Air Friction on Rapidly Moving Surfaces

The friction on a surface moving through air at high speed has been measured over a pressure range from 0.5 to 200 microns. A rotating cylinder type apparatus was used in which the inner cylinder was rotated at very high speed by means of an air turbine arrangement similar to that used in an air driven vacuum type ultracentrifuge. The outer cylinder completely surrounded the inner cylinder and was supported coaxially with it by means of a torsion fiber so that the frictional torque on the rotating cylinder could be measured with precision. Observations were made at rotor speeds varying from 400 r.p.s. to 1700 r.p.s. which corresponds to a linear velocity at the periphery of the rotor varying from 400 to 1700 ft./sec. It was found that at all pressures the observed torque varied in an essentially linear fashion with the rotor speed. The relationship between torque and pressure at constant speed yielded a smooth curve indicative of ordinary viscosity, and this general form held for all speeds tested. However, the results could be explained by kinetic theory treatment of viscosity only at pressures lower than 10 microns; i.e., below the point where the intermolecular collision frequency predominates over that of the collisions between the molecules and the walls.