Rarefied Gas Dynamics

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Rarefied Gas Dynamics

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The study of the phenomena related to the molecular or noncontinuum nature of gas flow at densities where

λ / l > 0.01

where λ is molecular mean free path and l is a characteristic dimension of the flow field.
Flow with λ / l > 0.01 is called molecular flow.
Flow with λ / l < 0.01 is called continuum flow.
Flow with λ / l of approximately 0.01 to 0.1 is called slip flow.
Flow with λ / l of approximately 0.1 to 10 is called transition flow.
Flow with λ / l > 10 is called free molecule flow.
Slip flow and transition flow are not always distinguished from each other. The value 1 is sometimes used instead of 10 as the boundary value for transition flow and free molecule flow. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use