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Re: [Phys-l] question about Bernoulli

Let me take a stab at the original question - as far as I can remember it. As usual, the thread evolved into something different - probably because no one had a good response to the question.

Pressure comes from the momentum change of the molecules of a gas when they hit the walls of a container. The question was "what is happening at the molecular level to explain the reduction in pressure as a gas flows from a large diameter pipe into a smaller one?". Specifically, I think the question was asking about the component of velocity of the molecules perpendicular to the pipe walls - was it reduced? - and by what mechanism was it reduced?

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Tuesday, November 23, 2010 8:07 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] question about Bernoulli

On 11/23/2010 03:49 AM, William Robertson wrote:
Is it safe to say that most of the "everyday" examples of the
Bernoulli effect you read about are not examples of the Bernoulli
effect?

I have no idea what that is trying to ask. Maybe I'm old-fashioned,
but I prefer to think that if it is an example of the Bernoulli
effect then it is an example of the Bernoulli effect.

If it's not a change in cross sectional area that results in a
change in velocity due to a pressure difference, it's not Bernoulli?

Again, I have no idea where that is coming from. I don't recall
anybody suggesting that. I cannot imagine why anybody would
suggest that. The example of steady flow through a changing
cross-section is pretty much the poster child for a conventional
and reasonable example of the Bernoulli effect.

And with the syringe, air will go out the side hole, not in. By
depressing the plunger, you are increasing the air pressure inside
the
syringe compared to the outside air, not decreasing it.

Yeah, and that's why you need the term involving ∂P/∂t|x in
the equation, when the pressure distribution is time-dependent.

Note that this notion of time-dependence is not Galilean
invariant. Even the ultra-simple example of a vortex that
just sits there in one reference frame will move in any other
reference frame. There is not in general any such thing as
the "rest frame" of the overall fluid, so you'd better be
able to handle time-dependent pressure distributions.

The physics is what it is and does what it does. It is easy
enough to write down an equation that does a good job of
modeling the physics.
http://www.av8n.com/physics/bernoulli.htm
Whether you want to call the resulting equation the "Bernoulli"
equation is not a question that interests me. I am much more
interested in understanding the physics than in fussing over
what name to give the equation.

I would hope everybody knows that the stagnation pressure is a
conserved quantity for a given packet flowing along a streamline
when the pressure distribution is independent of time. I would
hope everybody knows that under other conditions it is easy to
change the stagnation pressure, e.g. by using a syringe e.g.
bicycle pump or whatever. I would hope everybody knows that
you can't use Bernoulli to compare two different parcels unless
there is some special reason to believe they started out with
the same stagnation pressure.

Fluid dynamics has a number of exceedingly tricky points, but
the idea of being able to change the pressure using a pump
does not register very high on the trickometer.

Is there a question on the table? I've been lurking this thread
for a week and I still can't figure out what the discussion is
about. Perhaps somebody could ask a more specific question.
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