Kinetic Theory of Drag on Objects in Nearly Free Molecular Flow

TL;DR

This paper develops a kinetic theory framework to calculate drag forces on objects in nearly free molecular flow, extending the free-molecular limit with correction terms based on density expansions.

Contribution

It introduces a novel method to compute first-order corrections to free-molecular drag using collision integrals and density expansion analogies.

Findings

Derived collision integrals for drag correction terms.

Calculated drag coefficients for a sphere and disc as functions of speed ratio.

Established a theoretical link between gas density and drag in nearly free molecular flow.

Abstract

Using an analogy between the density expansion of the transport coefficients of moderately dense gases and the inverse-Knudsen-number expansion of the drag on objects in nearly free molecular flows, we formulate the collision integrals that determine the first correction term to the free-molecular drag limit. We then show how the procedure can be applied to calculate the drag coefficients of an oriented disc and a sphere as a function of the speed ratio.