31 October 2012

Those little bumps on the wings of airplanes

I found the explanation for them at Fuck Yeah Fluid Dynamics:
Ever look out an airplane’s window and wondered why a row of little fins runs along the upper side of the wing? These vortex generators help prevent a wing from stalling at high angle of attack by keeping flow attached to the surface. Airflow over the vanes creates a tip vortex that transports the higher-momentum fluid from the freestream closer to the wing’s surface, increasing the momentum in the boundary layer. As a result of this momentum exchange, the boundary layer remains attached over a greater chordwise distance. This also increases the effectiveness of trailing-edge control surfaces, like ailerons, on the wing.
...these vortex generators serve another, arguably more economical purpose: They can be very good at reducing drag, which is why you can see them on the roofs of some cars and trucks. One of my aerodynamics teachers even glued them to skaters' legs to make them go faster.

That may sound like a joke but as a result the dutch speed skating team in the Nagano Olympics had special 'go-faster stripes' on their calves. Incidentally, that year the dutch Gianni Romme won the two events he entered (the 5,000 and 10,000m) and broke the world record in both cases. So maybe there was something to this weird hobby.

Anyway, they do this basically in the same way the dimples on golf balls do, by delaying the transition to turbulent flow. Turbulent flow behind a wing/car/ball/leg creates a low pressure region which 'sucks' it back. In the case of airplane wings, the mini-vortices also create a sort of barrier to the spanwise flow of air on top of the wing, which reduces the size of the big vortices at the wingtip, which are responsible for induced drag.

1. I've always been fascinated by the small details in the design of aircraft, and why certain ones had certain things that others didn't. This obsession made me a nerd obviously, so I can tell that this is the wing of a 737. It also made my first years of studying aerospace engineering much easier -- so kids: be nerds, it really helps.

But more to the point; these vortex generators serve another, arguably more economical purpose: They can be very good at reducing drag, which is why you can see them on the roofs of some cars and trucks. One of my aerodynamics teachers even glued them to skaters' legs to make them go faster.

That may sound like a joke but as a result the dutch speed skating team in the Nagano Olympics had special 'go-faster stripes' on their calves. Incidentally, that year the dutch Gianni Romme won the two events he entered (the 5,000 and 10,000m) and broke the world record in both cases. So maybe there was something to this weird hobby.

Anyway, they do this basically in the same way the dimples on golf balls do, by delaying the transition to turbulent flow. Turbulent flow behind a wing/car/ball/leg creates a low pressure region which 'sucks' it back.
In the case of airplane wings, the mini-vortices also create a sort of barrier to the spanwise flow of air on top of the wing, which reduces the size of the big vortices at the wingtip, which are responsible for induced drag.

I'm not sure if anyone actually wanted to read what I've just written, but now it's done so I hope I haven't bored too many people.

1. To the contrary, that's an excellent comment. What you describe may be implied in the source comment, but it's obscured by jargon there, while your explanation is refreshingly accessible.

I've added part of your comment to the post; thanks for making the blog better.

Stan

2. Speaking of which, I can recommend the Mythbusters episode where they "golfball" a car, reducing drag significantly.
http://dsc.discovery.com/tv-shows/mythbusters/videos/dimpled-car-minimyth.htm

2. Just wanted to add a caveat to Danny M's comment: they do this basically in the same way the dimples on golf balls do, by delaying the transition to turbulent flow. Vortex generators (and indeed golf balls) don't delay the transition to turbulence--they promote it! The reason that this is desirable is the same that I gave in my original entry: the vortices help keep flow attached to the surface. It's generally true that a laminar flow will have lower drag than a turbulent flow, except in situations where flow detaches from the object. A turbulent boundary layer is better able to remain attached to an object in the face of an adverse pressure gradient, like the one experienced as air flows toward the trailing edge of a wing or golf ball. Keeping the flow attached to the object prevents the low pressure region in the wake that Danny describes as sucking an object back and increasing its drag. Thus, objects prone to flow separation can have less drag in turbulent flow than in laminar flow.

1. From the blogger at the original source. Thanks, FYFD.