Kinetic theory of the drag force on objects in rarefied gas flows

A kinetic theory is presented for the drag force on objects in rarefied gas flows as a function of the inverse Knudsen number. It is demonstrated that an expansion of the drag force around the free molecular limit contains terms that depend logarithmically on the inverse Knudsen number. It is shown how the coefficients of this expansion are related to various sequences of collisions among the gas molecules and the object. The analogy between the inverse Knudsen number expansion for the drag force on objects in rarefied gases and the density expansion of the transport properties of moderately dense gases is elucidated.