The bernoulli theory of starting a long thread

In article , Jan-Olov
Newborg writes
David CL Francis wrote in message
...
On Sat, 11 Oct 2003 at 00:39:56 in message
, Jan-Olov Newborg
wrote:

You should explain that all pressure differentials only comes from
"turning the airflow", just as NASA Glenn Research shows he

That is just not true. Even in a simplistic inviscid incompressible
potential flow there are pressure differences around an aerofoil
section. They just all cancel out to a zero overall effect!

Ofcause it´s true!

I'll try once more, then I give up. Slight misunderstanding here as I
thought your comment referred to the overall deflection of the airflow
that generates lift and not to local movements. Local pressures and
local changes in airflow direction are of course interconnected.

Stanford Aero shows here how "turning the fluidflow causes local
pressure gradients":

http://www.scienceweb.org/movies/aero.htm

Yes and no. There are a number of different equations that can be
formulated about flows. Flows away from the boundary layer can be
calculated from potential flow theory and locally you can write
equations relating to the movement of individual elements of air. Fine.
Within a given stream tube Bernoulli will give you good answers - it is
based on conservation of energy. The illustration gives a balance of the
element forces across the flow but omits the effects of the forces along
the length of the flow where the air is also accelerating or
decelerating with the velocity changes.

All these equations are ways of making predictions, some of them work
better than others under different conditions. However they are all
imperfect descriptions of reality. Neither equations or 'explanations'
_are_ reality.

In a real flow pressures and velocities are what they are and you cannot
say that one causes the other except in the sense that changing the
environment (changing the wing section or the angle of attack) give rise
to different results.
[Snip]

http://www.lerc.nasa.gov/www/K-12/airplane/right2.html

Well that is a nice page, but as far as I can see the math on it is pure
Newton although it does nicely show some simple flow patterns. The
pressure results on the demo though appear to be based on Bernoulli! So
he appears to show pressure changes by Bernoulli conversion from
velocity changes!

[Snip]

Lets get rid of "The Reversed Bernoulli use"!


What on earth is that? Since Bernoulli is an energy conserving equation
it is, by its nature, reversible.

Thats when people write " high airflow speed causes low pressure"!

A change in velocity can never causes a change of a force (pressure)!
[1]

As you may have guessed I don't go for chicken and egg explanations but
I would just ask how you equate that statement with what happens in a
pitot tube? Some of the air comes to rest and the pressure in the pitot
tube rises to the total pressure of the air stream. Or take a venturi
where the cross section changes slowly and smoothly? The centrifugal
forces that you liked on the web page are then very small but the
velocity and pressure will still change in almost exactly the same way
as Bernoulli predicts (better actually).

The literal acceptance of your statement above [1] is that Newton's
equations are wrong. Yet the equations you prefer are based on Newton's
laws of motion! Do you also assert the converse; that a change in force
cannot cause a change in velocity? Or that flow meters cannot work? The
introduction of a venturi provokes a velocity change in an
incompressible fluid and at the point of maximum velocity the static
pressure drops and the result can be used to measure the flow.

Generalisations like [1] are, in my humble opinion, not very helpful. If
you were to say that in a flow the pressures velocities and
accelerations are all interrelated I would be much happier.

--

David CL Francis E-Mail reply to