Re: [Phys-L] Bernoulli's equation

[John Denker <jsd@av8n.com>]

On 12/21/2014 04:42 PM, Bernard Cleyet wrote:
> [...] read the blown foolscap demo. is not an example of Bernoulli.  And
> doesn’t know why.

0) Here's why:  Bernoulli's principle applies to a /particular parcel/ 
of fluid as it flows along a streamline.

1) If you want to compare parcel A with parcel B, you need to establish
that they started off with the same head (or some other known relationship).
Demos that involve a blower producing a free jet rarely (if ever) qualify.
You can't work forward from the blower, because it gives a higher head
(aka enthalpy) to the air.  You can't work backwards from downstream,
because the jet becomes turbulent before it becomes comparable to
anything you know.

2) The only reason the blowing demo produces lift is because of the 
Coanda effect, which is very complicated and nonlinear.    It has 
nothing to do with the normal operation of airfoils.  The required 
velocity profile is wildly different.

Evidence for both (1) and (2):  If it really were a Bernoulli effect,
blowing on a /flat card/ would work equally well ... which is not 
what we observe.
  https://www.av8n.com/physics/bernoulli.htm
  https://www.av8n.com/how/htm/airfoils.html#sec-bernoulli
  https://www.av8n.com/how/htm/spins.html#sec-coanda

Evidence for (0):  The rotating bowl experiment.