On 26 Okt, 08:24, Bertie the Bunyip wrote:
jon wrote groups.com:





On 17 Okt, 01:48, 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.htmforacloser examination
of
the physics behind the aerodynamicliftand drag.

You might want to actually _include_Bernoulli'stheorem somewhere
in y
our
pages. You talk aboutBernoulli'sequation,Bernoulli'sprinciple,
and
Bernoulli'slaw. And yet none of them are actually presented. Are
you
saying they all the same or all different? Why not use the
terminolog
y used
by the professionals and stick with "Bernoulli'stheorem"? How
about including references to relevant texts on your pages? It's
not like s
erious
texts and lab experiments haven't been done on the subject for a
zill
ion
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 l
abeled
arrows. Then present the assumptions and math needed to show your
wor
k and
why you think the vertical magnitudes sum to zero. Just saying
they d
o, or
they only yield a torque, isn't good enough. It is more useful to
_sh
ow_ -
not pontificate and hand-wave.

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

Bernoulli'stheorem is not a fundamental physical law and thus not
required to understand the principle behind the aerodynamiclift.
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 invokeBernoulliis 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.

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.
SoBernoulli'stheorem 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.

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.

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The venturi pipe is mostly misunderstood. To get through the narrow
section, the fluid must be pressed against the convergent part with a
higher pressure. The Coanda effect forces the fluid to follow the
walls in the divergent part.

All early speed sensors in 1920 used only the divergent part of the
venturi pipe.

Look att Bleriot and other planes. Look at Piper Colt 1953 model with
its backpart venturi. The front convergent part was not needed.

Piper Colt 1953 model?

Unh unh. First flew in 1960 you fjukkwit.

Backpart Venturi?

Bwawhahwhahwhahwhahwhahwha!

Changing the airflow direction over and under the wing, creates the
local pressure gradients + or - .

Nope.

Bertie



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The backpart of a venturi is sitting also on the Piper Colt 1960-64
models, like it does on the 1953 TriPacer of the same family.

One does not need the convergent entrance part, to measure the
airspeed.