Radiation-Driven Acoustic Waves in a Confined Gas

Abstract

Preliminary to a projected experimental program, the linearized theory of radiative gas dynamics is applied to the problem of the standing acoustic waves produced inside a closed cylindrical tube by a sinusoidal input of radiation at one end of the tube. The gas in the tube is taken to be perfect, optically grey, and in local thermodynamic equilibrium; the radiative transfer is treated on the basis of the differential approximation. The results of dimensionless, parametric calculations are analyzed in terms of the two types of simultaneous harmonic acoustic waves present in the theory: (1) the modified classical wave and (2) the radiation-induced wave. For temperatures and pressures attainable in the laboratory, the results show a marked resonance effect from tuning of the tube length with respect to the wavelength of the modified classical wave. The resonance condition exhibits equal levels of pressure and velocity response with marked spatial variation in both quantities. In certain situations the off-resonance condition is dominated by the radiation-induced wave. Here the pressure response is spatially uniform and several orders of magnitude larger than the nonuniform velocity response.