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maggiomail
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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?
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.Drakkith said:The heat a spacecraft is exposed to during re-entry is solely a result of its velocity, not its altitude.
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.Also, the heating produced during entry into the atmosphere is NOT because of friction. It is because of compression and forced convection.
See above comments - those linked articled do not support your contention.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
Don't we all at times!Drakkith said:I guess I stand corrected.
Q-reeus said:Don't we all at times!
Drakkith said:I blame everyone but myself! Where's Phinds at!?
phinds said: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 !
If we were to float high enough wouldn't the same thing happen?
Naty1 said: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.
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 traveling very rapidly through it. Or does it come to the same thing?Chestermiller said: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.
haruspex said: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 traveling very rapidly through it. Or does it come to the same thing?
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.Chestermiller said: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).
haruspex said: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.
Chestermiller said: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.
cjl said: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.
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 . 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:Chestermiller said: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).
haruspex said: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 . 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.
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.cjl said: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.
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.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.
The rate of descent refers to the speed at which an object is falling towards the Earth's surface. It is typically measured in meters per second or kilometers per hour.
The rate of descent can have a significant impact on the heating and burning up of objects in Earth's atmosphere. As an object falls towards the Earth, it encounters increasing levels of atmospheric pressure and friction, which can cause it to heat up and potentially burn up if it is traveling at a high enough rate of descent.
The rate of descent can be influenced by a variety of factors, including the object's mass, shape, and surface area, as well as the density and composition of the Earth's atmosphere at the object's altitude.
In most cases, the rate of descent of an object falling towards Earth cannot be controlled. However, in some cases, objects such as spacecraft or parachutes are designed to have a specific rate of descent in order to safely land on the Earth's surface.
Objects are more likely to burn up at higher rates of descent, as the increased atmospheric pressure and friction cause more heat to be generated. However, the exact rate of descent at which an object will burn up depends on a variety of factors, including the object's size, composition, and the density of the Earth's atmosphere at its altitude.