Re: [Phys-L] [Pinhole] Wake left by a plane through fog

[John Denker <jsd@av8n.com>]

On 04/23/2013 05:56 PM, Bernard Cleyet wrote:
> >  https://www.youtube.com/watch?feature=player_embedded&v=QZkggFzAtEc

Yes, that's a pretty video.

There are tons of other wake vortex videos.  Some of them are messier
but more detailed, if you know what to look for, e.g.
  https://www.youtube.com/watch?v=uy0hgG2pkUs



> Now one "sees"  the purpose of wing tip devices:

Some people might look at such video and imagine that winglets are supposed
to eliminate or reduce the wake vortices ... which would be entirely wrong

The key to understanding what's going on is the Kutta-Zhukovsky theorem
  http://www.av8n.com/how/htm/airfoils.html#sec-k-z

It says that the lift is equal to the circulation, times the airspeed, times
the wingspan, times the density of the air.  Period.

There is also a rule that says that vortex lines cannot end.  This is sort
of like the rule for magnetic field lines ... except that the rule for
vortices is not even a law of physics;  it is a mathematical theorem.

This absolutely guarantees that each wing will shed a wake vortex, whenever
the wing is producing lift.  Using winglets et cetera, you can fiddle with 
/where/ the vorticity will be shed.  However, I guarantee you that *anything* 
you do that eliminates the wake vortex will also eliminate the lift.

In physics terms, the vortex is the boundary between the air that has been
yanked down by the wing and the relatively undisturbed ambient air.

> From this we conclude:  As a general rule, adding a pair of six-foot-tall 
winglets has no aerodynamic advantage compared to adding six feet of regular,
horizontal wing on each side.

  There may be non-aerodynamic advantages, e.g. in cases where extra span 
  would make it difficult to taxi and/or park at congested airports.

For more on this, see
  http://www.av8n.com/how/htm/airfoils.html#sec-circulation-vortices
 
> http://en.wikipedia.org/wiki/Wingtip_device

That article does not mention the Kutta-Zhukovsky theorem, and therefore
must be considered fundamentally clueless.