Re: The bernoulli theory of starting a long thread

[David CL Francis]

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 here:
>
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!

I stick to the concept that a wing section in a real flow at a lifting
angle of attack generates lift and pressures around the section. The
airflow is, of course, turned as well. Summing the pressures around the
wing will give you the lift as will finding the total deflection of the
flow (which is more difficult to measure). Mathematical relationships
between variables do not indicate cause and effect, useful as they might
be. The input variables are the angle of attack and the flow velocity.
After that it all just happens. :-)

>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!

Most of us know that the brute force solutions using powerful computers
can now give the best answers. As I understand it these calculations are
essentially step by step iterations and gradually approach closer and
closer to reality but will never be reality.

>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.
--
Francis E-Mail reply to >

[Jan-Olov Newborg]

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 here:
> >
> 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!

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

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


> I stick to the concept that a wing section in a real flow at a lifting
> angle of attack generates lift and pressures around the section. The
> airflow is, of course, turned as well. Summing the pressures around the
> wing will give you the lift as will finding the total deflection of the
> flow (which is more difficult to measure). Mathematical relationships
> between variables do not indicate cause and effect, useful as they might
> be. The input variables are the angle of attack and the flow velocity.
> After that it all just happens. :-)
>
> >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!
>
> Most of us know that the brute force solutions using powerful computers
> can now give the best answers. As I understand it these calculations are
> essentially step by step iterations and gradually approach closer and
> closer to reality but will never be reality.
>
> >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)!


Jan-Olov Newborg

[David CL Francis]

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 here:
>> >
>> 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 >

[Jan-Olov Newborg]

David CL Francis > wrote in message >...

>You should explain that all pressure differentials only comes from
> >> >"turning the airflow", just as NASA Glenn Research shows here:
> >> >
> >> 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
> >
>
> 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.
>
Still airflow velocity can never "cause" a change in Pressure (force)!
[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).
>
Professor em. of Aerodynamics John D. Anderson, Maryland University
writes in one of his books of aerodynamics:

"Strictly speaking,the 1 dimensional Bernoulli equation is only valid
along one streamline in the venturi pipe, namely the centerline and
then we are neglecting all the compressible effects taking place ( and
the ignorance of viscous effects)!"



> 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?

I think you my postings like The Devil reads the Bible!

How can you say that I write that Newton`s laws are wrong!



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.
>
As you well know, you can cut off the entrance part of the venturi
pipe, because it´s only the divergent backpart that lowers the
pressure due to THE COANDA EFFECT!

If you look at the speedsensor of the Piper Colt, PA22, you can see
this type of "cut venturi pipe"!

All old german aircraft used this device in the 1920!

> 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.

Can you explain the behavior of the Windsock, seen at every airport,
using Bernoulli and the continity equation?

The harder it blows, the higher the pressure gets inside the windsock!

All constrictions works this way in a pipe for real flow
(ideal/perfect flows is a mathematical, non existing, flow model)!


Jan-Olov Newborg

[David CL Francis]

On Wed, 22 Oct 2003 at 13:40:11 in message
>, Jan-Olov Newborg
> wrote:
[snip - Your posts would be better if you cut out unnecessary
repetition]
>>
>Professor em. of Aerodynamics John D. Anderson, Maryland University
>writes in one of his books of aerodynamics:
>
>"Strictly speaking,the 1 dimensional Bernoulli equation is only valid
>along one streamline in the venturi pipe, namely the centerline and
>then we are neglecting all the compressible effects taking place ( and
>the ignorance of viscous effects)!"
>
A nice statement, and true, 'strictly speaking' but ,as far as I am
concerned, irrelevant to what I was saying or what we were discussing.
>
>I think you my postings like The Devil reads the Bible!
>
>How can you say that I write that Newton`s laws are wrong!
>
I realise from the above that you are not writing in your first
language. I cannot criticise that as am fluent only in English (and not
perfect even in that!). Your English is good but sometimes fails.
>
>>
>As you well know, you can cut off the entrance part of the venturi
>pipe, because it´s only the divergent backpart that lowers the
>pressure due to THE COANDA EFFECT!
>
Sorry, that is wrong and seems to show that you do not understand what
happens in a normal venturi. You do not seem able to differentiate
between details at the molecular level, the effects of viscosity and
compressibility and the range of situations where broader principles
explain a large percentage of fluid phenomena.

>If you look at the speedsensor of the Piper Colt, PA22, you can see
>this type of "cut venturi pipe"!
>
I am not familiar with that design.

>All old german aircraft used this device in the 1920!
>
I accept what you say about that. However you have not explained the
purpose or principle behind these devices.
>
>Can you explain the behavior of the Windsock, seen at every airport,
>using Bernoulli and the continity equation?
>
Well first think of it like a very small parachute with a large hole in
the top! If there was no opening the pressure inside would rise to the
total pressure. A small opening in this case where there is no expansion
section will restrict the flow due to the inevitable back pressure.

>The harder it blows, the higher the pressure gets inside the windsock!
>
With an upper limit of the total air pressure of the wind.

>All constrictions works this way in a pipe for real flow
>(ideal/perfect flows is a mathematical, non existing, flow model)!
>
No they don't. Your part in brackets is strictly true but so what? What
matters is the deviation from the simple explanation in each case.
Because relativity explains things that Newton doesn't it does not mean
that we run about using it in engineering designs or that we abandon
Newton as inaccurate.

I give up altogether now as we seem doomed never to understand each
other.
--
Francis E-Mail reply to >

[Todd Pattist]

David CL Francis > wrote:

>I give up altogether now as we seem doomed never to understand each
>other.

You lasted longer than I. :-) There seems to be a
fundamental disconnect with Jan-Olov, probably exacerbated
by the language difference. I was never able to decide
quite what his point was. If it's any consolation to you, I
found your posts quite helpful.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
___
Make a commitment to learn something from every flight.
Share what you learn.

[Jan-Olov Newborg]

David CL Francis > wrote in message >...
> On Wed, 22 Oct 2003 at 13:40:11 in message
> >, Jan-Olov Newborg
> > wrote:
> [snip - Your posts would be better if you cut out unnecessary
> repetition]
> >>
> >Professor em. of Aerodynamics John D. Anderson, Maryland University
> >writes in one of his books of aerodynamics:
> >
> >"Strictly speaking,the 1 dimensional Bernoulli equation is only valid
> >along one streamline in the venturi pipe, namely the centerline and
> >then we are neglecting all the compressible effects taking place ( and
> >the ignorance of viscous effects)!"
> >
> A nice statement, and true, 'strictly speaking' but ,as far as I am
> concerned, irrelevant to what I was saying or what we were discussing.
> >
> >I think you my postings like The Devil reads the Bible!
> >
> >How can you say that I write that Newton`s laws are wrong!
> >
> I realise from the above that you are not writing in your first
> language. I cannot criticise that as am fluent only in English (and not
> perfect even in that!). Your English is good but sometimes fails.
> >
> >>
> >As you well know, you can cut off the entrance part of the venturi
> >pipe, because it´s only the divergent backpart that lowers the
> >pressure due to THE COANDA EFFECT!
> >
> Sorry, that is wrong and seems to show that you do not understand what
> happens in a normal venturi. You do not seem able to differentiate
> between details at the molecular level, the effects of viscosity and
> compressibility and the range of situations where broader principles
> explain a large percentage of fluid phenomena.
>

David you should read this article about "The Venturi Pipe"!

http://www.mitypiac.net/

> >If you look at the speedsensor of the Piper Colt, PA22, you can see
> >this type of "cut venturi pipe"!
> >
> I am not familiar with that design.
>
All pilots in the aviation world knows about Piper Aircraft Coperation
and their different aircraft. Piper Colt was a 2 seater built before
the long Piper PA28 Cherokee period!

Have you ever flown a aircraft?

I have been flying 4300 hours!

> >All old german aircraft used this device in the 1920!
> >
> I accept what you say about that. However you have not explained the
> purpose or principle behind these devices.
> >

You don´t need the front part of the venturi to measure the
pressuredifference between total and static pressure. The cut venturi
was used before the pitot pipe!

I would like to to see the Continuity equation and Bernoulli relation
applied on a Windsock!

> >Can you explain the behavior of the Windsock, seen at every airport,
> >using Bernoulli and the continity equation?
> >
> Well first think of it like a very small parachute with a large hole in
> the top! If there was no opening the pressure inside would rise to the
> total pressure. A small opening in this case where there is no expansion
> section will restrict the flow due to the inevitable back pressure.
>
> >The harder it blows, the higher the pressure gets inside the windsock!
> >
> With an upper limit of the total air pressure of the wind.
>
> >All constrictions works this way in a pipe for real flow
> >(ideal/perfect flows is a mathematical, non existing, flow model)!
> >
> No they don't. Your part in brackets is strictly true but so what? What
> matters is the deviation from the simple explanation in each case.
> Because relativity explains things that Newton doesn't it does not mean
> that we run about using it in engineering designs or that we abandon
> Newton as inaccurate.
>
All constrictions in normal pipes for real fluids, slows down the
speed of the flow due to friction and turbulence!



Jan-Olov Newborg

[David]

On Fri, 24 Oct 2003 at 08:56:43 in message
>, Todd Pattist
> wrote:
>David CL Francis > wrote:
>
>>I give up altogether now as we seem doomed never to understand each
>>other.
>
>You lasted longer than I. :-) There seems to be a
>fundamental disconnect with Jan-Olov, probably exacerbated
>by the language difference. I was never able to decide
>quite what his point was. If it's any consolation to you, I
>found your posts quite helpful.
>
Thank you Todd! I have got caught before by those who come back with
isolated facts that they seem to have collected.

Sometimes their arguments feel like this:

The very existence of quantum mechanics means that weighing a few pounds
of potatoes on a grocer's scales is now invalid without taking account
of the quantum effects!
--
Francis E-Mail reply to >