Lowhyphen;Temperature Viscosity Cross Sections Measured in a Supersonic Argon Beam

摘要

An exact translationalhyphen;relaxation equation is derived from the Boltzmann equation for the special case in which (a) the flow is spherically symmetric source flow, (b) the velocity distribution is ellipsoidal, and (c) the atoms are hard spheres. It is shown that, at least for small temperature differences, the appropriate cross section is the thermalhyphen;conductivity (or viscosity) transport cross section. This relaxation equation is used then to calculate values of argon cross sections from velocity distributions near the centerline of a free jet measured by molecularhyphen;beam techniques. These values (a) agree with the values obtained from viscosity measurements in the temperature range in which the two sets of data overlap, (b) continue to increase as the temperature decreases to about 25deg;K, and (c) decrease as the temperature decreases to about 10deg;K. The observed decrease in values as the temperature decreases from about 25deg;K was not expected and has not been explained.