I Explain Bernoulli at the molecular level?

[user079622]

@jbriggs444
@Dale
@A.T.
@boneh3ad
..
and others...

This is not homework question, I use numbers to easier explain my question.

Static pressure is caused by bouncing molecules in normal direction at walls, the faster air travel, the less force at normal direction(static pressure) and more force in direction of flow(dynamic pressure). Is this correct?

Atmospheric pressure is 101325 Pa, car drive at two speeds, first at 10km/ and then at 300km/h at flat road, at same altitude.
Is static pressure:
a) for both speeds same as atmospheric pressure
b) for both speeds lower than atmospheric pressure, and lower at 300km/h than at 10km/h

I think a) is correct answer, Bernulli cant apply here(faster speed = lower static pressure)

Is this correct and why here pressure dont drop with speed(as Bernoulli state)?

 

[russ_watters]

A. Is correct. Bernoulli says this: p is just p.

 

[user079622]

A. Is correct. Bernoulli says this: p is just p.

So it isnt less bouncing force from molecules in normal direction at higher speeds?

 

[russ_watters]

So it isnt less bouncing force from molecules in normal direction at higher speeds?

No. Unless you have something else in mind that you haven't stated you seem to be describing this:

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/pitot-static-tube-speedometer/

 

[user079622]

No. Unless you have something else in mind that you haven't stated you seem to be describing this:

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/pitot-static-tube-speedometer/

Yes static port at pitot tube show same static pressure for every speeds, so it show that static pressure dont depend on airflow speed.
I am try to find out why different speeds have same static pressure and why accelerated flow reduce static pressure, using explanation of pressure at molecular level("molecules hitting walls")?

 

[jbriggs444]

I am try to find out why different speeds have same static pressure and why accelerated flow reduce static pressure, using explanation of pressure at molecular level("molecules hitting walls")?

It is my firm belief that "molecules hitting walls" is a hindrance to understanding rather than an aid.

 

[russ_watters]

Yes static port at pitot tube show same static pressure for every speeds, so it show that static pressure dont depend on airflow speed. I am try to find out why...

That's the definition/goal of the device.

 

[user079622]

It is my firm belief that "molecules hitting walls" is a hindrance to understanding rather than an aid.

Then we have to answer the question of what static pressure actually is, how is it physically transmitted at the molecular level to the walls?
Bernoulli doesn't answer this, he just set correlation between something, that it turns out that we dont know what it is..

 

[user079622]

That's the definition/goal of the device.

It is property of constant velocity, accelerated flow reduce pressure.
If you blow between two flat plates it will not comes together, but if you blow between two curved surface it will comes together because flow accelerated=reduce s. pressure.

 

[A.T.]

I am try to find out why different speeds have same static pressure ...

Speed is frame dependent, while pressure differences are frame invariant. So it wound not make sense if the later was determined by the former.

... and why accelerated flow reduce static pressure, using explanation of pressure at molecular level("molecules hitting walls")?

The more important part here is molecules hitting each other. In order to accelerate you need an a force imbalance. In order for bouncing molecules to accelerate in bulk you need an imbalance in momentum transferred to them over time. That imbalance in momentum transfer rate (force) is a pressure gradient.

 

[russ_watters]

Then we have to answer the question of what static pressure actually is, how is it physically transmitted at the molecular level to the walls?
Bernoulli doesn't answer this, he just set correlation between something, that it turns out that we dont know what it is..

While I agree with @jbriggs444 that this is a potentially unnecessary and unhelpful complication, I'll give it a go:

If you have X number of molecules per square meter bouncing off a horizontal surface per second, that number doesn't change just because the surface is moving horizontally. That would be like the weight of a block changing just because you're pushing it horizontally along the ground. Obviously it doesn't.

It is property of constant velocity, accelerated flow reduce pressure.
If you blow between two flat plates it will not comes together, but if you blow between two curved surface it will comes together because flow accelerated=reduce s. pressure.

OK?

 

[user079622]

While I agree with @jbriggs444 that this is a potentially unnecessary complication, I'll give it a go:

If you have X number of molecules per square meter bouncing off a horizontal surface per second, that number doesn't change just because the surface is moving horizontally. That would be like the weight of a block changing just because you're pushing it horizontally along the ground. Obviously it doesn't.

Yes that is good explanation. How can we use this analogy to explain pressure drop in accelerated flow?

OK?

Yes that is correct, I can post video from these experiments.

Speed is frame dependent, while pressure differences are frame invariant. So it wound not make sense if the later was determined by the former.

P1+1/2pv1^2=P2+1/2pv2^2

If I enter here v1=500km/h and v2=200km/h, p=1.2kg/m3,
P2 would be not same as P1

Why here Bernoulli is not valid?

 

[russ_watters]

Yes that is good explanation. How can we use this analogy to explain pressure drop in accelerated flow?

We can't. Different situations are different.

 

[user079622]

It is my firm belief that "molecules hitting walls" is a hindrance to understanding rather than an aid.

How would you explain physically, what is static pressure and why accelerated flow reduce pressure?
If we do F=ma for every molecule that hit a wall, will this correspond to pressure?

 

[russ_watters]

How would you explain physically, what is static pressure and why accelerated flow reduce pressure?

What is "accelerated flow"? What are you claiming here?

 

[user079622]

What is "accelerated flow"? What are you claiming here?

Airflow that increase speed, for example over wing upper surface.

 

[russ_watters]

Airflow that increase speed, for example over wing upper surface.

A wing and venturi are not flat surfaces. Do you recognize that this question is completely different from what you were asking before?

 

[user079622]

A wing and venturi are not flat surfaces. Do you recognize that this question is completely different from what you were asking before?

Yes, I want to find where can I use Bernoulli and where not

 

[jbriggs444]

How would you explain physically, what is static pressure and why accelerated flow reduce pressure?

If the flow accelerates, there must be a pressure gradient consistent with the acceleration.

$F=ma$ You cannot have acceleration without a net force.

 

[russ_watters]

Yes...

So when I said "Unless you have something else in mind that you haven't stated...", this was it?

I want to find where can I use Bernoulli, why I cant use it...

What exactly is it that you don't understand? Bernoulli's principle obviously applies to both situations if you use it correctly.

 

[user079622]

So when I said "Unless you have something else in mind that you haven't stated...", this was it?

What exactly is it that you don't understand? Bernoulli's principle obviously applies to both situations if you use it correctly.

This was what?
How would use it Bernoulli correctly for my first post?

 

[russ_watters]

This was what?

The different thing you wanted to discuss that you weren't saying.

How would use it Bernoulli correctly for my first post?

It's in the link I gave you in post #4.

 

[user079622]

@jbriggs444

1. So there is not good explanation with molecules, what happen with them when static pressure drop, with constant density and temperature?
(In case density reduce, than is easy, distance between molecules increase..)

2. If we sum all normal component of hits from molecules into the wall, is this correspond to static pressure at that location? Is this mathematically correct or this is pure explantion?

3. If pressure is caused by molecules bouncing the walls, why then in zero gravity, pressure is zero?

It's in the link I gave you in post #4.

There are lots of questions like my, that are confused by Bernoulli
https://www.pprune.org/tech-log/88839-why-isn-t-static-pressure-speed-dependent.html

 

[jbriggs444]

3. If pressure is caused by molecules bouncing the walls, why then in zero gravity, pressure is zero?

You will need to specify the scenario better than than just "zero gravity".

A tank full of compressed gas has pressure in zero gravity.

The same mass of gas allowed to spread through an essentially unlimited volume in interstellar space has essentially zero pressure.

 

[russ_watters]

There are lots of questions like my, that are confused by Bernoulli
https://www.pprune.org/tech-log/88839-why-isn-t-static-pressure-speed-dependent.html

Ok, and?

 

[user079622]

You will need to specify the scenario better than than just "zero gravity".

A tank full of compressed gas has pressure in zero gravity.

The same mass of gas allowed to spread through an essentially unlimited volume in interstellar space has essentially zero pressure.

question 1. and 2.?

Ok, and?

just say

 

[russ_watters]

just say

Say what? I answered your question. You don't seem to like the answer but you haven't said why or where we go from here. Do you understand the answer/that link? Are we finished with the question you asked in the OP/"How would use it Bernoulli correctly for my first post?"?

 

[erobz]

Imagine you are compressing fluid in a piston cylinder assembly with the end closed . You push on the plunger; the water is pushing in any way it can to relive the elastic potential being stored in it. It cant move much(compress) itself, and the pipe is trying to contain it (so it is expanding). If the nozzle opens under this force of the plunger a flow begins to accelerate (from rest) as that elastic potential in the pipe walls and fluid is converted to kinetic energy of the flow. So the two things are coupled. Accelerating flow is the outcome of internal energy change to kinetic energy. In that scenario to $t=0$ the pressure will start high and decline as the flow accelerates out of the end of the tube. What I'm talking about is a time varying Bernoulli's to get a handle on how pressure is generally converted to kinetic energy, its easier to get a handle on this way. It's not "Classic Bernoulli" you encounter in steady flow (flow not varying in time) but it's an extension that I think helps clarify. Why this translates to steady incompressible flows that experience a change in their container shape is because of mass conservation. So for steady flow is spatially varying pressure, mass is entering across some surface at some rate and unless it's being stored or temporarily compressed it's got to exit. So it's using that internal pressure energy to accelerate the flow to satisfy the mass conservation constraint. So it seems that that mass constraint "decides" what must happen, and it takes from the available energy reserve it can alter.

$$ 0 = \frac{ d}{dt} \int_{cv} \rho d V\llap{-} + \int_{cs} \rho ( \boldsymbol {V}\cdot d \boldsymbol{A} ) $$

Bernoulli's is a special case of the First Law of Thermodynamics for a control volume:

$$ \dot Q - \dot W_s = \frac{d}{dt} \int_{cv} \rho \left( \frac{V^2}{2} + gz + u \right) d V\llap{-} + \int_{cs} \left( \frac{V^2}{2} + gz + u + \frac{p}{\rho} \right) \rho \boldsymbol {V}\cdot d \boldsymbol{A} $$

It's not hard to be "deceived" by it, there is much to study. I've been trying to chip away at it for many years myself.

 

[A.T.]

1. So there is not good explanation with molecules, what happen with them when static pressure drop, with constant density and temperature?

Depends on what is considered a 'good explanation'

2. If we sum all normal component of hits from molecules into the wall, is this correspond to static pressure at that location?

That's the total absolute pressure.

3. If pressure is caused by molecules bouncing the walls, why then in zero gravity, pressure is zero?

Pressure doesn't have to be zero in zero gravity.

 

[user079622]

Say what? I answered your question. You don't seem to like the answer but you haven't said why or where we go from here. Do you understand the answer/that link? Are we finished with the question you asked in the OP/"How would use it Bernoulli correctly for my first post?"?

I didnt tell that I dont agree with Bernoulli, I just say that Bernoulli tell nothing about how pressure is exerted on walls, at molecular level(real physics behind pressure).
Yes I understand that total pressure = dynamic p. + static p. , Do I know how and when to use Bernoulli correctly? No

Depends on what is considered a 'good explanation'

Can you write explanation?