Knudsen number

The Knudsen number is a dimensionless number that helps us understand how gas particles behave relative to a physical boundary or container. It compares the average distance a particle travels between collisions (mean free path) to the size of the object or space it interacts with. A small Knudsen number indicates many collisions and fluid-like behavior, suitable for traditional fluid dynamics. A large number suggests particles mostly move freely without hitting others, leading to rarefied gas effects. This concept is essential in fields like aerospace and vacuum technology to determine whether continuum assumptions or kinetic models are appropriate.