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Burnt up

  1. Oct 15, 2012 #1
    Objects burn up entering earth's atmosphere. If we were to float high enough wouldn't the same thing happen? Or does` the rate of descent play a role?
     
  2. jcsd
  3. Oct 15, 2012 #2

    haruspex

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    They burn up because they are travelling so fast when they hit the atmosphere.
     
  4. Oct 15, 2012 #3
    The heat generated due to air friction is highly dependent on the speed of descent, and the terminal velocity of a falling human is considerably less than that of an asteroid or satellite.
     
  5. Oct 15, 2012 #4

    Drakkith

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    The heat a spacecraft is exposed to during re-entry is solely a result of its velocity, not its altitude. One thing you may not know is that the vast majority of fuel that a launch vehicle expends is not to get the spacecraft up to the proper altitude but to get it to a high enough velocity to stay in orbit. The ISS orbits at about 17,000 mph at an altitude of about 250 miles.

    As for non-manmade objects such as meteors and such, that is solely due to the velocity of the Earth in it's orbit, as we orbit the sun at around 66,600 mph. Objects that cross the Earth's orbit are not moving entirely in the same direction as we are even if they are moving as fast or faster than we are. The Earth actually moves into them, not the other way around. It is extremely rare for objects to hit us that are moving against our orbital motion, as the vast majority of objects are orbiting the same direction around the Sun.

    Also, the heating produced during entry into the atmosphere is NOT because of friction. It is because of compression and forced convection. When an object enters the atmosphere it is moving so fast that air simply doesn't have time to move out of the way. The objects slams into the air molecules and compresses them, heating them up. A stagnant layer of air forms, preventing airflow over the object, so friction can't even happen as there is no movement of air at the surface of the object. This is different from friction as friction is the result of objects or layers sliding against each other.

    http://en.wikipedia.org/wiki/Aerodynamic_heating
    http://en.wikipedia.org/wiki/Forced_convection
     
    Last edited: Oct 15, 2012
  6. Oct 16, 2012 #5
    That needs heavy qualification. Air density is very much a function of altitude and aerodynamic heating is very much a function of fluid density. Meteorites do not burn up in outer space after all.
    This is not true. It is fluid friction owing to 'slippage' between successive layers of air that primarily causes heating for a re-enty vehicle. That the very innermost boundary layer is stagnant is beside the point - source of heat is overwhelmingly fluid friction. Heat then conducts/convects/radiates between successive layers. There is an initial compressive heating component, but will be a fleeting effect compared to frictional contribution. Reverse may be the case for say head-on collision with a truly massive killer-asteroid - not sure. Forced convection normally refers to things like fan-assist, but here may be referring to turbulent mixing in boundary layers[but probably just the high speed flow that is occurring between layers]. That does no imply any lack of frictional influence.
    See above comments - those linked articled do not support your contention.
     
    Last edited: Oct 16, 2012
  7. Oct 16, 2012 #6

    Drakkith

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    I guess I stand corrected. :wink:
     
  8. Oct 16, 2012 #7
    Don't we all at times! :smile:
     
  9. Oct 16, 2012 #8

    Drakkith

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    I blame everyone but myself! Where's Phinds at!?
     
  10. Oct 16, 2012 #9

    phinds

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    NOW what have I done?

    WAIT ... this one is on YOU. You should not only stand corrected, you should do it on one leg while whistling Dixie and drinking some kind of fluid as a reminder of your mistake !
     
  11. Oct 16, 2012 #10

    Drakkith

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    Buahahaha!
     
  12. Oct 16, 2012 #11
    If you float high enough, there is little to no air, so no friction and no associated frictional heating. That's why astronauts can go outside and repair space stations and such and no burn up.
     
  13. Oct 16, 2012 #12
    When we talk about "frictional heating", what we are really talking about is viscous heat generation associated with the very high deformation rate of the air within the boundary layer.
     
  14. Oct 16, 2012 #13

    Drakkith

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    I assume that by "float high enough" the OP literally means floating up that high, like in a balloon. In such a case you won't experience heating because you are not moving through the air at a high velocity.
     
  15. Oct 16, 2012 #14

    haruspex

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    I've always assumed it was that the temperature of a gas is determined by the speed of the molecules, so it would feel much hotter to a body travelling very rapidly through it. Or does it come to the same thing?
     
  16. Oct 16, 2012 #15
    It's not the same thing. The viscous mechanism involves the dissipation of mechanical energy to heat. The dissipation rate goes as the square of the velocity gradient (not velocity), and is also proportional to the viscosity (which represents the resistance of the fluid to deformation).
     
  17. Oct 17, 2012 #16

    haruspex

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    The mechanism I described is also dissipation of kinetic energy. When exposed to a hot gas, a surface warms and slows down the gas molecules. This is what it looks like at the molecular level from the reference frame of the object.
     
  18. Oct 17, 2012 #17
    From the frame of reference of an object passing through the stratosphere, the gas approaching it is not hot (at least not hot like the surface of the object gets). It is not the relative kinetic energy that is important, but the layers of air shearing over each other, caused by the no-slip boundary condition at the surface. The velocity of the air relative to the object is zero at its surface. The shear rate at the wall generates a shear stress at the wall, which acts to slow down the object and does work on the air in the boundary layer. This causes the boundary layer to heat up. The rate of viscous heating is proportional to the viscosity times the square of the shear rate.
     
  19. Oct 17, 2012 #18
    Viscous heating occurs even in systems where kinetic energy is not a factor. An example is viscous heating in fluid flow through a pipe, where the flow velocity is a constant. This is a particularly big factor in processing of polymer melts where the viscosity is much higher than that of air (although the wall shear rates are much lower than at the surface of a re-entry vehicle). Viscous heating will occur in pipe flow even for gases, although the effect is small because of the low viscosity. See any book on fluid mechanics and transport phenomena, such as Transport Phenomena by Bird, Stewart, and Lightfoot, which gives ample treatment to viscous heating in various situations.
     
  20. Oct 17, 2012 #19

    cjl

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    That depends on the shape of the reentry vehicle. For sharp pointed objects, your description is correct. For blunt objects however (and most things entering the atmosphere are fairly blunt), the heating occurs due to a combination of compression and viscous dissipation in the shock, some distance in front of the object itself. This heating is sufficient (if the object is reentering from orbit) to dissociate the gas, and thus the object gets a plasma cloud in front of it. The velocity gradient next to the object is small, since most of the gas velocity is bled off in the shock, but the extremely high pressure and temperature still cause substantial heating.
     
  21. Oct 17, 2012 #20
    Thanks. This sounds very reasonable. I was also going to mention compressional heating in my response, but my focus was on ruling out kinetic energy as the primary mechanism (and I wanted to keep it simple).
     
  22. Oct 17, 2012 #21

    haruspex

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    There's a good discussion at http://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.4.1.7%20Returning%20from%20Space.pdf [Broken]. See re-entry trajectory and heating. It would appear that the KE explanation operates up to about 15000 m/s. They word it differently, but it sounds essentially the same to me:
    As the shock wave slams into the air molecules in front of the re-entering vehicle, they go from a cool, dormant state to an excited state, acquiring heat energy.
    That expresses it from the frame of reference of the air. From the object's perspective, the molecules are just moving faster, i.e. they're hotter.
    Above 15000 m/s, the excitation is so great that radiation is given as the dominant cause.
     
    Last edited by a moderator: May 6, 2017
  23. Oct 17, 2012 #22

    cjl

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    Although that explanation gets some of the basics right (since the KE of the air is turned into a combination of pressure and heat), the problem is that the temperature is based on the KE of the molecules after you remove the bulk motion. By definition, temperature already has the bulk motion removed. As a result, it's incorrect to say that the molecules have a high temperature because of their velocity. They have a high energy, sure, but the pedantic side of me really prefers calling it a high energy flow rather than a high temperature flow. Also, as I said above, the molecules slow down in the shock wave, not on impact with the surface, so the surface never sees the high bulk velocity of the fluid. At 7-11 km/s (typical reentry speeds for manned vehicles), the heating is a mix of radiation from the glowing plasma, and conduction from contact with the glowing plasma, and conduction is more significant. Above 15km/s, as your link states, the radiation from the glowing plasma is more significant than the conduction - but the plasma is still created because of the viscous dissipation and compression in the shock.
     
    Last edited by a moderator: May 6, 2017
  24. Oct 18, 2012 #23

    haruspex

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    You can call it what you like, but it all comes down to KE of molecules. The object experiences high speed collisions of air molecules, and whether that's down to random or bulk motion is somewhat irrelevant to the object. My point is not that the bulk motion is a higher temperature but that at the molecular level it has exactly the same consequence.
    True, though one could argue that the shock wave is merely acting as a surface layer, partly insulating the object, similarly to the stationary case.
     
  25. Oct 18, 2012 #24
    @Drakkith: I have to take back in part my criticism in #5 of your #4 - you should have stood your ground on shock heating bit (but not in claiming friction is absent)!:redface: Some further study and I agree with #19 - certainly at Hypersonic (Mach 5+) speeds, compression dominates over friction. A case of having imbibed so much repetition of the frictional heating 'myth' it became indisputable 'fact'. Only comfort is there is plenty of company:
    http://tinyurl.com/9nghtvo - official publication!
    http://community.discovery.com/eve/forums/a/tpc/f/7501919888/m/4241946599/p/2 [Broken]
    http://indianapublicmedia.org/amomentofscience/reentry-heat/
    http://wiki.answers.com/Q/Why_do_spaceships_get_hot_when_they_return_to_earth - 'best' answer.

    More accurate picture:
    http://www.newton.dep.anl.gov/askasci/eng99/eng99460.htm
    http://www.daviddarling.info/encyclopedia/R/reentry_thermal_protection.html
    http://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system#Reentry_heating

    hypersonic flow is complex and it seems still not fully understood as this article on experience with X-15 points out: http://history.nasa.gov/SP-60/ch-5.html
    And here: http://en.citizendium.org/wiki/Atmospheric_reentry
     
    Last edited by a moderator: May 6, 2017
  26. Oct 18, 2012 #25

    Drakkith

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    No worries. It seems like a fairly difficult concept. I'm just a guy who can read wikipedia.
     
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