Physics Question

[RDY4WAR]

Say you have a closed off cylinder with a gas inside. The gas is spinning in the cylinder at a given angular velocity. You start to compress the gas in the cylinder with a piston. Would the compression increase, decrease, or have no effect on the angular momentum and angular velocity of the gas?

 

[ABN_CBT_ENGR]

Would the compression increase, decrease, or have no effect

For accuracy sake, by "compression" are you meaning the force to compress the gas or the pressurization of the gas?

What exactly are you referring to as angular velocity/momentum ( in a closed system?)

 

[ecotourist]

Thinking out loud:

1) Gas molecules are somewhat like balls circling on a series of stacked plates. Pushing the plates closer together would have no effect on their angular momentum.

So there should be no effect.

2) In addition, the force being applied is not in the direction of motion.

So there should be no effect.

But I could be wrong. I'd like to hear someone else's analysis.

 

[SuperSalad]

I think it would change the original angular velocity because whatever the piston is pushing on would then have a velocity in that vector in addition to the original velocity. With the decrease in volume, the interaction between gas molecules would increase and there should be a positive change in momentum and velocity in the direction of the piston movement.

It should come to equilibrium though once the "normal" force of the immovable bottom of the cylinder cancels out the piston's input, assuming the piston remains unmoved following the initial compression, and it is a fully closed system with no energy losses.

@ecotourist, the only thing I would argue about your thought and where our responses disagree is that your assuming the "plates" wouldn't interact with one another. Like you though, I'd like to hear why my thoughts on it are wrong.

 

[ABN_CBT_ENGR]

The question is somewhat confusing ( which is why I asked what I did and may need further clarification)

Gas itself has very little kinetic ability for "momentum" ( it relies on pressure to move it) so theoretically in a "closed cylinder" ( would have to be closed to compress) it wouldn't have a prime mover to be spinning. ( angular or otherwise)

If hypothetically it was spinning ( moving in a series of arc tangents based on ricocheting because energy cant turn in a circle) it would be decreasing ( losing energy at a rate) which would slow to stop it in short order.

Then as density changed ( Mass / volume) on the compression that energy would surely overwhelm any remaining counter energy ( if it ever existed) and movement would cease.

 

[Davejam]

The speed of a gas is proportional to temperature.
The answer is it depends. In most situations the temperature will rise when compressing gas, but in physics we can pretend isothermal compression is possible.

 

[ecotourist]

I think it would change the original angular velocity because whatever the piston is pushing on would then have a velocity in that vector in addition to the original velocity. With the decrease in volume, the interaction between gas molecules would increase and there should be a positive change in momentum and velocity in the direction of the piston movement.

It should come to equilibrium though once the "normal" force of the immovable bottom of the cylinder cancels out the piston's input, assuming the piston remains unmoved following the initial compression, and it is a fully closed system with no energy losses.

@ecotourist, the only thing I would argue about your thought and where our responses disagree is that your assuming the "plates" wouldn't interact with one another. Like you though, I'd like to hear why my thoughts on it are wrong.

My (unstated) assumption was that the piston pushing down on the gas was moving very slowly (or quasi-statically if you prefer).

But even if the piston was moving quickly, the force being applied would not be in the direction of gas movement (at least as I envision the apparatus). I could imagine an apparatus however where the movement was in or against the direction of rotation and of course the answer then would be completely different.

FT=MV. If there is no force (F), there would be no change in momentum (MV). [I'm not sure that you can apply that formula to rotational movement but you probably can.]

I agree that there would be more interaction between the molecules when the gas is compressed, but assuming those interactions are efficient they would (in aggregate) just cancel out. And they are efficient because a cylinder of compressed gas stays compressed if there is no leakage or cooling.

An interesting discussion just the same.

 

[ABN_CBT_ENGR]

Again, "closed cylinder" ( that puts a hard stop in there)

At that point the "gas" would be at a zero potential state thus no movement ( other that the nano second it would take for all previous movement to normalize)

Then starting "compression", it would "increase" as the piston moves forward ( changing the volume of the area) affecting the density of the gas.

Anything beyond that would have to have more info on the gas

 

[Leo99]

My guess is that as the volume of the container decreased, the angular velocity would increase to keep the kinetic energy level constant and conserve angular momentum. That's assuming frictionless. There is friction and nothing to keep the angular velocity going, it's going to slow down on its own. More compression will increase friction and hasten this. That would make the angular velocity decrease. Hmmm. I see why barely passed college physics.

 

[kschachn]

This discussion reminds me of the old Ford HSC engine.

 

[RDY4WAR]

The question is somewhat confusing ( which is why I asked what I did and may need further clarification)

Gas itself has very little kinetic ability for "momentum" ( it relies on pressure to move it) so theoretically in a "closed cylinder" ( would have to be closed to compress) it wouldn't have a prime mover to be spinning. ( angular or otherwise)

If hypothetically it was spinning ( moving in a series of arc tangents based on ricocheting because energy cant turn in a circle) it would be decreasing ( losing energy at a rate) which would slow to stop it in short order.

Then as density changed ( Mass / volume) on the compression that energy would surely overwhelm any remaining counter energy ( if it ever existed) and movement would cease.

The question relates to a cylinder in an engine and the induction of "swirl" from the intake port, valve, and chamber on the incoming air/fuel mixture. The mixture swirls around the cylinder as it enters and then the valve closes as the piston is on its way back up.

My theory is that... as volume decreases from the accelerating piston up the bore, the mass (per cubic inch of remaining volume) increases. As mass increases, so does angular momentum. Therefore I feel the angular velocity would increase. Realistically though, with irregular shapes in the chamber and piston top, changes in piston velocity, temperature, etc... the cylinder is a chaotic mosh pit and an accurate answer is much deeper in quantum mechanics.

 

[tcp71]

Designing a new dyson vacuum?

 

[A_Harman]

The question relates to a cylinder in an engine and the induction of "swirl" from the intake port, valve, and chamber on the incoming air/fuel mixture. The mixture swirls around the cylinder as it enters and then the valve closes as the piston is on its way back up.

My theory is that... as volume decreases from the accelerating piston up the bore, the mass (per cubic inch of remaining volume) increases. As mass increases, so does angular momentum. Therefore I feel the angular velocity would increase. Realistically though, with irregular shapes in the chamber and piston top, changes in piston velocity, temperature, etc... the cylinder is a chaotic mosh pit and an accurate answer is much deeper in quantum mechanics.

After the intake valve closes mass trapped in the cylinder is fixed. As the piston comes up during the compression stroke, pressure, temperature, and density all increase. Angular momentum remains constant as the volume of the cylinder decreases, but since the charge is being compressed into a smaller volume, conservation of angular momentum dictates that the rate of swirl has to increase. Therefore the angular velocity of the charge will increase.

 

[JohnnyG]

My theory is that... as volume decreases from the accelerating piston up the bore, the mass (per cubic inch of remaining volume) increases. As mass increases, so does angular momentum. Therefore I feel the angular velocity would increase. Realistically though, with irregular shapes in the chamber and piston top, changes in piston velocity, temperature, etc... the cylinder is a chaotic mosh pit and an accurate answer is much deeper in quantum mechanics.

I kinda figured you were referring to swirl charge, but that's not really a closed cylinder. Also it was tried and tested and found somewhat effective. Computer design later improved on that idea. So, I'm sure the "Mosh Pit" refers to the design of the top of the piston and head. Take the Northstar engine for example. The "swirl" was not induced by the shape or direction of the intake ports, but rather the shape of the top of the piston and the matching cylinder head. Parts of the combustion chamber had compression ratios greater that 25:1, but other parts not so much. Therefore each compression stroke resulted in the charge moving or "swirling" if you prefer, around in the compressed chamber. This caused the ignition of those gases to be very complete. It also allowed that engine to run perfectly fine on 87 octane fuel, so many advantages. Additionally, it qualified the Northstar as an ULEV engine, without EGR!

 

[ABN_CBT_ENGR]

The question relates to a cylinder in an engine and the induction of "swirl" from the intake port, valve, and chamber on the incoming air/fuel mixture. The mixture swirls around the cylinder as it enters and then the valve closes as the piston is on its way back up.

OK, that's a bit clearer.

This is not a "gas' but a solid aerosol vapor contained in air

I thought so but that was not what you were actually describing

 

[JimPghPA]

Angular momentum of rotation of each induvial gas molecule would remain unchanged with respect to the direction of rotation. Even though more gas molecules would occupy any given space, and there temperature would increase. There mass and energy as far as energy of rotation momentum would remain unchanged. Rotational energy (of each molecule) is not lost, or increased, and therefore angular momentum (of each molecule) or rotation remains the same. And since the mass of each gas molecule remains the same and the angular momentum of each gas molecule remains the same, the angular velocity of each gas molecule remains the same.

What you have to realize is that although the rotational energy in the remaining areas increased, the increase in mass because of the additional molecules, means that the rotational speed will remain the same.

If rotational momentum were to increase, that increase would be an input of force in the direction of rotation, and that was not supplied, because if molecules in the top portion of the cylinder were to increase in rotational momentum then that means molecules that previous were lower in the cylinder added rotational momentum to those molecules, but in order to do that they would of given up rotational energy because that rotational energy has to have come from somewhere. And if they gave up rotational momentum they would slow down because of loss of rotational momentum. Therefore the average rotational momentum of each gas molecule remains the same. And again since the mass of each molecule remains the same, the angular rotational velocity remains the same.

Now what would really make it interesting would be if the piston was not flat on top, but the top of the cylinder was flat. Because if the piston had a dome (higher in the center) then on average, and initially the angular rotation speed was the same regardless of the radius of each molecule, then after the piston moved up, on average there would be more molecules that previously had short radius (and lower rotational momentum) migrating outward to an area with more radius, and on average the momentum of rotation of the molecules that are already there interacting with these molecules so the average rotational momentum in any given radius had the same momentum, the average momentum of the molecules with greater radius would decrease, and the angular rotation speed of the outer molecules would decrease.

And if the piston had a cavity in the center, and the top of the cylinder was flat, then as the molecules that were previously at a greater radius and were above the piston, migrated towards the center, because there rotational momentum would remain the same but the radius decreased, there rotational speed would increase.

 

[ollie]

Say you have a closed off cylinder with a gas inside. The gas is spinning in the cylinder at a given angular velocity. You start to compress the gas in the cylinder with a piston. Would the compression increase, decrease, or have no effect on the angular momentum and angular velocity of the gas?

Not having done too well in chemistry and physics I'll still take a stab at this.

Compression will increase

The momentum and velocity of gas molecules will not slow down nor speed up

Temperature of the gas will increase

 

[JimPghPA]

On an atomic level the velocity of the gas molecules increases proportionately to temperature, but on a higher level the physical momentum of rotation remain the same.

 

[PandaBear]

In theory the piston top and the chamber these days are not flat so when you compress, more air will be slowed down by the dome of the pistons. In practice there are more important things to care about like pumping loss of the other active cylinders to worry about what's going on in the closed off cylinders.

 

[CR94]

Zero change, if we ignore the increase in kinetic energy of the average molecule due to temperature increase, and ignore deceleration due to viscosity.