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Knudsen number - Wikipedia, the free encyclopedia (188 words) |
 | The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. |
| The Knudsen number is useful for determining whether statistical mechanics or the continuum mechanics formulation of fluid dynamics should be used. |
| If the Knudsen number is near or greater than one, the mean free path of a molecule is comparable to a length scale of the problem, and the continuum assumption of fluid mechanics is no longer a good approximation. |
| Aeronautic and Aerospace Flight Limits (598 words) |
| The Mach number (introduced by J. Ackeret, 1929) defines the appearance of compressibility effects and the changes associated with the shock waves. |
| The Reynolds and Mach numbers are independent and are both needed to define the characteristics of speeds in the transonic regime. |
| Flows at Knudsen numbers Kn >> 1 are basically collisionless flows (artificial artificial satellites in orbital motion above the Earth); flows at Kn < 1 are in a regime of slight rarefaction, and are called slip flows; flows at intermediate Knudsen numbers are called transitional flows. |
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