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Old 14th August 2019, 04:26 AM   #201
Robin
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Originally Posted by GnaGnaMan View Post
Now let's conclude ...
When you are a little more yourself perhaps you can try to explain yourself better here.

As I understand it, you are claiming that when the air passes through the cone, the pressure inside remains the same as the pressure outside? Yes? You have concluded this from blowing through a straw covered with cling film, as I recall.

If I have misunderstood you I apologise. Please clarify, when the air passes through the cone, does the pressure on the inside surfaces drop or not?
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Old 14th August 2019, 04:32 AM   #202
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Originally Posted by GnaGnaMan View Post
First of all, excuse my hyperbole. I am aware that he is not proposing that. His theory implies it but he is not aware of that. Simply pointing it out and asking about it received no reaction, so I tried with some sting.

Here is RBF's theory:

Now let's conclude. If we take away the forward push of the "simulated rocket engine" then the push of the atmosphere will push the whole contraption backwards.
We can just pipe the escaping air back into the balloon (with a pump to restore the pressure). That eliminates the thrust generated by "simulated rocket engine" but leaves the push of the atmosphere.

This is not rocket science (pun intended). IMHO, this is just a primitive fridge.
Finally you quoted what I said instead of rude strawmen and lies. Good.

Your conclusion is still flawed because you missed something. Try and figure that out. I'll be back to check your work tonight.
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Old 14th August 2019, 04:35 AM   #203
GnaGnaMan
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Originally Posted by Robin View Post
I was obviously talking about your continued childish strawmanning of RBF.
I don't know what you refer to here.

Quote:
Perhaps I can charitably assume that you are still blotto, in which case perhaps we can continue this when you have a clearer head.
I am quite sober but, unfortunately, I still don't realize "that the difference between speed and velocity here is not if the air streams into or out of the balloon."
I have to continue to await clarification.
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Old 14th August 2019, 04:42 AM   #204
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Originally Posted by GnaGnaMan View Post
I am quite sober but, unfortunately, I still don't realize "that the difference between speed and velocity here is not if the air streams into or out of the balloon."
I have to continue to await clarification.
As I said before, any high school text will explain the difference between speed and velocity. Speed is a scalar, velocity is a vector.

But you are jumping back to another topic.

I was asking you to explain your more recent comment. When the air passes through the cone, does the pressure inside drop? Or are you saying that it remains the same?
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Old 14th August 2019, 04:50 AM   #205
GnaGnaMan
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Originally Posted by Robin View Post
When you are a little more yourself perhaps you can try to explain yourself better here.

As I understand it, you are claiming that when the air passes through the cone, the pressure inside remains the same as the pressure outside? Yes? You have concluded this from blowing through a straw covered with cling film, as I recall.

If I have misunderstood you I apologise. Please clarify, when the air passes through the cone, does the pressure on the inside surfaces drop or not?
I went back to pull out what I asked RBF.
Originally Posted by GnaGnaMan View Post
The air at the narrow end of the cone is faster than in the balloon, so by Bernoulli it is at lower pressure. It is also faster than at the wide end of the cone, so again lower pressure.
We know that the pressure at the wide end is higher than atmosphere because air exits.
That means we know that the air at the narrow end of the cone is lower than above atmosphere. So how do we get from below above atmosphere to simply below atmosphere?
I believe this answers your question and I am now able to answer my own question there. So that is cleared up?
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Old 14th August 2019, 04:51 AM   #206
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Originally Posted by GnaGnaMan View Post
Please explain.
For example, connect it to the central branch of one of these
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Old 14th August 2019, 04:52 AM   #207
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Originally Posted by Robin View Post
As I said before, any high school text will explain the difference between speed and velocity. Speed is a scalar, velocity is a vector.
Ok. And is the only confusion this might cause about whether air is flowing into or out of the balloon, as drunk me so muddily believed?
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Old 14th August 2019, 04:55 AM   #208
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Originally Posted by GnaGnaMan View Post
I went back to pull out what I asked RBF.


I believe this answers your question and I am now able to answer my own question there. So that is cleared up?
Perhaps you could just answer the question.
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Old 14th August 2019, 04:56 AM   #209
GnaGnaMan
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Originally Posted by Robin View Post
For example, connect it to the central branch of one of these
A vital component of the RBF drive. But what does it have to do with my argument?
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Old 14th August 2019, 04:56 AM   #210
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Originally Posted by GnaGnaMan View Post
Ok. And is the only confusion this might cause about whether air is flowing into or out of the balloon, as drunk me so muddily believed?
No of course that is not the only confusion it can cause.

The fact that you think that you can account for the behaviour of that system using only scalars tells me a lot.
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Old 14th August 2019, 04:58 AM   #211
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Originally Posted by Robin View Post
Perhaps you could just answer the question.
I thought I had but ok...

The question is:
As I understand it, you are claiming that when the air passes through the cone, the pressure inside remains the same as the pressure outside?
No.
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Old 14th August 2019, 05:00 AM   #212
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Originally Posted by Robin View Post
No of course that is not the only confusion it can cause.
Can you please give an example?

Originally Posted by Robin View Post
The fact that you think that you can account for the behaviour of that system using only scalars tells me a lot.
That I looked it up in a fluid mechanics textbook?
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Old 14th August 2019, 05:09 AM   #213
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Originally Posted by GnaGnaMan View Post
That I looked it up in a fluid mechanics textbook?
Perhaps you could cite the part that said you can model the forces on a three dimensional object using only scalars. That would be an interesting textbook.
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Old 14th August 2019, 05:15 AM   #214
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Originally Posted by GnaGnaMan View Post
Can you please give an example?
For example what happens when a stream of air expands into a cone where there is already air at atmospheric pressure. There is not just forces in one direction.

In any case you have made a conceptual mistake. A scalar doesn't have a single direction, it doesn't have a direction at all.
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Old 14th August 2019, 05:19 AM   #215
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Originally Posted by GnaGnaMan View Post
I thought I had but ok...

The question is:
As I understand it, you are claiming that when the air passes through the cone, the pressure inside remains the same as the pressure outside?
No.
Good. But now I don't understand your strange interpretation of what RBF said.

Do you not think it possible that if the pressure on the inside surfaces of the cone was less than the pressure on the outside surfaces of the cone that the sum of the forces acting on the cone might point in the direction of the large opening?
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Old 14th August 2019, 06:41 AM   #216
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The part I am trying to understand is the pressure drop when the fluid passes through the constricted area of a cone. More molecules are being forced into a small and smaller cross sectional area yet the pressure drops. I get that the velocity increases. I'm just trying to get my head round it.

As a bit of a relevant derail, what happens if the cone in the video is replaced with a cylinder with a diameter the same as the large end of the cone.
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Old 14th August 2019, 06:54 AM   #217
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Originally Posted by The Common Potato View Post
The part I am trying to understand is the pressure drop when the fluid passes through the constricted area of a cone. More molecules are being forced into a small and smaller cross sectional area yet the pressure drops. I get that the velocity increases. I'm just trying to get my head round it.

As a bit of a relevant derail, what happens if the cone in the video is replaced with a cylinder with a diameter the same as the large end of the cone.
The same, basically, because there is still a section in which the airflow expands laterally to fill the cylinder and hence the pressure increases. If you replace the first half of the cone with a cone that has a steeper sidewall angle and the second half with a cylinder equal to the large end of the original cone, then repeat that with the shorter cone, and so on, ultimately you end up with the full length cylinder and a zero length cone.

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Old 14th August 2019, 07:24 AM   #218
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Originally Posted by The Common Potato View Post
The part I am trying to understand is the pressure drop when the fluid passes through the constricted area of a cone. More molecules are being forced into a small and smaller cross sectional area yet the pressure drops. I get that the velocity increases. I'm just trying to get my head round it.
This is Bernoulli's principle. It is really a fundamental property of fluid and can be difficult to grasp. But, it is worth focusing on until you grasp it, because once you do, fluid just makes more sense.
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Old 14th August 2019, 07:42 AM   #219
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Originally Posted by Robin View Post
Good. But now I don't understand your strange interpretation of what RBF said.

Do you not think it possible that if the pressure on the inside surfaces of the cone was less than the pressure on the outside surfaces of the cone that the sum of the forces acting on the cone might point in the direction of the large opening?
I can't really make sense of that question. If you sucked the air out of the cone with a vacuum cleaner or something, then the air pressure would probably push the cone backwards. The sum of forces will point towards the large opening But you don't have that here. There's no vacuum cleaner or something.
When you push against an object there will be an equal and opposite force pushing back. If that push comes from air pressure against some vessel then the material of that vessel will supply the push-back force (or collapse). There will not be any net force.
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Old 14th August 2019, 08:22 AM   #220
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Originally Posted by The Common Potato View Post
The part I am trying to understand is the pressure drop when the fluid passes through the constricted area of a cone. More molecules are being forced into a small and smaller cross sectional area yet the pressure drops. I get that the velocity increases. I'm just trying to get my head round it.
What we feel as gas pressure is lots and lots of molecules bouncing around and hammering against the wall of the vessel/pipe/whatever. On the whole all the molecules there will be having the same energy (actually it's distributed in a specific way).
Those that go at speed thru the narrow part are those that happened to be bouncing that way. So their sideways motion actually subtracts from their "bouncing around". While they are going faster through the narrow part, they cannot be bouncing as hard against the walls.

Quote:
As a bit of a relevant derail, what happens if the cone in the video is replaced with a cylinder with a diameter the same as the large end of the cone.
I think you'll get flow separation and turbulence where the tube suddenly turns into the bigger cylinder.


It's probably best to ask these questions on a dedicated science forum. Particularly the last one is tricky.
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Old 14th August 2019, 03:02 PM   #221
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Originally Posted by The Common Potato View Post
The part I am trying to understand is the pressure drop when the fluid passes through the constricted area of a cone. More molecules are being forced into a small and smaller cross sectional area yet the pressure drops. I get that the velocity increases. I'm just trying to get my head round it.

As a bit of a relevant derail, what happens if the cone in the video is replaced with a cylinder with a diameter the same as the large end of the cone.
You return to thrust like in the original but with slightly more friction and drag.....but the car should move again like before the cone was applied.
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Old 14th August 2019, 03:14 PM   #222
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Originally Posted by GnaGnaMan View Post
I can't really make sense of that question. If you sucked the air out of the cone with a vacuum cleaner or something, then the air pressure would probably push the cone backwards. The sum of forces will point towards the large opening But you don't have that here. There's no vacuum cleaner or something.
When you push against an object there will be an equal and opposite force pushing back. If that push comes from air pressure against some vessel then the material of that vessel will supply the push-back force (or collapse). There will not be any net force.
and that is the same woo that multiple times has people confused in this thread. I did ask if you would please pay attention and work your way through this, before confusing people actually trying to learn.

There is less pressure and at an angle, so a portion of that delta can be broken down to a vector that will be directly opposing thrust and the other vertically inwards. What % depends on the angle. If there is less pressure and it is horizontal, then the vector opposing thrust would be zero as the entire delta would be vertical.
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Old 14th August 2019, 03:50 PM   #223
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Originally Posted by GnaGnaMan View Post
I can't really make sense of that question.
Yes, I can see that you had trouble understanding it, but I am not sure which part of it you didn't get.

The sum of forces acting on the cone due to air pressure will either be in equilibrium and there will be no net force or else there will be a net force and it will point in a particular direction.

I am saying that the evidence is that the sum of forces acting on the cone due to air pressure points towards the large opening of the cone.

You have said that you find this possibility highly improbable, so I am asking you to explain why you think it improbable that the net force exerted on the cone by air pressure points backwards.

I am also asking which direction you think that net force points.

Just to clear up something, the force exerted back by those surfaces is not a force acting on the cone.
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Old 14th August 2019, 04:38 PM   #224
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Originally Posted by GnaGnaMan View Post
But what does it have to do with my argument?
If you have forgotten the point, why did you even bring it up again?

You asked me to explain how the balloon could be deflated with the velocity of the air pressure being zero and I showed you.

You were claiming that conservation of energy must (for reasons you didn't explain) imply conservation of velocity. I showed you a counter example.
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Old 14th August 2019, 04:56 PM   #225
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Originally Posted by The Common Potato View Post
The part I am trying to understand is the pressure drop when the fluid passes through the constricted area of a cone. More molecules are being forced into a small and smaller cross sectional area yet the pressure drops. I get that the velocity increases. I'm just trying to get my head round it.
Yes, it is counter intuitive. We can see from experiments that it is the case, but that doesn't help us get our heads around it.
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Old 14th August 2019, 05:58 PM   #226
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Originally Posted by The Common Potato View Post
The part I am trying to understand is the pressure drop when the fluid passes through the constricted area of a cone. More molecules are being forced into a small and smaller cross sectional area yet the pressure drops. I get that the velocity increases. I'm just trying to get my head round it.
Here is a simplified model I did of a venturi meter a while ago when I was trying to understand this.

https://www.youtube.com/watch?v=64dZFi9p4nA

To begin with it does what you would expect and the higher pressure is in the constricted part. After a while the flow settles down and the higher pressure is in the larger pipe.

As close as I can understand this, in the faster flow the momentum of each particle is more likely to be along the line of flow and therefore the collisions with the wall of the pipe. In the slower flow the particles are relatively more likely to be heading in the direction of the wall of the pipe, therefore the collisions are going to have more force.

Don't take this as an expert opinion. Like you I am trying to get my head around it.
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Old 15th August 2019, 03:41 AM   #227
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Originally Posted by Robin View Post
If you have forgotten the point, why did you even bring it up again?

You asked me to explain how the balloon could be deflated with the velocity of the air pressure being zero and I showed you.

You were claiming that conservation of energy must (for reasons you didn't explain) imply conservation of velocity. I showed you a counter example.
Should have been " .... velocity of the air escaping ... "
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Old 15th August 2019, 03:59 PM   #228
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OK, here is a point that I think is causing a lot of confusion.

Let's take the question posed by RBF, using the video from the UCB.

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The straws are all vertical.

Suppose we add another straw at A and at the same height as the straw at A but have the end curved over so that the end of it is at right angles to the ends of the other straws, with the opening towards the source of flow.

My question is, would the pressure detected by this straw be the same as A, higher than A or lower than A?

My guess is that the pressure would be higher than the original straw at A.

In other words "pressure" is not a completely non-directional quantity, even if it is often useful to model it as such.

So the pressure on the surfaces of the cone can be lower than atmospheric pressure while the pressure over the plane of the opening can be higher than atmospheric pressure.

Any comments?

Edit - I have put the suggested modification at A in blue:
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Old 15th August 2019, 05:11 PM   #229
Red Baron Farms
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Originally Posted by Robin View Post
OK, here is a point that I think is causing a lot of confusion.
snip

So the pressure on the surfaces of the cone can be lower than atmospheric pressure while the pressure over the plane of the opening can be higher than atmospheric pressure.

Any comments?

Edit - I have put the suggested modification at A in blue:
https://robinsrevision.files.wordpre...08/image-2.png
Don't be fooled by GnaGnaMan woo Robin.

The pressure in the cone varies from lower than atmospheric to atmospheric as it leaves the cone. His premise that the pressure in the cone must be higher in order to leave the cone is a false premise and it was designed as a troll to obfuscate.

The pressure in the cone is less than or equal to atmospheric. It is not greater than atmospheric.
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Old 15th August 2019, 06:36 PM   #230
Robin
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Originally Posted by Red Baron Farms View Post
Don't be fooled by GnaGnaMan woo Robin.
This does not come from anything that GnaGnaMan has said.

He and I have disagreed on just about everything so far.
Quote:
The pressure in the cone varies from lower than atmospheric to atmospheric as it leaves the cone. His premise that the pressure in the cone must be higher in order to leave the cone is a false premise and it was designed as a troll to obfuscate.

The pressure in the cone is less than or equal to atmospheric. It is not greater than atmospheric.
So the modified straw at A in my diagram will raise or lower the water to just the same level as the unmodified straw at A in the original diagram? (edit to say that the opening is supposed to be the same diameter).

Also, the question comes from something Dave Rogers said. He said that the pressure must exceed atmospheric pressure at the exit, otherwise the air would never leave the cone.
Originally Posted by Dave Rogers
As the cone widens, the air slows and its pressure increases, to the point where it exceeds atmospheric pressure at the end of the cone, and therefore is expelled, but its pressure doesn't need to exceed atmospheric at every point in the cone in order for this to be the case.
The air still has velocity when it exits the cone which suggests that the pressure on the inside surface of the cone just before the exit has still not matched atmospheric pressure.
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Old 15th August 2019, 06:39 PM   #231
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And also, to return to the question, would the air leave the cone if it just matched atmospheric pressure?
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Old 15th August 2019, 07:12 PM   #232
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And, looking around a bit, it sounds like I am talking about the difference between static pressure and dynamic pressure, which is the half rho v squared portion of the equation.
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Old 15th August 2019, 07:36 PM   #233
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A description from Dr Yan Wong's BBC blog (OK, he is an evolutionary biologist, not a rocket scientist but this sounds about right):
Quote:
The key is that Bernoulli's principle is referring to 'static pressure'. That's the pressure you notice building up within a hosepipe when you seal off the end with your thumb. Static pressure pushes equally in all directions, which is why water in the tube will start squirting out from any punctures.

This pressure is quite different from the directional 'dynamic pressure' with which a stream of water knocks over your flowers, after you've removed your thumb from the end of the hosepipe [see Note 1, below].

In fact, Bernoulli's principle tells us that there's exactly the same total pressure at any point in a flowing stream. That means if the dynamic 'forward-streaming' pressure (ie the speed) goes down, the static 'sideways-pushing' pressure must increase to make up for it

http://www.bbc.co.uk/bang/bernoulli_principle.shtml
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Old 15th August 2019, 07:55 PM   #234
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If I am on the right track, then this resolves the problem. The air in the nozzle near the exit can be below atmospheric pressure and still exit the nozzle and there is no contradiction.

Say the air just inside the plane of the nozzle is at the same pressure of the atmosphere outside.

But outside that pressure is exerted in all directions, but inside it is exerted in one direction, and so it will push outwards.

So if the air inside the plane of the nozzle is below atmospheric pressure it can still push out for the same reason.
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Old 15th August 2019, 08:09 PM   #235
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So if I have a large box filled with air with even pressure throughout and I select a volume of air and transform the velocities of the particles by taking the product of their magnitudes with a unit vector pointing in a particular direction (or I rotate them all until they point the same direction) then I will not have altered the pressure in that area (I think).

(EDIT: Well obviously not Robin - I would have decreased the pressure in one sense and increased the pressure in another)

However that volume of air would move in the direction of the unit vector pushing the surrounding air out of the way.

If I do the same thing but also take the product of the vectors and 0.75 then I would have reduced the pressure in that area, but still it would move in the direction of the unit vector, pushing the surrounding air out of the way.
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Old 15th August 2019, 10:15 PM   #236
Robin
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Incidentally this doesn't change anything that has come before, Ziggurat and RBF are still correct about the reasons the car does not move with the addition of the cone.

The exit velocity of the air is lower with the cone.

The overall force vector for just the cone points towards the back of the car because of the lower air pressure on the inside surfaces of the cone compared to that on the outside of the cone.

And either of these straightforwardly explains why adding the cone will reduce the forward force of the car overall so that it cannot overcome friction.

There was just a confusion about why air would exit the cone if it is not above atmospheric pressure and this would clear that up too.
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Old 16th August 2019, 01:16 AM   #237
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Originally Posted by Robin View Post
Also, the question comes from something Dave Rogers said. He said that the pressure must exceed atmospheric pressure at the exit, otherwise the air would never leave the cone.
Yeah, not sure I got that right. The air leaving the cone has to have momentum, but then as it hits ambient pressure it loses that momentum by the application of a force in the opposite direction, which I think on reflection must increase its pressure. Since its final pressure must be equal to that of the atmosphere, that implies that its pressure is below atmospheric at the exit. Not sure; hydrodynamics is horribly counter-intuitive, which is why I posted the link to the tripe cone injector[1].

Originally Posted by Robin View Post
The air still has velocity when it exits the cone which suggests that the pressure on the inside surface of the cone just before the exit has still not matched atmospheric pressure.
Yeah, I think that's right.

Dave

[1] Regardless of the fact that nobody seems to call it that on the Internet, that's what steam locomotive drivers I've met call it.
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