Search   Memberlist   Usergroups
 Page 1 of 1 [14 Posts]
Author Message
Mark Walter
science forum beginner

Joined: 22 Apr 2006
Posts: 7

Posted: Sun Jun 25, 2006 2:59 pm    Post subject: Bernulli Effect

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

Joined: 25 Mar 2005
Posts: 2835

Posted: Sun Jun 25, 2006 8:31 pm    Post subject: Re: Bernulli Effect

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
Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

Posted: Mon Jun 26, 2006 11:17 pm    Post subject: Re: Bernulli Effect

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

Joined: 25 Mar 2005
Posts: 2835

Posted: Tue Jun 27, 2006 12:15 am    Post subject: Re: Bernulli Effect

Dear Bret Cahill:

"Bret Cahill" <BretCahill@aol.com> wrote in message
 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
science forum beginner

Joined: 03 Dec 2005
Posts: 46

Posted: Tue Jun 27, 2006 9:34 am    Post subject: Re: Bernulli Effect

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)
science forum beginner

Joined: 03 Dec 2005
Posts: 46

Posted: Tue Jun 27, 2006 9:43 am    Post subject: Re: Bernulli Effect

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 \(dlzc\)" 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)
Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

 Posted: Tue Jun 27, 2006 7:06 pm    Post subject: Re: Bernulli Effect 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."
N:dlzc D:aol T:com (dlzc)
science forum Guru

Joined: 25 Mar 2005
Posts: 2835

Posted: Wed Jun 28, 2006 2:09 am    Post subject: Re: Bernulli Effect

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!)" wrote: On Mon, 26 Jun 2006 17:15:18 -0700, in sci.engr.mech "N:dlzc D:aol T:com \(dlzc\)" 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
Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

 Posted: Wed Jun 28, 2006 5:02 am    Post subject: Re: Bernulli Effect 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
Mark Walter
science forum beginner

Joined: 22 Apr 2006
Posts: 7

Posted: Thu Jun 29, 2006 2:05 am    Post subject: Re: Bernulli Effect

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

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

Joined: 25 Mar 2005
Posts: 2835

Posted: Thu Jun 29, 2006 5:03 am    Post subject: Re: Bernulli Effect

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
Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

Posted: Thu Jun 29, 2006 12:18 pm    Post subject: Re: Bernulli Effect

 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'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
Mark Walter
science forum beginner

Joined: 22 Apr 2006
Posts: 7

Posted: Thu Jun 29, 2006 10:43 pm    Post subject: Re: Bernulli Effect

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
Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

 Posted: Sat Jul 01, 2006 12:42 am    Post subject: Re: Bernulli Effect 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

 Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First
 Page 1 of 1 [14 Posts]
 The time now is Fri Sep 21, 2018 4:33 am | All times are GMT
 Jump to: Select a forum-------------------Forum index|___Science and Technology    |___Math    |   |___Research    |   |___num-analysis    |   |___Symbolic    |   |___Combinatorics    |   |___Probability    |   |   |___Prediction    |   |       |   |___Undergraduate    |   |___Recreational    |       |___Physics    |   |___Research    |   |___New Theories    |   |___Acoustics    |   |___Electromagnetics    |   |___Strings    |   |___Particle    |   |___Fusion    |   |___Relativity    |       |___Chem    |   |___Analytical    |   |___Electrochem    |   |   |___Battery    |   |       |   |___Coatings    |       |___Engineering        |___Control        |___Mechanics        |___Chemical

 Topic Author Forum Replies Last Post Similar Topics Effect of temperature on pH of acid? Paucceri Chem 0 Wed Aug 16, 2006 5:50 pm " f o f = g ; f a half effect of g " alain verghote Math 7 Mon Jul 17, 2006 3:02 pm HPLC/UV Effect of Flow rate on Area Count denise.pattavina@gmail.co Analytical 1 Thu Jul 06, 2006 3:43 pm Effect of Flow Rate on Area Count denise.pattavina@gmail.co Analytical 3 Thu Jul 06, 2006 2:32 pm The effect of convolution yezi Math 0 Fri Jun 30, 2006 10:50 pm