Thanks for a great explanation! My field is electrical engineering, though
I often wish I'd gone the aero path...


"Martin Gregorie" wrote in message
...
On Mon, 15 Aug 2011 08:06:35 -0600, Dan Marotta wrote:

"Martin Gregorie" wrote in message
...
On Sun, 14 Aug 2011 14:34:37 -0700, sisu1a wrote:

Interesting article about Duke engineers applying their passive,
engineered material light/sound cloaking techniques to water,
theoretically (substantially) reducing the amount of drag generated by
a boat's hull by 'tricking' the surrounding water into standing still.

http://www.eurekalert.org/pub_releas...-esw081111.php

It will be interesting to see if it works, but I don't think it will
help us much, apart from the possibility of reducing friction drag in
the fuselage. The reason: since the whole point of this cloaking seems
to be to prevent momentum transfer to the surrounding fluid while a
wing generated lift by transferring momentum to the surrounding air, it
ses to me if you put the stuff on a wing, it would stop the wing from
doing anything useful.


Are you saying, then, that the velocity of the air moving over the top
of the wing will be reduced relative to that over the bottom? What
would Bernoulli say?

In the most general case a wing is a device that generates lift by
imparting downward momentum to the surrounding air in such a way that
that the resulting accelerating force counterbalances the force of
gravity acting on the aircraft. As the main effect of this 'cloaking'
seems to be to reduce momentum transfer from the moving body to the
surrounding fluid it seems to me that the 'cloaking' would also reduce
the amount of lift the wing would produce under normal flow conditions.

I wasn't being so specific about velocities or saying anything about
Bernoulli.

I don't much care for the application of Bernoulli to wing sections: that
theory applies to flow inside a pipe while airflow over a wing is not so
enclosed. Better aerodynamicists than I think its application to wing
sections is questionable. AFAICR straight application of Bernoulli can't
account for the upflow field in front of a wing and it certainly can't
account for the measured flow over a wing being so much faster than that
under it that air molecules separated by the LE don't rejoin at the TE:
the molecules passing over the top surface are long gone before molecules
passing under the lower surface arrive at the TE.

OTOH, if you were applying Prandtl..... My guess is that the cloaking
effect would reduce circulation.


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