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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).
     
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