Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information 14. ABSTRACT Rarefied helium and nitrogen flow expanding into vacuum through 150µm high and 1.5 cm long channels is studied experimentally and numerically with the DSMC method. Different types of channel walls are examined, both polished and rough with well characterized roughness shaped as triangles and rectangles. The pressure varies from 200 to 13,000 Pa, with the gas mean free path being both much larger and much smaller than the roughness size of about 20µm. A conical surface roughness model applicable for the DSMC method is proposed. An expression relating this model to the Cercignani-Lampis scattering model is derived. Good agreement between the numerical and experimental results is observed for the rough walled channel. Abstract. Rarefied helium and nitrogen flow expanding into vacuum through 150µm high and 1.5 cm long channels is studied experimentally and numerically with the DSMC method. Different types of channel walls are examined, both polished and rough with well characterized roughness shaped as triangles and rectangles. The pressure varies from 200 to 13,000 Pa, with the gas mean free path being both much larger and much smaller than the roughness size of about 20µm. A conical surface roughness model applicable for the DSMC method is proposed. An expression relating this model to the Cercignani-Lampis scattering model is derived. Good agreement between the numerical and experimental results is observed for the rough walled channel.