Heat on heat shield. Friction or compression?

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Discussion Overview

The discussion revolves around the mechanisms of heat generation on a reentry vehicle's heat shield during atmospheric reentry, specifically examining the roles of air compression and friction in this process.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that the heat generated during reentry is primarily due to the compression of air in front of the vehicle, comparing it to the heat generated when filling tires.
  • Another participant introduces the concept of atmospheric heating, explaining that as fluid flow slows down, its kinetic energy is converted to heat, leading to an increase in temperature.
  • A third participant agrees that both compression and viscous dissipation (referred to as "friction") contribute to heat generation, noting that while compression is the primary source, friction plays a significant role in transferring heat to the surface.
  • A later reply expresses appreciation for the discussion and mentions finding useful information on "viscous dissipation."

Areas of Agreement / Disagreement

Participants acknowledge that both compression and friction contribute to heat generation, but there is no consensus on the relative contributions of each mechanism, leading to a contested understanding of the topic.

Contextual Notes

Some assumptions about the definitions of heat generation mechanisms and the specific contributions of compression versus friction remain unresolved.

thenewmans
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I always thought of a reentry vehicle compressing the air in front of it. And that would cause a lot of heat kind of like filling my tires. But I keep hearing friction even from NASA heat shield experts. OK, I guess maybe 10% of the heat is friction. I always assumed they said that to make the concept easier. But I’ve heard it so much recently that I’m starting to think I’m wrong. Or maybe they’re the same thing in a way that I hadn’t understood yet. Can you straiten me out?
 
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The jargon is atmospheric heating, and its explanation is
When fluid flow slows down its kinetic energyis converted to heat; in high speed flows, tremendous energy is represented by the mean motion of the flow. As the flow is slowed to near zero speed, its temperature increases, the gradient in the speed in a direction normal to the surface allows small scale mass transport effects to dissipate the temperature in the outward direction and thus the temperature at the surface is less than the stagnation temperature; the actual temperature is referred to as the recovery temperature. [ ... ]
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Both are contributors. The heat is mostly generated through the compression but there is some that comes from viscous dissipation as well (or what you might call "friction"). The "friction" plays a great role in transferring that heat to the surface, however.
 
That's what I love about PF. Thanks guys. I did found some good stuff on "viscous dissipation"
 

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