Absorption and Dispersion of Ultrasonic Waves by Karl F. Herzfeld

By Karl F. Herzfeld

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In diatomic and polyatomic gases, the mechanism is the slow energy transfer between internal and external degrees of freedom. For the transfer of vibrational energy, at least, the relaxation times can be fairly well calculated in simple molecules, using only molecular constants known from nonacoustic experiments. 22 INTRODUCTION In liquids, four mechanisms are known. For many organic liquids ("Kneser liquids") the mechanism is the same as in gases. The relaxation times are several hundred times shorter than for the vapor under standard conditions.

One can see t h a t here the finite size of the vessel comes in. , increases with decreasing frequency. 5b K. F. Herzfeld and F. O. Rice, Phys. 6 Rev. 31, 691 (1928). M. Greenspan, / . A const. Soc. Am. 22, 568 (1950). 46 A. GENERAL THEORY OF RELAXATION IN FLUIDS [7] is used by aerodynamicists, but is awkward to use in physics. Instead, we define a related number here and call it the Eucken number [Eu] λΜ [Eu] = — = y [ P r ] - i . (7-20") Then + (§ - ^ ) 2 [Re] {[Re] + . · ( ! + [Eu])} = [Re]·.

Lyon, and L. Peselnick, / . Acoust. Soc. Am. 26, 566 (1954). [8] I. THE STOKES-NAVIER EQUATIONS OF HYDRODYNAMICS 47 experiments on helium and argon (see Sec. 47) seem to indicate that it is applicable though not completely so. 12 8. Formal Introduction of Volume Viscosity Except in monatomic gases and possibly monatomic liquids, the experimentally found absorption is always larger than the classical absorption «*iass> as s h o w n 1 b Y E q· (7~17)· As Tisza has first pointed out, this excess absorption can be formally accounted for by a volume viscosity; see Eqs.

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