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The opening post is about the pressure of "atmosphere" outside the ISS.
This became an issue for configuration of a CFD (computational fluid dynamics) model for an engine intended to operate in space.
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Post #3: "pressure" of atmosphere outside ISS (Knudsen's number)
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From conversation with Google's Gemini:
Thank you for explaining the comparison, and I agree that 20 Pascal was an inappropriate value with which to launch a CFD model that is supposed to operate in space. We got surprising results in our first run, and the guess of 20 is clearly inappropriate.
***That makes perfect sense. The discrepancy between a pressure of 20 Pa and the actual pressure of 10−8 Pa in space is so vast that it would fundamentally change the physics of the fluid model, leading to those surprising and inaccurate results you saw.
For these kinds of ultra-low-pressure simulations, you'll often encounter a concept called the Knudsen number (Kn). It's the ratio of the molecular mean free path to a characteristic length scale of the problem (like the size of your engine). When the Knudsen number is high, as it would be in space, the gas flow behaves very differently from what we typically see on Earth; it's considered free molecular flow, where molecules rarely collide with each other and instead primarily interact with the surfaces of the engine. Standard CFD models often assume a continuous fluid, which doesn't hold true in this regime.
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