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Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Pinky's Brain wrote ...
I tried, but it didn't come across ... lets assume the insulation is 100% effective and the temperature of the surfaces are only dependent on the balance between convective heat transfer from the air to the tiles and the radiative heat loss. The air heated at the bow flows past the surfaces of the vehicle, losing some heat but conceivably can stay significantly hotter than those surfaces. Then when it gets turbulent it can transfer more heat (turbulence breaks up the boundary layers which act as insulation) thus shifting the energy balance and making the underlying surfaces hotter even if no new heating of the air occurs due to friction.
Yup, that's exactly my understanding. Also I read once that NASA found in practice that the place (both temporally and physically) where the turbulent flow developed depends on the smoothness of the tiles. During reentry I believe it starts off with purely laminar flow, and then during the descent the turbulence forms as the density of the air increases (i.e. the Reynolds number goes up). On some flights the turbulence developed at significantly higher speeds, and in a location that was further forwards, and the total heat transfer jumped up markedly, it was probably because they had some sticky out spacer strips between the tiles and perhaps other more subtle differences.
TLDR summary: it's all horrible heat flow stuff; and the Shuttle is a particularly complex shape which doesn't help
The compression is where most of the significant heat rise is coming from, with minor contribuitions from adhesison, viscous forces, mice flatus, waffles, friction and peanut butter. Thus, (as per BigBad and AndrewM's points) we can presume this process to be almost perfectly isentropic.
This might be true but it is not at all obvious. I'd expect the pressure in the shockwave to be roughly linearly dependent on the velocity of the air hitting it by the argument, that you need some force to keep the shockwave compressed. Adiabatic compression rules would imply a slower increase of temperature, e.g. p^2/7 for a diatomic gas. The energy of the air hitting the shockwave scales as v^2, v being the velocity of the air. So how can the temperature increase due to compression account for all the kinetic energy of the air?
Edit: I forgot the air mass per time factor in above relations, i.e. Shockwave pressure ~ mass/time * v ~ v^2 leading to a temperature rise proportional v^4/7 in the adiabatic limit Kinetic energy of air ~ mass/time * v^2 ~ v^3. To get an estimate on how hot the air would get, the speed of heat removal due to airflow needs to be taken into account, which would perhaps be T * mass/time, which would make the temperature proportional to v^2. I've seen the wiki article about Atmospheric entry, which states that the heat comes from isoentropic compression, but I don't understand, why that is so. Possibly the kinetic energy of the gas ic consumed mostly by molecular dissociation, so that the pressure in the shockwave is much lower than suggested by an ideal gas law.
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burn up on reentry
