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re: When an object enters the atmosphere it burns up from compression, not friction.
Posted on 7/26/22 at 8:10 am to GumboPot
Posted on 7/26/22 at 8:10 am to GumboPot
Without friction you have no compression. Rocks hit the moon all the time but don't blow up before they get there. That is because the moon has little to zero atmosphere. The atmospheric particles are what create friction and allow for compression to happen. It's a combination.
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Posted on 7/26/22 at 8:47 am to GumboPot
I believe it depends a bit on the composition but the initial entry into the atmosphere is the place where compression is the issue. That's the critical phase of reentry for a spacecraft as well. Frozen gasses probably play a part as well. Once it is in the atmosphere, friction should also factor in.
Very interesting stuff and you will find explanations that vary from friction or compression. Friction is simply a term used to explain the resistance to motion created by molecules 'rubbing' against each other. Those same molecule interaction are effectively what causes the compression. I think that may be why there is some disparity in how this is explained.
Whatever the cause, it is pretty cool
Very interesting stuff and you will find explanations that vary from friction or compression. Friction is simply a term used to explain the resistance to motion created by molecules 'rubbing' against each other. Those same molecule interaction are effectively what causes the compression. I think that may be why there is some disparity in how this is explained.
Whatever the cause, it is pretty cool
Posted on 7/26/22 at 9:59 am to jrodLSUke
quote:
Isn’t the friction causing the compression?
No. Friction contributes to it, but some (probably most) is due to head-on collisions. As an example, think of a basketball being thrown directly at a wall. It will compress, even if the vector is 90 degrees from the wall surface, and thus there is almost zero friction.
Posted on 7/26/22 at 10:11 am to GumboPot
Is that what stops a car when the brakes are pushed and the tires lose rubber on the road?
Posted on 7/26/22 at 10:26 am to GumboPot
It’s actually just the electrical current in the computer generating our simulation
Posted on 7/26/22 at 10:28 am to magildachunks
quote:
Who are we supposed to believe here? NASA or Slate?
Slate is a propaganda rag.
Posted on 7/26/22 at 11:10 am to mdomingue
There's a bunch of misconceptions in this thread as it relates to compressible aerodynamics. There's three sources of temperature increase that people are talking about and getting confused.
There's adiabatic heating of the air upon re-entry that is caused by compression of the air at the leading edge of the body. Adiabatic heating is strictly an inviscid phenomena meaning it has no attachment to irreversible processes (at least within the limits of typical modeling), a.k.a. there's no friction involved. At really high temperatures due to compression this probably becomes more complicated due to the ionization of air and loss of heat due to radiation of the plasma but for all intents and purposes, no friction here. Effects here are very well represented by the Euler equations of fluid motion for a compressible gas, you'll get shockwaves and the full she-bang.
Getting into the frictional effects, there's the conversion of dilatational work of the air into heat via the bulk compression of the fluid and the conversion of shear in the air to heat via your typical viscous behavior in fluids. I think what this person is trying to say is that the latter shear driven frictional effects on the temperature are small compared to the adiabatic temperature increase and the dilatational work which I'd imagine is completely true when your pushing re-entry Mach No. ~20.
Thank you for coming to my TED talk.
There's adiabatic heating of the air upon re-entry that is caused by compression of the air at the leading edge of the body. Adiabatic heating is strictly an inviscid phenomena meaning it has no attachment to irreversible processes (at least within the limits of typical modeling), a.k.a. there's no friction involved. At really high temperatures due to compression this probably becomes more complicated due to the ionization of air and loss of heat due to radiation of the plasma but for all intents and purposes, no friction here. Effects here are very well represented by the Euler equations of fluid motion for a compressible gas, you'll get shockwaves and the full she-bang.
Getting into the frictional effects, there's the conversion of dilatational work of the air into heat via the bulk compression of the fluid and the conversion of shear in the air to heat via your typical viscous behavior in fluids. I think what this person is trying to say is that the latter shear driven frictional effects on the temperature are small compared to the adiabatic temperature increase and the dilatational work which I'd imagine is completely true when your pushing re-entry Mach No. ~20.
Thank you for coming to my TED talk.
Posted on 7/26/22 at 11:16 am to Gaggle
quote:Gravity is an extremely weak force. If we could feel the gravity from other objects in space, we wouldn't be here to think about why it's imperceptible. The planet wouldn't hold itself together, or wouldn't have accreted in the first place.
Everything's always conveniently imperceptible.
It's not "convenient" or improbable that we happen to live on a rock in the universe where things are relatively stable. It's the only type of place that life like ours can exist.
If you want to attribute the value of the gravitational constant to god, that's fine. I can't argue why it is what it is. But he's not projecting heavenly bodies onto a dome over a flat earth. The earth is a sphere, and all that shite is really out there.
Posted on 7/26/22 at 12:05 pm to Korkstand
quote:
I don't know whether this is more semantics or pedantism, but aren't compression and shear both just work inputs which achieve the same result of increased molecular collisions and therefore temperature?
I don't mean to be argumentative, I'm just suggesting that maybe "friction" as the cause of increased temperature isn't entirely incorrect.
So a couple of things here..
First off, all of this is the end result of drag. Total drag is generally considered the sum of two components: skin friction drag and pressure drag.
Skin friction drag occurs because the moving object causes the fluid to shear at the boundary layer.
Pressure drag occurs due to pressure effects. For example, the high- and low-pressure areas of a lift surface. Pressure drag can also be the result of shockwaves at high (transonic and supersonic) speeds which probably play a significant part in the case of objects entering the atmosphere.
The overall point is that “friction” is a pretty well-defined term and it is caused by resistance to lateral movement. In the case of skin friction drag, the resistance comes from the viscosity of the fluid.
I think it’s likely that heat is generated by both friction and compression for objects entering the atmosphere, and the percentage of heat produced by each factor is going to depend largely on the shape and speed of the object. It could very well be completely different for a meteor vs. the space shuttle or a re-entry capsule.
Posted on 7/26/22 at 1:23 pm to Korkstand
quote:
Isn't it just semantics though? Doesn't compression just create more friction between air molecules, which creates heat?
Yes, what we should have said, to be more clear, is that it's not due to friction between the meteorite and the atmosphere.
Posted on 7/26/22 at 3:50 pm to Penrod
quote:
Isn't it just semantics though? Doesn't compression just create more friction between air molecules, which creates heat?
Nope not semantics, compression doesn't create more friction between air molecules.
If you're in a diesel piston before the compression stroke, air is sitting in the piston at some temperature and pressure. That pressure is the force resulting from air molecules colliding with each other and the walls of the cylinder. When you compress the piston, the number of molecules stays the same but you reduced the room that they have to live in so they hit the walls of the piston more often and it higher velocities and as a result the pressure is higher.
Temperature is more complicated in terms of what it actually is but its related to the local kinetic energy (translational and rotational) of the molecules. When you compress the piston, because the molecules have less room they're going to collide more often and their velocity RMS goes up. Overall the temperature is going to increase either way because you put work into the air when you compressed it.
This video helps visualize it
This post was edited on 7/26/22 at 3:52 pm
Posted on 7/26/22 at 3:53 pm to GumboPot
Learn new things everyday
Posted on 7/26/22 at 4:15 pm to Tiger Chemist
quote:
The system (meteor) is doing work (compression) on the surroundings (atmosphere).
That makes no sense. You can’t compress the atmospheric air from a little meteor enough to generate the heat necessary to melt iron. You have to have a finite space to be able to compress something. There is no way that the atmosphere is being displaced that quickly enough to compress it enough to generate that much heat.
An actual air compressor compressing 78 degree air to 125 psi we’ll see temperatures coming out of it around 420F. The melting point of iron is 2800F.
I have a hard time believing enough compression is happening to generate those temperatures.
Posted on 7/26/22 at 4:19 pm to stbtiger87
Yeah I get all that. I'm suggesting that it all boils down to semantics anyway.
For example, metals get hot when bent. Is that due to internal friction or compression? Does it matter? You can compress metal, too. We performed work which increased the KE of molecules.
What's the difference in working a metal vs working a gas? If the metal was hot enough to be a gas, would it not behave like any other gas and cool when allowed to fill a larger volume?
I'm just saying I don't see the difference at the molecular level between mechanically exciting molecules via compression and mechanically exciting molecules via friction. Seems to be the same process when the concept of "friction" is applied at the molecular level.
For example, metals get hot when bent. Is that due to internal friction or compression? Does it matter? You can compress metal, too. We performed work which increased the KE of molecules.
What's the difference in working a metal vs working a gas? If the metal was hot enough to be a gas, would it not behave like any other gas and cool when allowed to fill a larger volume?
I'm just saying I don't see the difference at the molecular level between mechanically exciting molecules via compression and mechanically exciting molecules via friction. Seems to be the same process when the concept of "friction" is applied at the molecular level.
Posted on 7/26/22 at 4:30 pm to GumboPot
quote:
thought it was friction all this time until a physics
Wait now - friction is just how you measure the heat generated by causing one hunk of matter rubbing against another hunk of matter.
It takes energy to overcome the resistance - where that resistance be from a mass of air to be displaced, or a rough surface to be ground down by displacing particles.
It is all the same thing - without the friction, there would be no resistance, and hence no heat.
Posted on 7/26/22 at 4:47 pm to ChineseBandit58
Throw a marble at a balloon. Then answer why it bounced back.
Posted on 7/26/22 at 4:50 pm to Penrod
quote:
surface, and thus there is almost zero friction.
Zero friction?
You sure about that Clark?
This post was edited on 7/26/22 at 4:51 pm
Posted on 7/26/22 at 5:20 pm to GumboPot
Space shuttle tiles were not strong at all. Super lightweight and you can poke a finger through one.
Not sure I understand how they are strong enough to compress the atmosphere in front of them. They get red hot but I understood it was due to friction.
Not sure I understand how they are strong enough to compress the atmosphere in front of them. They get red hot but I understood it was due to friction.
Posted on 7/26/22 at 6:25 pm to stbtiger87
quote:
There's a bunch of misconceptions in this thread as it relates to compressible aerodynamics. There's three sources of temperature increase that people are talking about and getting confused. There's adiabatic heating of the air upon re-entry that is caused by compression of the air at the leading edge of the body. Adiabatic heating is strictly an inviscid phenomena meaning it has no attachment to irreversible processes (at least within the limits of typical modeling), a.k.a. there's no friction involved. At really high temperatures due to compression this probably becomes more complicated due to the ionization of air and loss of heat due to radiation of the plasma but for all intents and purposes, no friction here. Effects here are very well represented by the Euler equations of fluid motion for a compressible gas, you'll get shockwaves and the full she-bang. Getting into the frictional effects, there's the conversion of dilatational work of the air into heat via the bulk compression of the fluid and the conversion of shear in the air to heat via your typical viscous behavior in fluids. I think what this person is trying to say is that the latter shear driven frictional effects on the temperature are small compared to the adiabatic temperature increase and the dilatational work which I'd imagine is completely true when your pushing re-entry Mach No. ~20. Thank you for coming to my TED talk.
You are welcome. Good stuff.
Posted on 7/26/22 at 6:46 pm to Tridentds
quote:
Not sure I understand how they are strong enough to compress the atmosphere in front of them.
The math tells me it’s due to very low starting pressure and temperature. Let’s say the temp is -100 degrees F. That is equal to 360 degrees R. The pressure is also very low. Lets assume 1 psi. So just using the ideal gas law as a very rough rule of thumb:
1/360=p2/21500 R. P2 is equal to approximately 60 psi. (21500 R is the approximate temp to ionize air).
The point is in the upper atmosphere you do not need a lot of compression to achieve a lot of temperature. It’s assumed, and I think this is a valid assumption, the film between the air and surface is adiabatic.
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