No one can explain WHY planes fly...

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jedi

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I know it works.
But the question is why is air meeting a bottleneck in a tube so different than cars on Interstate 5 when reduced to one lane?
Because the traffic on I-5 is compressible. I am quite sure someone will post an interesting picture of that but it is not my intent here.

Recall that the requirement of Bernoulli when applied to a gas or liquid was incompressible flow. The molecules vibrate in their own little space and bunch together to fill the total space available. For a gas this requires relatively low speeds and low mass for quick and easy accelerations.

If the speed limit on I-5 was 5 mph and the following distance was strictly enforced to 500 feet between cars there would be no compression when the pavement changed from 4 lanes to 2. Assume the average speed in the four lanes of two mph. The cars would speed up and travel at 4 mph in the two lane section in order to handle the desired (or required) flow rate and separation distance in the two lane section.

If the 4 lane speed increased to 5 mph, then you would get a bottleneck at the two lane section because the two lanes could not handle the flow with the speed and spacing restrictions imposed. This is what happens in a supersonic wind tunnel.
 

bmcj

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Hard to compare cars to air pressure reduction. For the cars, between outside access points (exits and entrances) the mass flow rate is defined. If four lanes are flowing past a datum line at a rate of 100 cars per minute, then a bottleneck down to two lanes would have to double their speed or halve their separation in order to sustain the 100 cars per minute flow rate (in real life, it doesn’t always work that way due to human nature/driving skills). This is how velocity and density interrelate in a compressible flow; pressure is not addressed here.

As for air flow on a surface, perhaps the molecules of the faster air don’t have as much time to interact and exert force (pressure) against the surface?
 

BBerson

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The cars on I-5 do get closer together at the bottleneck. The air molecules at the Venturi must apparently get further apart in the Venturi bottleneck to cause reduced pressure. The question is why do they get further apart to reduce pressure at the bottleneck?
 

Dan Thomas

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I know it works.
But the question is why is air meeting a bottleneck in a tube so different than cars on Interstate 5 when reduced to one lane?
Because the cars don't speed up through the bottleneck?
 

Dan Thomas

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Hard to compare cars to air pressure reduction. For the cars, between outside access points (exits and entrances) the mass flow rate is defined. If four lanes are flowing past a datum line at a rate of 100 cars per minute, then a bottleneck down to two lanes would have to double their speed or halve their separation in order to sustain the 100 cars per minute flow rate (in real life, it doesn’t always work that way due to human nature/driving skills). This is how velocity and density interrelate in a compressible flow; pressure is not addressed here.

As for air flow on a surface, perhaps the molecules of the faster air don’t have as much time to interact and exert force (pressure) against the surface?
It's more, I think, due to the fact that in physics, nothing is free. If you take the static and dynamic pressures and add them, you get total pressure, which is the same as static pressure when the air is not moving and there's no dynamic pressure.
 

Dan Thomas

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The cars on I-5 do get closer together at the bottleneck. The air molecules at the Venturi must apparently get further apart in the Venturi bottleneck to cause reduced pressure. The question is why do they get further apart to reduce pressure at the bottleneck?
This shows that they get further apart. Why? That's the question. And why do they speed up so much, far beyond the relative wind speed?



I'm just glad that they do, or we'd not be flying.
 

poormansairforce

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This shows that they get further apart. Why? That's the question. And why do they speed up so much, far beyond the relative wind speed?



I'm just glad that they do, or we'd not be flying.
That shows air being expanded and compressed around convex and inclined surfaces. Expanded air has to speed up because it occupies more space and the reverse for compressed air. There is the magic.
 

jedi

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Reference Post #141 at the top of this page. I guess I need to post my own photos.

The cars on I-5 do get closer together at the bottleneck. The air molecules at the Venturi must apparently get further apart in the Venturi bottleneck to cause reduced pressure. The question is why do they get further apart to reduce pressure at the bottleneck?
This is an example of compressible flow.

upload_2020-2-15_20-4-58.jpeg

Because the cars don't speed up through the bottleneck?
The one in front stopped for some reason and the one in the rear did not realize it in time. This is an example of supersonic flow.




This is the shock wave that results. Flow on either side of the shock wave is normal but it has switched from supersonic to sub sonic.



If the I-5 traffic were less dense and the drivers knew ahead of time what was happening they could adjust their speed and avoid the pile up. The density (distance between cars) would change but not enough to be a concern or cause an accident.

If you tell your kids you have been supersonic in a car they will be impressed provided you did not crash. Do not drive supersonic if there is a possible obstruction in the road ahead.

You asked, "The question is why do they get further apart to reduce pressure at the bottleneck?" The question could be not "why" but "how". Perhaps it is because they can or it could be because they adjust to fill the space available. The point is at subsonic speeds they know in advance that they need to adjust to the changing conditions ahead. How do they know? It is because the molecules in front, behind and to the sides are pushing and signaling thru the crowd that conditions are changing ahead and an adjustment is required. If the adjustment proceeds smoothly, there is no pile up.

BTW they do not get very much farther apart but they do go faster.

Ok, this is not a scientific example but for those who still want to know why here goes.

Imagine you are in one of the two center lanes at a comfortable speed when the road narrows to two lanes. The driver to your left slides in behind you at an uncomfortable distance, flashes his lights and lays on the horn so you speed up a little. Later, when the road goes back to four lanes you pull to the right out of his way and slow down to the same comfortable speed. That is subsonic flow. It is all rather slow and easy. There is little chance of a pile up.
 
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BBerson

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Imagine you are in one of the two center lanes at a comfortable speed when the road narrows to two lanes. The driver to your left slides in behind you at an uncomfortable distance, flashes his lights and lays on the horn so you speed up a little. Later, when the road goes back to four lanes you pull to the right out of his way and slow down to the same comfortable speed. That is subsonic flow. It is all rather slow and easy. There is little chance of a pile up.
He can lay on his horn all he wants.... no way I can make the guy in front of me go faster.
I guess that air in the Venturi is smarter than me.
 

jedi

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He can lay on his horn all he wants.... no way I can make the guy in front of me go faster.
I guess that air in the Venturi is smarter than me.
I drive I-5 between Bellevue and I-90. If i do it between 3 and 7 pm I experience the example given in Post #141. The flow is two MPH as it approaches the lane restriction to two lanes. As soon as everyone is established in their proper lane the flow rapidly accelerates to 4 mph. :););).

That is subsonic flow. Just try to continue going 2 mph and you will find out how serious the gun issue is in Seattle. :fear::fear::fear:
 
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BBerson

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It's not intuitive at all, but if this was not so, a turbine engine would not work. Not even a bit.
I think it might work because the port is just beyond the restriction.
I hadn't noticed that before.
Cars accelerate just beyond the accident, not at the accident. They get further apart and less pressure.:p
 
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jedi

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And now can we return to the Bernoulli equation of Post #136 copied below?



Recall that Daniel Bernoulli studied both hydrostatics and hydrodynamics related to the pressure in a tank or a reservoir behind a dam and the fluid flow in the associated pipes, etc.

The equation has two components, static and dynamic. The term with the velocity, V, is the dynamic component. If the velocity is zero, the dynamic term is zero and we are left with the static equation.

There may be some confusion over the static equation because the first term, p, is misunderstood.

Picture a reservoir behind a dam. There is some static pressure at the surface. We can call that zero or 15 psi depending upon whether we chose to deal with gauge or absolute pressure. We know from experience that as a diver goes deeper into the lake the pressure rises. If p were the surface pressure, how could the second term be added to that to equal a constant?

The first term, p, is the surface pressure when the equation is applied on the surface. The first term, p, is the local pressure at the diver as he descends into the lake. The last, or second term in the static equation, includes the depth of the diver but because the diver is descending into the lake the term "h" is negative and is subtracted from the first term so that the constant is equal to the surface pressure. The term h is the "head" and is a length measurement, the associated term is a pressure and is sometimes referred to at the "static head" or "static head pressure" neglecting the sign. Note that the term "h" is in reality a differential measurement or vertical distance from some reference height, typically the surface of the reservoir.

Note that when the static equation is applied to a gas, the density is very small and the head pressure is near zero so that the term with the "h" is typically omitted in aerodynamics. It is however important in aerostatics or ballooning and is what makes a balloon fly. In a smaller container the fluid head pressure is small and the head term may be omitted. In other words it does not make much difference whether you check the tire pressure with the valve stem at the top or the bottom of the tire.

The dynamic pressure or second term in the dynamic equation (the term containing velocity, V) will be discussed in a follow on post.
 
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