{"history":[],"article":{"id":"7ea5a0b1-8a6d-4a37-b7b2-123c710c8f80","title":"Free molecular flow","slug":"free-molecular-flow","content":":::summary\n:::\n\n# \n**Free molecular flow** describes the [fluid dynamics](https://bliptext.com/articles/fluid-dynamics) of gas where the [mean free path](https://bliptext.com/articles/mean-free-path) of the molecules is larger than the size of the chamber or of the object under test.\nFor tubes/objects of the size of several cm, this means pressures well below 10−3 [mbar](https://bliptext.com/articles/mbar).\nThis is also called the regime of high [vacuum](https://bliptext.com/articles/vacuum), or even ultra-high [vacuum](https://bliptext.com/articles/vacuum).\nThis is opposed to [viscous flow](https://bliptext.com/articles/viscous-flow) encountered at higher pressures.\nThe presence of free molecular flow can be calculated, at least in estimation, with the [Knudsen number](https://bliptext.com/articles/knudsen-number) (Kn).\nIf Kn > 10, the system is in free molecular flow, also known as Knudsen flow.\nKnudsen flow has been defined as the transitional range between viscous flow and molecular flow, which is significant in the medium vacuum range where λ ≈ d.\nGas flow can be grouped in four regimes: For Kn≤0.001, flow is continuous, and the [Navier–Stokes equations](https://bliptext.com/articles/navier-stokes-equations) are applicable, from 0.001 10, the system is in free molecular flow, also known as Knudsen flow.\nKnudsen flow has been defined as the transitional range between viscous flow and molecular flow, which is significant in the medium vacuum range where λ ≈ d.\nGas flow can be grouped in four regimes: For Kn≤0.001, flow is continuous, and the [Navier–Stokes equations](https://bliptext.com/articles/navier-stokes-equations) are applicable, from 0.001

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