The Knudsen iteration is shown to be valid to predict the near free molecular corrections to drag, heat flux, etc., for non-linear rarefied gas flow problems. Β§ 1. Introduction Consider a rarefied gas dynamics problem where the charactefistic body length L is rauch smaller than the mean free path/~ of the gas. The free molecular solution, i.e., the solution when interaction between molecules of the surroundings and those re-emitted from the body are neglected, can in principle be computed. One method to obtain the perturbation to free molecular solution is the Knudsen iteration method [l~. While this method is not valid for calculafing perturbations to the distribution function itself, it has been shown E2, 3] to yield the correct next order term of the drag, heat flux, etc,, for a linearized three-dimensional problem. With proper extrapolation this method can also yield correct next order term of the moments for a linearized two-dimensional problem [4, 51.
For a non-linear problem the validity of the method of Knudsen iteration so far has not been investigated, although this method is always assumed to be correct. For example, Willis [61 has used it to demonstrate the deficiency of the BGK-equation E7]. In this paΓer we will show that the method of Knudsen iteration is also applicable for a non-linear problem.