Thomas wrote in
oups.com:

On 17 Oct, 00:48, Le Chaud Lapin wrote:
On Oct 16, 3:31 pm, Thomas wrote:



On 16 Oct, 19:41, Jim Logajan wrote:

Thomas wrote:
You may want to check out my web pages
http://www.physicsmyths.org.uk/bernoulli.htmand
http://www.physicsmyths.org.uk/drag.htmfora closer examination
of
the physics behind the aerodynamic lift and drag.

You might want to actually _include_ Bernoulli's theorem
somewhere in your pages. You talk about Bernoulli's equation,
Bernoulli's principle, and Bernoulli's law. And yet none of them
are actually presented. Are you saying they all the same or all
different? Why not use the terminology used by the professionals
and stick with "Bernoulli's theorem"? How about including
references to relevant texts on your pages? It's not like serious
texts and lab experiments haven't been done on the subject for a
zillion years. It helps to show you know what you're talking
about by showing you've first read the professional literature on
the subject and done your own relevant research.

You might also want to redraw your figures so they include
vertical labeled arrows. Then present the assumptions and math
needed to show your work and why you think the vertical
magnitudes sum to zero. Just saying they do, or they only yield a
torque, isn't good enough. It is more useful to _show_ - not
pontificate and hand-wave.

P.S. Chapter section 40-3 in volume 2 of Feynman's Lectures on
Physics is as good a place as any to start.

Bernoulli's theorem is not a fundamental physical law and thus not
required to understand the principle behind the aerodynamic lift.
And its misinterpretation and misapplication quite evidently leads
to incorrect physical conclusions, like the claim that a moving gas
would inherently have a lower static pressure than a stationary
one. The net flow velocity of a gas has per se nothing to do with
the static pressure.

I so agree. The amout of hand-waving that goes on when (presumably
technically-inclined) individuals invoke Bernoulli is perplexing.
Oddly, my college physics book is almost as guilty - after chapters
and chapters of Newtonian mechanics that are quite clear, they seem
to imply just that.

It is not so much a case of 'hand waving' arguments, but of
insufficient and contradictory physical definitions (especially with
regard to the notion of an 'inviscid' gas). Applying some physical
equation to a situation where it can not be applied is bound to lead
to paradoxes and wrong results.




As a thought experiment, consider a large tank containing gas with
a pipe attached to it which leads into a vacuum space. Assume first
this pipe is closed at the end; then the flow velocity in the pipe
is zero because the molecules heading outwards will be reflected at
the end and reverse their velocity (assume for simplicity that the
molecules do not collide with each other but only with the walls of
the pipe and the tank). If one now opens the pipe, the only thing
that changes is that the molecules heading outwards will not be
reflected anymore at the end but simply carry on heading into the
vacuum space (with the corresponding loss of molecules being
replaced from the large tank). So we now have a net flow velocity
within the pipe without that either the density nor the speed of
the molecules has changed in any way. This means that the pressure
exerted on the inside wall of the pipe is unchanged despite the
fact that we now have a net flow velocity within it. So Bernoulli's
theorem would quite evidently give a wrong result here.

Hmmm...technically, someone could argue that, in the vicinity of the
exit hole of the tank, there would be resulting decrease in pressure,
which would be true.

As should be evident from what I said above already, for an inviscid
gas (i.e. assuming the molecules do not collide with each other but
only with the walls), it should not make any difference whatsoever if
the pipe is open or closed at the end. The rate with which the
molecules hit the inside wall (and thus the pressure on it) is exactly
the same anywhere within the pipe (assuming the lost molecules for the
open pipe situation are readily replaced from the tank).


The misapplication, I think, results from too much hand-waving and
not being very specific about what pressure decreases over what. A
venturi apparutus, for example, very clearly demonstrates a drop in
pressure, and that drop is real, but the points chosen to measure the
pressure in the apparutus is very specific.

The Venturi effect (like the paper sheet example, the Coanda effect
and the Magnus effect) is merely a result of the viscosity of the
medium. It does not occur for an ideally inviscid medium (i.e. if the
collisions of molecules amongst each other can be neglected), whereas
the aerodynamic lift does.

Jesus Christ you're boring.

You want to have something done about that.

Oh yes, also, you;re wrong.


Bertie