Keith A. Gillis
Michael R. Moldover
NIST, Thermophys. Div., Gaithersburg, MD 20899
James B. Mehl
Univ. of Delaware, Newark, DE 19716-2570
Acoustic resonators consisting of a duct coupling two cavities (Greenspan viscometers) have a low-frequency mode whose response is a sensitive function of the viscous diffusivity of the enclosed gas. The response function is determined experimentally by measuring the acoustic pressure in one cavity as a function of the frequency of a source in the other cavity. A theoretical response function, based on a model which includes viscous and thermal boundary layer phenomena in both the duct and the cavities, as well as inertial and resistive duct-end corrections, is fit to the data to determine the viscous diffusivity. Recent measurements in argon, helium, and propane near room temperature span a range of viscous diffusivity from 7x10[sup -7] to 2x10[sup -4] m/s[sup 2]. Experimental values determined with three different viscometers differ from reference values by average values of 2.7% (viscometer 0), 0.8% (viscometer 1), and 1.8% (viscometer 2). [Work supported by the Office of Naval Research.]