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Forum index » Science and Technology » Engineering » Mechanics
Bernulli Effect
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Mark Walter
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Joined: 22 Apr 2006
Posts: 7

PostPosted: Sun Jun 25, 2006 2:59 pm    Post subject: Bernulli Effect Reply with quote

I was thinking about the Bernulli effect, where the pressure of a fluid in
motion decreases as the velocity increases. I can understand that some
characteristic has to decrease in order to not have an overall increase in
energy: but why is it not the temperature of the fluid?

M Walter
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N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Sun Jun 25, 2006 8:31 pm    Post subject: Re: Bernulli Effect Reply with quote

Dear Mark Walter:

"Mark Walter" <MarkRWalter@SBCGlobal.net> wrote in message
news:qxxng.53210$fb2.38611@newssvr27.news.prodigy.net...
Quote:
I was thinking about the Bernulli effect, where the
pressure of a fluid in motion decreases as the
velocity increases. I can understand that some
characteristic has to decrease in order to not
have an overall increase in energy: but why is
it not the temperature of the fluid?

Bernoulli does not describe entropy. Temperature is a measure of
entropy.

Bernoulli describes the interchange between kinetic energy
(velocity) and potential energy (pressure). Temperature is along
the lines of frictional losses... energy that is no longer
available as either kinetic or potential.

David A. Smith
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Bret Cahill
science forum Guru


Joined: 05 May 2005
Posts: 480

PostPosted: Mon Jun 26, 2006 11:17 pm    Post subject: Re: Bernulli Effect Reply with quote

Quote:
I was thinking about the Bernulli effect, where the pressure of a fluid in
motion decreases as the velocity increases. I can understand that some
characteristic has to decrease in order to not have an overall increase in
energy: but why is it not the temperature of the fluid?

Gas cools going through a nozzle and warms going through a diffuser in
isentropic flow. The temperature can calculated using the ideal gas
law and the heat capacity ratio Cp/Cv of the gas.

http://www.tech.plym.ac.uk/sme/THER204A-web/entropy_8.pdf

Liquids are easier because they are incompressible so the PV term is
zero and there is no temperature change in adiabatic flow.


Bret Cahill
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N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Tue Jun 27, 2006 12:15 am    Post subject: Re: Bernulli Effect Reply with quote

Dear Bret Cahill:

"Bret Cahill" <BretCahill@aol.com> wrote in message
news:1151363821.586801.67450@i40g2000cwc.googlegroups.com...
Quote:
I was thinking about the Bernulli effect, where the
pressure of a fluid in motion decreases as the
velocity increases. I can understand that some
characteristic has to decrease in order to not
have an overall increase in energy: but why is it
not the temperature of the fluid?

Gas cools going through a nozzle and warms
going through a diffuser in isentropic flow.

"vortex coolers"
http://www.iprocessmart.com/Exair/cabinet_coolers_page.htm

Quote:
The temperature can calculated using the ideal gas
law and the heat capacity ratio Cp/Cv of the gas.

http://www.tech.plym.ac.uk/sme/THER204A-web/entropy_8.pdf

But Bernoulli is inviscid, so yields NO temperature change.

Quote:
Liquids are easier because they are incompressible
so the PV term is zero and there is no temperature
change in adiabatic flow.

David A. Smith
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Ed Ruf (REPLY to E-MAIL1
science forum beginner


Joined: 03 Dec 2005
Posts: 46

PostPosted: Tue Jun 27, 2006 9:34 am    Post subject: Re: Bernulli Effect Reply with quote

On Mon, 26 Jun 2006 17:15:18 -0700, in sci.engr.mech "N:dlzc D:aol T:com
\(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote:


Quote:
But Bernoulli is inviscid, so yields NO temperature change.

Not so, look at the compressible form of bernoulli's eqn. There is no
absolute requirement for inviscid flow.
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)
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Ed Ruf (REPLY to E-MAIL1
science forum beginner


Joined: 03 Dec 2005
Posts: 46

PostPosted: Tue Jun 27, 2006 9:43 am    Post subject: Re: Bernulli Effect Reply with quote

On Tue, 27 Jun 2006 05:34:22 -0400, in sci.engr.mech "Ed Ruf (REPLY to
E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote:

Quote:
On Mon, 26 Jun 2006 17:15:18 -0700, in sci.engr.mech "N:dlzc D:aol T:com
\(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote:


But Bernoulli is inviscid, so yields NO temperature change.

Not so, look at the compressible form of bernoulli's eqn. There is no
absolute requirement for inviscid flow.

For example:
http://www.fluidmech.net/tutorials/bernoulli/compressible-bernoulli.htm
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)
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Bret Cahill
science forum Guru


Joined: 05 May 2005
Posts: 480

PostPosted: Tue Jun 27, 2006 7:06 pm    Post subject: Re: Bernulli Effect Reply with quote

The mechanical energy balance also holds for supersonic gas flow.
Here's a fun applet:

http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html


Bret Cahill


Engineers: Why does a vacuum only hold water down to 28 feet?

Galileo: "Maybe nature abhors a vacuum down to 28 feet and after that
nature no longer abhors a vacuum."
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N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Wed Jun 28, 2006 2:09 am    Post subject: Re: Bernulli Effect Reply with quote

Dear Ed Ruf:

"Ed Ruf (REPLY to E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote
in message news:u3v1a2dtpb13bf6ab6ksmr4kl7991ri4ln@4ax.com...
Quote:
On Tue, 27 Jun 2006 05:34:22 -0400, in sci.engr.mech "Ed Ruf
(REPLY to
E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote:

On Mon, 26 Jun 2006 17:15:18 -0700, in sci.engr.mech "N:dlzc
D:aol T:com
\(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote:


But Bernoulli is inviscid, so yields NO temperature
change.

Not so, look at the compressible form of bernoulli's
eqn. There is no absolute requirement for inviscid flow.

For example:
http://www.fluidmech.net/tutorials/bernoulli/compressible-bernoulli.htm

There are caveats to what you say:
http://www.lmnoeng.com/Flow/bernoulli.htm
http://www.navier-stokes.net/nsbege.htm
http://www.diam.unige.it/~irro/richiami1_e.html
http://www.britannica.com/eb/article-77484

It is *common* and *expected* that the flow is inviscid for
Bernoulli. This is what is presented in most literature.
Inviscid equates to "frictionless" in the "block and trolley"
world of dynamic modelling.

Along with this is the general form where "potential" equates to
"kinetic" along streamlines. No frictional loss term for
increasing temperature.

David A. Smith
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Bret Cahill
science forum Guru


Joined: 05 May 2005
Posts: 480

PostPosted: Wed Jun 28, 2006 5:02 am    Post subject: Re: Bernulli Effect Reply with quote

True. Bernoulli is for ideal flow -- no viscosity, etc.

The mechanical energy balance is the first thing you use to determine a
situation. If that suggests that there is significant shear or
turbulence then you must go to the equation of motion and friction
factor charts to account momentum transfer.


Bret Cahill
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Mark Walter
science forum beginner


Joined: 22 Apr 2006
Posts: 7

PostPosted: Thu Jun 29, 2006 2:05 am    Post subject: Re: Bernulli Effect Reply with quote

Bret Cahill wrote:

Quote:
True. Bernoulli is for ideal flow -- no viscosity, etc.

The mechanical energy balance is the first thing you use to determine a
situation. If that suggests that there is significant shear or
turbulence then you must go to the equation of motion and friction
factor charts to account momentum transfer.


Bret Cahill
Everyone is I think missing what my question is. We all agree that there is

an increase in energy with the increased velocity. Since the total energy
can't change something has to decrease. Bernoulli's equation (which I have
no doubts about it's validity) says that the pressre decreases. Why doesn't
the temperature of the fluid decrease to make up for the decrease in the
velocity (instead of pressure)?

M Walter
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N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Thu Jun 29, 2006 5:03 am    Post subject: Re: Bernulli Effect Reply with quote

Dear Mark Walter:

"Mark Walter" <MarkRWalter@SBCGlobal.net> wrote in message
news:KzGog.161543$F_3.45281@newssvr29.news.prodigy.net...
Quote:
Bret Cahill wrote:

True. Bernoulli is for ideal flow -- no viscosity, etc.

The mechanical energy balance is the first thing
you use to determine a situation. If that suggests
that there is significant shear or turbulence then
you must go to the equation of motion and friction
factor charts to account momentum transfer.

Everyone is I think missing what my question is.
We all agree that there is an increase in

*kinetic*

Quote:
energy with the increased velocity.
Since the total energy can't change something
has to decrease.

It *can* change. The change is usually small.

Quote:
Bernoulli's equation (which I have no doubts
about it's validity) says that the pressre
decreases. Why doesn't the temperature of
the fluid decrease to make up for the decrease
in the velocity (instead of pressure)?

It does, but *very* slightly for Mach numbers less than 0.3.

Think "Second Law of Thermodynamics"... and what temperature
represents. In fluids, it represents a measure of entropy. In a
closed system (such as along Bernoulli's streamlines) entropy is
required to stay the same (fixed temperature) or increase
(increasing temperature). So if the streamline is closed,
entropy cannot leave the system...

David A. Smith
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Bret Cahill
science forum Guru


Joined: 05 May 2005
Posts: 480

PostPosted: Thu Jun 29, 2006 12:18 pm    Post subject: Re: Bernulli Effect Reply with quote

Quote:
the temperature of the fluid decrease to make up for the decrease in the
velocity (instead of pressure)?

You're wondering why the temperature of a liquid doesn't drop to give
you a velocity head?

You're wondering why a nozzle cannot suck heat out of sea water to run
an impulse turbine?

EVERYBODY wants to do THAT but we can't.

The reason is the Second Law of Thermo.

It's kind of like the Catch - 22 of engineering.


Bret Cahill


"There was only one catch and that was Catch - 22."

-- Heller
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Mark Walter
science forum beginner


Joined: 22 Apr 2006
Posts: 7

PostPosted: Thu Jun 29, 2006 10:43 pm    Post subject: Re: Bernulli Effect Reply with quote

Thanks all. I knew there was some obvious reason (wasn't to me but..)

M Walter



Bret Cahill wrote:

Quote:
the temperature of the fluid decrease to make up for the decrease in the
velocity (instead of pressure)?

You're wondering why the temperature of a liquid doesn't drop to give
you a velocity head?

You're wondering why a nozzle cannot suck heat out of sea water to run
an impulse turbine?

EVERYBODY wants to do THAT but we can't.

The reason is the Second Law of Thermo.

It's kind of like the Catch - 22 of engineering.


Bret Cahill


"There was only one catch and that was Catch - 22."

-- Heller
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Bret Cahill
science forum Guru


Joined: 05 May 2005
Posts: 480

PostPosted: Sat Jul 01, 2006 12:42 am    Post subject: Re: Bernulli Effect Reply with quote

Most people call Bernoulli the mechanical energy balance. Mechanical
energy is the good stuff.

You can have a good time with mechanical energy.

Unless you are freezing heat is the bad stuff, even though it is
measured in the same units as mechanical energy.

Just staying warm isn't my idea of a good time.

Just as you can go from democracy to despotism but not the other way,
you can go from mechanical energy to heat energy but not the other way.

Whole forests have been leveled trying to keep mechanical energy from
turning into heat energy.

What am I saying? The future of human life on this planet is 100%
dependent on engineers preventing mechanical energy from becoming heat
energy.

Even a cure for cancer cannot make that claim.


Bret Cahill
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