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 …
Dimpling introduces turbulence, thus delaying the onset and magnitude of flow separation on bluff bodies (e.g. a spherical golf ball). For bodies that are already streamlined, such as an airfoil, the advantages to dimpling become trivial.
I don’t thing that is entirely accurate. The effect is not trivial; but a dimple is not the best way to arrive at that ‘trip’ introducing turbulence. Dimples are useful if you do not know the orientation of the object as it travels through the air; or if it is a changing orientation of the object. 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. There are even YouTube videos online which show how to test the placement of the strips to get the maximum effect using oil (there must be some flyers on this list who know more about this than me). Anyway, with this given, dimples would be good for golf balls, bicycle wheels, but not really frames, helmets, or bodies. Maybe cranks; but that I think gets more into the realm of marketing.
Generally agreed – Reynolds number comparison is key! A 50 ft wide plane going 200 mph, a Prius going 55 mph, and a triathlete on an aero bike going 20 mph see entirely different Reynolds Numbers.
There is a place for ridges and dimples in the bike world but generally not as they’ve been used in the past 10 years.
Something else that’s not been mentioned thus far; because dimples transform flow from laminar to turbulent, a greater amount of airflow is affected by the direction of movement of the surface (a turbulent flow is ‘deeper’ than laminar).
Let me explain in terms of a wheel. On the front of a wheel, the rim moves downwards relative to the bike. In doing so, it re-directs a small amount of airflow downwards. If you make the rim dimpled, it re-directs more air downwards.
On a side note, in a previous life I was a competitive whitewater kayaker. As I was getting out of the sport several companies were experimenting with dimpled and ridged hulls on both slalom race boats (ridges/rails became very popular but I don’t think this dimples ever gained much traction) as well as freestyle boats. ON freestyle boats you could plane the hull on a green wave very easily and could really feel the dimples disturb the water. I think the principal was that it broke the surface tension (or suck) against the hull and loosened the hull on the wave so that the boat could be spun very easily while surfing. The feeling was really different and you could either carve the boat on edge or spin it flat. On the right wave (green wave, not a hydraulic), you could actually surf a boat sideways down the face of a wave, because there was so little surface drag. Really cool feeling and was quite revolutionary in terms of the freestyle tricks that were opened up based on this technology.
Not sure what it adds here, but just another manner in which this concept has been applied.
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.
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
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.
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
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.
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.
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)
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
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.
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:
“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.
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.