The flow through orifices and slits is of great technical interest. The gas can flow either from one gas reservoir into another or into vacuum. The gas reservoir is serparated from the second reservoir or the vacuum by an orifice. The mass flow rate per unit area is the quantity of interest.
The image below shows two gas reservoirs separated by a wall. The lower edge of the image is the axis of radial symetry.
PI-DSMC was used to study various variants of this geometry. The results of this work were presented at the 64th IUVSTA workshop.
The presentation given at the workshop can be downloaded here.