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Slowtwitch Forums: Triathlon Forum:
Mythbusters to test aerodynamics of dimples

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fstbckadct

Oct 22, 09 9:51

Post #26 of 65 (1131 views)

Re: Mythbusters to test aerodynamics of dimples [bryan913] [In reply to]	Can't Post

In Reply To:

Wow, when are they going to dimple actual bikes?

I should patent that before Cervelo does. Damn

In Reply To:

11% increase. pretty significant advantage with the dimpled car.

djciii

Oct 22, 09 9:56

Post #27 of 65 (1128 views)

Re: Mythbusters to test aerodynamics of dimples [MITaerobike] [In reply to]	Can't Post

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
--------
My Ride: http://www.kestrelbicycles.com/
My Power: http://www.saris.com/t-CycleopsPower.aspx
My Lid: http://www.rudyprojectusa.com/
My Protector: http://www.thatbuttstuff.com/

Rappstar

Oct 22, 09 10:13

Post #28 of 65 (1107 views)

Re: Mythbusters to test aerodynamics of dimples [djciii] [In reply to]	Can't Post

In Reply To:

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.

Toby Tri

Oct 22, 09 10:25

Post #29 of 65 (1091 views)

Re: Mythbusters to test aerodynamics of dimples [Sully] [In reply to]	Can't Post

Dimpled wheels, hubs, tires . . . BAWAHAHAHAHAHAHAHAHA! Fools . . . and their money are soon parted.

angryirish

Oct 22, 09 10:27

Post #30 of 65 (1087 views)

Re: Mythbusters to test aerodynamics of dimples [stephen J] [In reply to]	Can't Post

In Reply To:

If you have a object that has the same orientation in relation to travel direction, then you are better off with trip lines; which I have most often seen a chevrons. These are available as 'stickers' that you can put on your glider, Cessna, small aircraft, etc. Going from my foggy memory, you put them just before the maximum diameter of your airfoil for maximum effect.
Stephen J

stephen J

Oct 22, 09 10:43

Post #31 of 65 (1066 views)

Re: Mythbusters to test aerodynamics of dimples [angryirish] [In reply to]	Can't Post

In Reply To:

____________________________________________
Genius is recognizing when you are damn lucky

MITaerobike

Oct 22, 09 13:12

Post #32 of 65 (1005 views)

Re: Mythbusters to test aerodynamics of dimples [Rappstar] [In reply to]	Can't Post

In Reply To:

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

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
--
Mark Cote
MITAerobike (ST, Twitter)
Specialized Bicycle Components
Road Engineer/Aerodynamicist

gholmes

Oct 22, 09 13:29

Post #33 of 65 (987 views)

Re: Mythbusters to test aerodynamics of dimples [burnman] [In reply to]	Can't Post

In Reply To:

I work in the fuel/energy industry, and I'm continually amazed by the trends of corporate R&D, and appropriated gov't funds. Auto manufacturers spend billions of dollars developing alternative fuel systems and balance of plant equipment, all in an effort to gain some marginal improvement in fuel economy. If they just payed Joe Schmo from Detroit $30 an hour to stand on the assembly line with a hammer and beat the hell out of their sheet metal forms, they could nearly reach their goal. Annual cost to employer would be under $100k.

On the other hand, the Joe Schmos of the world could very well put me out of business ...

Except unfortunately one reason its been difficult to make automobiles more aerodynamic has been the appearnce of those cars to the consumer.
Better today but the most aero dynamic automobiles lacked visual appeal to the consumer thats changing somewhat but how many people who would buy
an automobile with hand made diy dimples in it..

does it have to be dimples can some other means attachments bends curves in the lines actually do the job just as well?
Plus the dimples wouldn't have to be over the entire automobile I would think not like mythbusters just where the air seemed to flow.(front,top)

stephen J

Oct 22, 09 13:37

Post #34 of 65 (978 views)

Re: Mythbusters to test aerodynamics of dimples [angryirish] [In reply to]	Can't Post

In Reply To:

You're speaking of Vortex Generators. As simple as a rectangular verticle fin, placed at an angle to the airflow. Used for airfoil effeciency, and commonly to improve airflow on the top surface that has been f'd up by an engine, flaps, etc.

That's all why I was asking about the speeds involved. It does seem that more would be gained, on an otherwise streamlined vehicle, by strategically placing the appropriate "device".....VG's on areas like sidemirrors, and the tops of rear quarter panels, dimples on hubcaps, straight ridges on flat curves perpindicular to the direction of travel......good luck selling that ugly car though.

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No, This is not what I was speaking of at all. I just did a quick search, and made a post which did not show what I looked up; so here it is again hopefully:

Turbulators:

http://www.mh-aerotools.de/airfoils/turbulat.htm
http://www.standardcirrus.org/Turbulators.html
http://www.wingsandwheels.com/page29.htm
http://www.streifly.de/shop%20baender-e.htm

____________________________________________
Genius is recognizing when you are damn lucky

stephen J

Oct 22, 09 13:38

Post #35 of 65 (976 views)

Re: Mythbusters to test aerodynamics of dimples [gholmes] [In reply to]	Can't Post

see my other post:

Turbulators

____________________________________________
Genius is recognizing when you are damn lucky

MITaerobike

Oct 22, 09 16:38

Post #36 of 65 (908 views)

Re: Mythbusters to test aerodynamics of dimples [stephen J] [In reply to]	Can't Post

From that second link:

"For the low Reynolds numbers of model aircraft, forced transition by means of a turbulator seems to be more recommendable. A small drag increase in the high speed regime is the cost which has to be paid for the bigger improvements at low flight speed. It is not a result of a bad design, when an airfoil needs a turbulator for best performance - at least not at Reynolds numbers below 1 million."

To put it in perspective, the Re # over a bike tube (even a HT) is about 10,000 to 50,000 max at 13.4 m/s (30 mph). That's x100 less than low speed flow over an airplane wing (assume 10x the size of a DT airfoil, 10x the speed = 30cm tall wing, chord of 1.5 m, going through 300 mph wind. Very different flow cases.

Interesting forum thread -- this is fun stuff.

MC
--
Mark Cote
MITAerobike (ST, Twitter)
Specialized Bicycle Components
Road Engineer/Aerodynamicist

djciii

Oct 22, 09 16:39

Post #37 of 65 (907 views)

Re: Mythbusters to test aerodynamics of dimples [gholmes] [In reply to]	Can't Post

I think that Adam and Jamie were right, this might have some legs in nascar..they don't give a damn what those cars look like.they just want speed
--------
My Ride: http://www.kestrelbicycles.com/
My Power: http://www.saris.com/t-CycleopsPower.aspx
My Lid: http://www.rudyprojectusa.com/
My Protector: http://www.thatbuttstuff.com/

gurufrank

Oct 22, 09 16:53

Post #38 of 65 (895 views)

Re: Mythbusters to test aerodynamics of dimples [slimfast] [In reply to]	Can't Post

If I understood you correctly, it seems that we should start adding dimples/ridges to undersides of our wetsuits...

Mr. Tibbs

Oct 22, 09 16:57

Post #39 of 65 (886 views)

Re: Mythbusters to test aerodynamics of dimples [Sully] [In reply to]	Can't Post

Anyone else want to do naked stuff with Carry Byron?

Mr. Tibbs will justify your sins.

stephen J

Oct 22, 09 17:19

Post #40 of 65 (878 views)

Re: Mythbusters to test aerodynamics of dimples [MITaerobike] [In reply to]	Can't Post

HI Mark,

The way I read that passage is that the use of turbulators are more ammenable to lower speed applications. The are using it on sailplanes extensively. The speed of sailplanes is a bit closer to what we see in cycling (I would say that unless you are diving, it will be less than 100 mph...right?). So in that case, it should be useful to use them in a cycling application.

Now, one of the things that seem to pop up all the time with these discussions is average speed of cyclists, and how these methods dont relate to most people. Well, I think that to an extent this is hogwash. This is because many courses have hills. Hills slow people down going up, and speed them up going down. So if you live in a place with hills that make you go 40-50mph for part of the race (distance at a certain speed vs distance at a slower speed going up them) then you will be in a zone where most people say they are not concerned about because they never go that fast...What I am so convolutedly trying to say is that people in races DO go in excess of 30mph; and the argument that aerodynamics dont apply to them because they dont go fast enough to take advantage of phenomenon that occur at faster speeds is not correct. One thing that I am probably missing on is whether there are actually things that happen differently between 15mph and over 30mph. This part I just dont know about. I would suspect that there are things different that happen; at the very least on the microenvironmental scale...but how this relates to the overall aerodynamics of a system, that is something that I unfortunately are much less versed at.

With this said, if you look at fluid dynamics, trip lines/ turbulators, etc, have also been used in boats (both above and below the water) which is a fluid much denser than air. They are a little different in this medium, but the definately exist.

I have seen data published with the use of these types of devices in peer reviewed literature. If some of the other people here really want to know about it, they will have to do the searching for themselves, as it is interesting stuff, and much more valuable if you connect the dots yourself.

Mark- on your final point--I will have to defer to your expertise that that a wing traveling at 300mph is very different than a bicycle downtube traveling at 50mph; but I would have to argue that the information that is available would tend to argue that this is not a very good comparison for the issue of turbulators. I would suggest a sailplane and a bicycle downtube would be a better example...But I will be the very first to admit that I am a novice in this area.

Stephen J

____________________________________________
Genius is recognizing when you are damn lucky

ndenezzo

Oct 22, 09 20:22

Post #41 of 65 (818 views)

Re: Mythbusters to test aerodynamics of dimples [gurufrank] [In reply to]	Can't Post

In Reply To:

If I understood you correctly, it seems that we should start adding dimples/ridges to undersides of our wetsuits...

Rocket Science sports actually has vortex generators on the back of their wetsuits and trisuits and dimples on the legs. They claim the vortex generators reduce swimming drag by up to 6%.
http://www.rocketsciencesports.com/...Race_20BPM_Page.php#
http://www.rocketsciencesports.com/...t_Race_Suit_Page.php
http://www.rocketsciencesports.com/...et_Wet_Suit_Page.php

Trirunner

Oct 22, 09 22:29

Post #42 of 65 (786 views)

Re: Mythbusters to test aerodynamics of dimples [Toby Tri] [In reply to]	Can't Post

No problem when you have a dimpled wallet!

chrisyu

Oct 22, 09 22:56

Post #43 of 65 (780 views)

Re: Mythbusters to test aerodynamics of dimples [MITaerobike] [In reply to]	Can't Post

In Reply To:

To put it in perspective, the Re # over a bike tube (even a HT) is about 10,000 to 50,000 max at 13.4 m/s (30 mph). That's x100 less than low speed flow over an airplane wing (assume 10x the size of a DT airfoil, 10x the speed = 30cm tall wing, chord of 1.5 m, going through 300 mph wind. Very different flow cases.

Interesting forum thread -- this is fun stuff.

MC

Hi Mark,

Do you see any transitional areas (including rider) in your tests or simulations? Given that a Re based on tube member scale is of order 10^4, it seems that a Re based on a longer length scale (ie; rider or treating the whole bike as a slender body) would easily reach 10^5 to 10^6. I suppose this may explain the gains you saw with a dimpled suit on the rider. This is assuming, of course, that laminar separation has not already occurred - admittedly a poor assumption at higher yaw.

Chris

MITaerobike

Oct 22, 09 23:52

Post #44 of 65 (767 views)

Re: Mythbusters to test aerodynamics of dimples [chrisyu] [In reply to]	Can't Post

R10C

Oct 22, 09 23:58

Post #45 of 65 (763 views)

Re: Mythbusters to test aerodynamics of dimples [MITaerobike] [In reply to]	Can't Post

But...when they are "prefect"....are we looking at a 3 hour IM bike split?

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f.k.a - Record9, Record9ti Record10, Record10ti, Record10Carbon, but not SuperRecord11 as there are no bar ends.

zebragonzo

Oct 23, 09 2:49

Post #46 of 65 (726 views)

Re: Mythbusters to test aerodynamics of dimples [Record10Carbon] [In reply to]	Can't Post

A fun sum I did a while back:
If a cyclist were to achieve the same aerodynamic shape (Cd) as a speed skier, they'd go from 25mph to 63mph.

stephen J

Oct 23, 09 4:19

Post #47 of 65 (704 views)

Re: Mythbusters to test aerodynamics of dimples [MITaerobike] [In reply to]	Can't Post

Oh yes, the rider's body is entirely different. Airflow remains attached much longer over a rider's back. Over a good back shape, you'll see airflow attached all the way to the rider's butt (usually at the large seam of the skinsuit). Dimple-ing/tripping of airflow is really only needed on the parts of the body that don't run tangential to airflow (the upper arm, upper leg, calf). The back and shoulders are a good top edge of an airfoil and see Re up in a much higher regime. Thus airflow attaches longer on the back and doesn't separate at the tallest point (like it would on a HT with a much lower Re).

Chris, I read your PM and as a Caltech aero guy, I'm sure you know much more of the theory here than I do. What I can say is that from my experience, the theory, CFD, wind tunnel flow visualization, and wind tunnel drag data all line up really well. There's still a lot to learn and I don't think the 3D aero shapes are perfect yet. I'd be interested in having a conversation about your research and would be glad to share mine as well. Our CFD models are pretty primitive and generally just show problem areas in the 3D rather than a home brew code that might yield an ideal airfoil shape. The CFD I do now is very different than the 2D I did in college.

Mark
--
Mark Cote
MITAerobike (ST, Twitter)
Specialized Bicycle Components
Road Engineer/Aerodynamicist

Ok,

I see what you are saying now... My very basic issue is not really understanding the variables that go into the reynolds numbers (mark one for the engineer and zero for the molecular biologist). One thing from my previous thought that I still maintain is the part about boats (both above and below water)...do you know much about reynolds numbers of boat hulls. From what you are describing the reynolds numbers of a wing, then the reynolds number of a hull would be even higher....and they definately make use of a tripping mechanism to decrease drag (by a quite increadible amount). Most is available in peer review literature, and some can be inferred from unclassified military literature...you just need to know how and were to look. What are your thoughts?

Stephen J

____________________________________________
Genius is recognizing when you are damn lucky

dalessit

Oct 23, 09 6:57

Post #48 of 65 (638 views)

Re: Mythbusters to test aerodynamics of dimples [Mr. Tibbs] [In reply to]	Can't Post

In Reply To:

Anyone else want to do naked stuff with Carry Byron?

yes

Spoon

Oct 23, 09 7:25

Post #49 of 65 (613 views)

Re: Mythbusters to test aerodynamics of dimples [Mr. Tibbs] [In reply to]	Can't Post

Isn't that why everyone watches the show?

jens

Oct 23, 09 8:24

Post #50 of 65 (582 views)

Re: Mythbusters to test aerodynamics of dimples [MITaerobike] [In reply to]	Can't Post

In Reply To:

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

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

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