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Old 11-05-2008, 03:42 PM   #31
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Originally Posted by Pete View Post
That said I have yet to see any such dimples / hexagons or anything similar appearing on 747's etc which suggests to me a smoother panel may be a better option in this instance.

Cheers , Pete.
From what I understand, airplanes are shaped so the airstream remains attached from tip to tail. For shapes where airflow detaches, dimples/zigzag tape help the air stay attached a little longer.
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Old 12-06-2008, 09:52 AM   #32
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Trip the boundary layer

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Originally Posted by Pete View Post
<snips>

That said I have yet to see any such dimples / hexagons or anything similar appearing on 747's etc which suggests to me a smoother panel may be a better option in this instance.

Cheers , Pete.
The big airliners don't have them because the passengers pay for the fuel and fuel has been cheap for a long time... cheaper than the extra engineering that boundary layer trips would cost added to the purchase of a big plane.

Ribbing, zig-zag strips, and dimples work on most trailing surfaces, even with designs that have laminar flow designs.

Anything that can trip the boundary layer and get it to stay attached to the form will help with the aerodynamic (or hydrodynamic) performance.

I've seen the effects of this myself in my streamlined recumbent racing vehicle development.

Best regards,

Jeff Bales
Tucson, AZ
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Old 12-10-2008, 05:07 PM   #33
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I was looking at the Festiva today and the back hatch isn't that dramatic, I thought it was almost vertical, but it looks like maybe a 60 deg slope. Granted this is more than on a round golf ball, but it still might help to have some dimples at the transition and halfway down the hatch. If only I could find some clear, stick, dimpled tape. I'm assuming larger dimples would be better, maybe 1/2" diameter, rather than tiny pinhole size on golf balls.

Any thoughts?

And yes, I was saying earlier that dimples will actually hurt compared to smoother or bubblier front sections. The point is that they create turbulence, but this turbulence helps keep the flow attached and seperate further down the shape of the object. Good turbulence. But only in the rear where it otherwise would be seperated and more turbulent, not in the front where it'd have been smooth anyway.
I think I've read that flow only attaches at a max of 30 degrees (from horizontal). I don't think there is any way you will get it to stay attached at 60
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Old 12-11-2008, 08:36 PM   #34
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I think I've read that flow only attaches at a max of 30 degrees (from horizontal). I don't think there is any way you will get it to stay attached at 60
I agree. I did some tuft testing on my car and found that airflow became turbulent at around 1/2 way down the back of the rear hatch on my car. Here's a nice side view of my car, http://www.gassavers.org/garage_imag...0pum2sbw4z.jpg
The angle of the hatch is around 20 degrees at that point. Something else to keep in mind is that the boundary layer grown thicker farther back on the vehicle, so I might have been seeing the tufts fluttering from that affect, too.

I want to try adding a trip layer to the rear window on my car, but it would be a b i t c h! to clean, and I'm sure it would collect dirt and look really nasty in no time.
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Old 12-12-2008, 03:48 AM   #35
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Quote:
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I want to try adding a trip layer to the rear window
What is a trip layer?
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Old 12-12-2008, 06:04 AM   #36
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Golf ball dimples don't reduce drag. The dimples increase drag.

So why do dimples make golf balls travel farther?

It's because the flying balls have backspin imparted by the angled club head. The backspin creates a low pressure area above the ball and a high pressure area under the ball, so that it stays in the air longer. Longer time in the air = longer distance. Dimples accentuate this "Bernouli" effect.

Dimples will increase automobile drag.
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Old 12-13-2008, 08:47 AM   #37
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Originally Posted by Sludgy View Post
Golf ball dimples don't reduce drag. The dimples increase drag.

So why do dimples make golf balls travel farther?

It's because the flying balls have backspin imparted by the angled club head. The backspin creates a low pressure area above the ball and a high pressure area under the ball, so that it stays in the air longer. Longer time in the air = longer distance. Dimples accentuate this "Bernouli" effect.

Dimples will increase automobile drag.
I was wondering about this. I could have sworn I heard that the dimples help a golf ball because it spins, but then I couldn't think of what would make a golf ball spin... So it's the angled club.
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Old 12-13-2008, 10:00 AM   #38
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Semantically, the issue of dimples vs. drag really isn't so black and white. I think that in terms of automotive application it pretty much is, however it cannot be ignored that when utilized in the right way, they can reduce the wake of an object, and consequently, the drag. Utilized incorrectly, they will increase drag.

It's also worth noting that dimples work for objects with varied orientation dynamics to the direction of airflow. In other words, on a golf ball, it doesn't matter which side is facing the airflow, the dimple pattern is always oriented in a way to create the desired effect. Cars, boats, and planes in contrast are always more or less oriented the same way into the flow, so there is no need to use the more dynamically effective dimple design, and instead the more directional linear boundary trip could provide greater benefit in the right place.
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Old 12-13-2008, 11:22 PM   #39
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So instead of dimples, what about a strip of thick tape just before a rear window drops off? That's the idea with the zigzag tape right?
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Old 12-15-2008, 11:05 PM   #40
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What is a trip layer?
Perhaps I don't have the correct term?
I meant that I wanted to add something to trip the boundary layer and help airflow remain more closely attached to the rear of the car, something like zig-zag tape or?

I was thinking of using clear silicone caulking. It can be applied almost anywhere, in various shapes and patterns, the thickness can vary, it's not too noticeable, and it's removable. The drawback is that, as I mentioned, if applied to a window it would collect dirt and be tough to clean for good visibility.
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