a smaller cross section to get a smaller wake.

Does that seem right?
This one is in a separate post so the information doesn't get confused.

Yes, anything with a flat back is going to be worse than something that is smaller and has even a virtually tapered rear end, such as a sedan. If the roof isn't tapered down as it leads rearward, it's going to create a larger wake than one that does taper downward.

In theory, something that would allow flow to stay attached around those rearmost edges would also reduce wake area, but not necessarily reduce drag, as I understand it.

In essence, while you're technically still decreasing the area that the wake occupies, you're increasing drag versus clean separation, so the decreased wake will probably be canceled out by the increased drag.

If one were to use a "duckbill" type spoiler, which is mid-mounted just below the windshield, and it didn't quite stick out far enough to make the 12* virtual slope, one could probably use dimples to train the flow downward, like a vortex generator. That's all they really are.

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