#21  
Old 09-02-2013, 10:12 AM
ToddL ToddL is offline
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Originally Posted by atvan View Post
As far as I know, the physics behind it is that with smooth surface the boundary layer (the air that the disc is pulling along with it) remains laminar (the air is flowing uniformly) for a longer period of time. This, in itself, would reduce drag, but when the boundary layer becomes turbulent, it can detach from the surface and greatly reduce drag. The matte surface forces the boundary layer to become turbulent sooner, and thus prevents separation. This same principle is important to aircraft wings, and is why some model airplanes will have sandpaper on the leading edge or their wings.
Yea, more or less. (despite your mixup in there)

A smooth surface will create a laminar boundary layer. A laminar BL has lower skin friction drag but suffers from the disadvantage of separating into a detached wake behind the object. The wake creates high amounts of pressure drag.
A rough surface promotes a turbulent boundary layer. This has higher skin friction, but the turbulent BL will remain attached to the body surface for a longer distance and will thus decrease the size of the wake, reducing pressure drag.

Aerodynamic design is a tradeoff between the two. With golf balls, it's clearly advantageous to sacrifice friction drag for the benefit of pressure drag. With a disc ... I'm not so sure. I doubt the separated wake is very large.

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  #22  
Old 09-02-2013, 10:37 AM
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Curtis_Valk Curtis_Valk is offline
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Quote:
Originally Posted by ToddL View Post
A smooth surface will create a laminar boundary layer. A laminar BL has lower skin friction drag but suffers from the disadvantage of separating into a detached wake behind the object. The wake creates high amounts of pressure drag.
A rough surface promotes a turbulent boundary layer. This has higher skin friction, but the turbulent BL will remain attached to the body surface for a longer distance and will thus decrease the size of the wake, reducing pressure drag.

Aerodynamic design is a tradeoff between the two. With golf balls, it's clearly advantageous to sacrifice friction drag for the benefit of pressure drag. With a disc ... I'm not so sure. I doubt the separated wake is very large.
Thanks for posting that, it cleared up a long held misconception for me.

Curtis
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  #23  
Old 09-02-2013, 11:55 AM
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ohtobediscing ohtobediscing is offline
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Yeah, one kayak company tried the dimpled surface on the bottom of their play boats to get the "boundary layer" effect while surfing in hydraulic water features. Because the speeds were so slow, however, it didn't make any difference, and the dimples hooked up on rocks/snags and tore open.
That boat, of course, no longer exists.
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  #24  
Old 09-02-2013, 12:36 PM
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Lewis Lewis is offline
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Quote:
Originally Posted by Curtis_Valk View Post
Thanks for posting that, it cleared up a long held misconception for me.
It also explains why dimples help a golf ball more than they do a car: a sphere is a less aerodynamic shape than an automobile body, and suffers more from the drag created by its wake. This would suggest that P&A discs might benefit more from dimples than drivers, but people don't throw discs as hard as they hit golf balls, so maybe the difference isn't significant. On the other hand, what, other than texture, accounts for the different flight characteristics of, say, DX vs. Champion plastic, or in the difference between a beat up Roc and a new one? And in Gateway's case, it still might make a good sales pitch for SSS discs, even if most of us don't notice any added distance vs. the competition. I throw Gateway and Vibram putters for the feel of the rim and grip of the surface rather than the aerodynamics at high speed.

Last edited by Lewis; 09-02-2013 at 12:38 PM.
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  #25  
Old 09-02-2013, 01:28 PM
ToddL ToddL is offline
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Originally Posted by Lewis View Post
It also explains why dimples help a golf ball more than they do a car: a sphere is a less aerodynamic shape than an automobile body, and suffers more from the drag created by its wake.
I would argue against that.
A regular car will probably have some points that will cause separation no matter what type of boundary layer exists. A flat roof with a sharp corner to the rear window will separate no matter what. If we have good, attached flow along the roof, we'll see separation at that 30deg or 45deg corner.

We might even see some separation at the forward edge of the roof from the flow coming off the windshield. And we can't really add turbulators to the windshield (neverminding that the wiper blades actually do a good job of that at the bottom of the windshield).

The back end of a car will be an enormous wake (very high pressure drag) no matter what you do to the skin. The only way to alleviate that would be to completely reshape the entire car. Big and round up front, smooth transition to a thin point at the back, like a raindrop or an airfoil. (search for "Aptera Car" to see a good example)
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  #26  
Old 09-02-2013, 01:41 PM
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Quote:
Originally Posted by ToddL View Post
I would argue against that.
A regular car will probably have some points that will cause separation no matter what type of boundary layer exists. A flat roof with a sharp corner to the rear window will separate no matter what. If we have good, attached flow along the roof, we'll see separation at that 30deg or 45deg corner.
...
The back end of a car will be an enormous wake (very high pressure drag) no matter what you do to the skin. The only way to alleviate that would be to completely reshape the entire car. Big and round up front, smooth transition to a thin point at the back, like a raindrop or an airfoil. (search for "Aptera Car" to see a good example)
For sure, some auto bodies are better than others. A 1970s Oldsmobile isn't as aerodynamic as a Prius. But without being an aeronautical engineer, I have to rely on the Mythbusters' results, and suppose that spheres have a worse wake than recent model car bodies.
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  #27  
Old 09-03-2013, 09:22 AM
garublador garublador is online now
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It's my guess that the difference the finish of a disc makes in flight is eclipsed by differences in shape between discs. In other words, the way a disc comes out of the mold makes way more difference than whether or not the disc is rough or smooth. Champ type discs seem to retain "sharper" features and that's why they act more overstable. However, because they take damage differently (they don't scuff as easy, but can be bent) the aerodynamic difference can be seen when the discs beat up, but it's more because of the beating they take than the initial surface finish of the disc.

It's my understanding that the surface finish theory came about when they started making discs that were designed in low end plastics in higher end plastics. They didn't glide as well and were more overstable. However, discs in Pro/X type plastic that were designed in that plastic had a smoother finish and the same, or better, glide than their D/DX type counterparts so some people started rethinking that theory. Now there are more and more discs designed in and made out of higher end plastics and they seem to glide better than discs designed in low end plastics, but made out of high end plastics.
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