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.htmfora closer examination of
the physics behind the aerodynamicliftand drag.

You might want to actually _include_Bernoulli'stheorem somewhere in your
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 terminology used
by the professionals and stick with "Bernoulli'stheorem"? 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'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.

-Le Chaud Lapin-- Dölj citerad text -

- Visa citerad text -- Dölj citerad text -

- Visa citerad text -

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.

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