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Mythbusters to test aerodynamics of dimples
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MITaerobike
Oct 23, 09 8:39
Post #51 of 65 (539 views)
Re: Mythbusters to test aerodynamics of dimples [stephen J]
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http://en.wikipedia.org/wiki/Reynolds_number
Re= (density*Velocity*Length)/(dynamic viscosity) is the equation I use most.
--
Mark Cote
MITAerobike (ST, Twitter)
Specialized Bicycle Components
Road Engineer/Aerodynamicist
Rappstar
Oct 23, 09 10:10
Post #52 of 65 (498 views)
Re: Mythbusters to test aerodynamics of dimples [jens]
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Wouldn't make the most sense to have dimpled tri suits and helmets before dimpling the frames...I mean we are the largest force in play with drag
Been done: Troxel helmet and Nike Spinsuit. Neither one showed any measurable improvement, and in some cases, tested worse. The Spinsuit, especially, tested quite poorly when Jens tested one, and IIRC, when they actually did get good numbers with it on Lance, it was held in place using all kinds of impractical methods according the folks at LSWT.
I've tested the Nike Swift Skinsuit and had excellent results. This was many years ago, but the suit was roughly 100 gF savings over a stock, normal lycra skinsuit. The key was that it needed to fit the athlete. A smaller (stretched out) skinsuit actually tested worse than one that was properly sized -- pulled open the fabric too much was our hypothesis.
Sure there have been other great suits designed in the past decade, but that Nike suit really was a step forward for cycling aerodynamics.
MC
I would not say that it tested poorly. It's just that a Pearl Izumi suit I bought from the late Steve Larsen was a bit faster.
-jens
Sorry, that's my bad Jens. I think the new custom PI suits for the Garmin Team are wicked. Apparently they have non-stretch panels that pull the shoulders into a hunch. That's cool.
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chrisyu
Oct 23, 09 10:11
Post #53 of 65 (496 views)
Re: Mythbusters to test aerodynamics of dimples [MITaerobike]
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...as a Caltech aero guy, I'm sure you know much more of the theory here than I do...
Is this a subtle dig of some sort? =P.
Once again, thanks for your willingness to share info! I'm curious if you guys have investigated whether transition to turbulence is happening over a rider's back (maybe with a white skin suit and oil film). I ask because given that the flow stays attached so far downstream and Re at mid-back is of order ~10^5, it seems like a pretty robust BL would be needed to navigate the back curvature (perhaps I am over estimating the actual amount of curvature on a "good" back shape). I'm interested because if this is indeed happening, then maybe it's possible to realize similar benefits by forcing earlier transition on people with less "optimal" back shapes. Anyways, my ramblings are becoming off topic in this thread...I'll PM you with my contact info so we can discuss further. I'm definitely interested in your research experience (I have some more CFD questions as well).
Chris
jackmott
Oct 23, 09 10:11
Post #54 of 65 (495 views)
Re: Mythbusters to test aerodynamics of dimples [Rappstar]
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i want to set my suit up with velcro to hold the tail of the aero helmet to my back
----
What is good in life? - To crush your enemie's dreams, see them dropped behind you, and to hear the lamentation of their women!
Sunglasses:
http://www.framesdirect.com/sunglasses
Tri Gear:
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Cheap Disc Wheel -
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jackmott
Oct 23, 09 10:11
Post #55 of 65 (495 views)
Re: Mythbusters to test aerodynamics of dimples [Rappstar]
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i want to set my suit up with velcro to hold the tail of the aero helmet to my back
----
What is good in life? - To crush your enemie's dreams, see them dropped behind you, and to hear the lamentation of their women!
Sunglasses:
http://www.framesdirect.com/sunglasses
Tri Gear:
http://www.atctrishop.com
Cheap Disc Wheel -
http://www.wheelbuilder.com
Tire Crr Data -
http://www.biketechreview.com
Rappstar
Oct 23, 09 10:14
Post #56 of 65 (493 views)
Re: Mythbusters to test aerodynamics of dimples [jackmott]
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i want to set my suit up with velcro to hold the tail of the aero helmet to my back
Honestly, I'd use rare earth magnets. Sew one into your suit and then glue one into the tail of your helmet. Use ones about the size of a quarter and they'd hold really well. Plus they will find each other much easier than velcro will. Just put them in proximity and they'll snap into place. Voila.
"BAM!" - Larry Williams
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jackmott
Oct 23, 09 10:16
Post #57 of 65 (490 views)
Re: Mythbusters to test aerodynamics of dimples [Rappstar]
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good idea, easier to pop them loose when you need to turn yer head
----
What is good in life? - To crush your enemie's dreams, see them dropped behind you, and to hear the lamentation of their women!
Sunglasses:
http://www.framesdirect.com/sunglasses
Tri Gear:
http://www.atctrishop.com
Cheap Disc Wheel -
http://www.wheelbuilder.com
Tire Crr Data -
http://www.biketechreview.com
zebragonzo
Oct 23, 09 10:30
Post #58 of 65 (482 views)
Re: Mythbusters to test aerodynamics of dimples [jackmott]
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How about putting a magnet in the jersey and a ferrous emergency multi-tool in the helmet?
Oh and Chris and MIT - don't discuss in a PM; it was just getting interesting!
jackmott
Oct 23, 09 10:33
Post #59 of 65 (477 views)
Re: Mythbusters to test aerodynamics of dimples [zebragonzo]
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ive said this before but, a multitool just behind the base of my head.....no!
=)
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How about putting a magnet in the jersey and a ferrous emergency multi-tool in the helmet?
Oh and Chris and MIT - don't discuss in a PM; it was just getting interesting!
----
What is good in life? - To crush your enemie's dreams, see them dropped behind you, and to hear the lamentation of their women!
Sunglasses:
http://www.framesdirect.com/sunglasses
Tri Gear:
http://www.atctrishop.com
Cheap Disc Wheel -
http://www.wheelbuilder.com
Tire Crr Data -
http://www.biketechreview.com
MITaerobike
Oct 23, 09 10:43
Post #60 of 65 (470 views)
Re: Mythbusters to test aerodynamics of dimples [Rappstar]
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The magnet idea is brillian, Jordan. I'm gonna give that a go with a TT2.
And no Chris, no Caltech dig -- I meant what I said. I did very little theory in college, mostly just experimental stuff in the tunnel. You'll have to talk slowly when we sit down and discuss aero.
BTW -- I watched this episode last night because of this thread. Really sparked some interest but it could confuse a lot of people because of the blanket statments being placed on the concept of dimple-ing. And I don't know what others think, but I couldn't see any appreciable difference between the cars in the water tunnel (btw running at 1 m/s).
Mark
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Mark Cote
MITAerobike (ST, Twitter)
Specialized Bicycle Components
Road Engineer/Aerodynamicist
zebragonzo
Oct 23, 09 10:46
Post #61 of 65 (463 views)
Re: Mythbusters to test aerodynamics of dimples [jackmott]
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Multi-tool under the jersey? Magnetic car keys in helmet?
stephen J
Oct 23, 09 11:22
Post #62 of 65 (448 views)
Re: Mythbusters to test aerodynamics of dimples [MITaerobike]
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The magnet idea is brillian, Jordan. I'm gonna give that a go with a TT2.
And no Chris, no Caltech dig -- I meant what I said. I did very little theory in college, mostly just experimental stuff in the tunnel. You'll have to talk slowly when we sit down and discuss aero.
BTW -- I watched this episode last night because of this thread. Really sparked some interest but it could confuse a lot of people because of the blanket statments being placed on the concept of dimple-ing. And I don't know what others think, but I couldn't see any appreciable difference between the cars in the water tunnel (btw running at 1 m/s).
Mark
--
Mark Cote
MITAerobike (ST, Twitter)
Specialized Bicycle Components
Road Engineer/Aerodynamicist
Im not so sure about rare earth magnets...real ones would either rip right out of the jersey, the helmet, or prevent you from looking down or to the side, soas to cause a crash. I would go with maybe one rare earth, and the other just a strong magnet. You would also have to put a disclaimer in with that...have you seen some of the things which have happened to children who get into their dads rare earth magnets? It is pretty gruesome, and would make you think twice about having any of those in your house if you have kids (now admittedly most of the really bad stuff happened with magnets about the size of a cell phone or larger).
Stephen J
____________________________________________
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chrisyu
Oct 23, 09 12:46
Post #63 of 65 (414 views)
Re: Mythbusters to test aerodynamics of dimples [MITaerobike]
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I'll need to sit down and watch the episode too. I agree that dimpling and trips are too often thought of as a global drag reduction technique. It is a tool (and one that is only applicable in a narrow Re regime, as Mark pointed out earlier) to delay onset of separation (via forced transition) and is only useful if stall would be imminent without an artificial trip. In the case of the car, this would have to be applied in very specific locations on the body (ie; mud covering everything is shotgun approach). Again, I need to watch the show and see what they concluded. I think the big problem lies in the fact that it is very difficult to say whether dimpling or trips are necessary on a complex geometry without a pretty detailed idea of what the local flow field looks like (and how it dynamically changes)....something that very few people/companies have the resources to investigate. Canonical examples like spheres, cylinders, or even airfoils can be used to gain insight but interactions on something like a bike or car can appreciably change the flow field.
Chris
Rappstar
Oct 23, 09 14:10
Post #64 of 65 (379 views)
Re: Mythbusters to test aerodynamics of dimples [stephen J]
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As with many things, size matters. You can get a really thin but wide ones. Or just very small one. You are likely thinking of the relatively thick neodymium magnets, which are a *type* of rare earth magnet, one of the strongest. There are other types that are not nearly as strong, and again, you just get a smaller magnet. Using reasonably sized neodymium would be a bit of overkill. You can basically hang your body weight off those...
"BAM!" - Larry Williams
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blog.rappstar.com
Ask me about:
Specialized
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PBN
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MITaerobike
Oct 23, 09 14:10
Post #65 of 65 (379 views)
Re: Mythbusters to test aerodynamics of dimples [chrisyu]
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I'll need to sit down and watch the episode too. I agree that dimpling and trips are too often thought of as a global drag reduction technique. It is a tool (and one that is only applicable in a narrow Re regime, as Mark pointed out earlier) to delay onset of separation (via forced transition) and is only useful if stall would be imminent without an artificial trip. In the case of the car, this would have to be applied in very specific locations on the body (ie; mud covering everything is shotgun approach). Again, I need to watch the show and see what they concluded. I think the big problem lies in the fact that it is very difficult to say whether dimpling or trips are necessary on a complex geometry without a pretty detailed idea of what the local flow field looks like (and how it dynamically changes)....something that very few people/companies have the resources to investigate. Canonical examples like spheres, cylinders, or even airfoils can be used to gain insight but interactions on something like a bike or car can appreciably change the flow field.
Chris
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Mark Cote
MITAerobike (ST, Twitter)
Specialized Bicycle Components
Road Engineer/Aerodynamicist
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