Investigation of Gaseous Acoustic Damping Rates by Transient Grating Spectroscopy

Abstract

An investigation of acoustic damping ratesin a pressurized gaseous medium by analyzing thetemporal behavior of laser-induced gratings is reported. Experiments were performed in various nonresonant gas samples as a function of pressure and grating spacing. Acoustic damping rates were determined through model e ts to the acquired signals. The results were compared with theoretical calculations using both classical acoustic damping rates and a more comprehensive model that includes rotational and vibrational energy transfer mechanisms. The relationships between the measured acoustic damping rate and molecular structure and pressure and grating spacing are discussed. The utility of exploiting the temporal signature from laser-induced gratings to determine acoustic damping rates in high-pressure gases is identie ed. Nomenclature cp = heat capacity at constant pressure crot = heat capacity of rotational modes cs = local speed of sound cv = heat capacity at constant volume cvib = heat capacity of vibrational modes Dth = thermal diffusivity f = acoustic frequency G = geometric factor I = signal intensity M = molecular weight P = pressure q = grating wave vector, 2 o=3 R = gas constant T = e uid temperature t = time Z = collision number ¯ = damping factor due to energy transfer 0 = acoustic damping coefe cient ° = specie c heat ratio 1n = local modulation in index of refraction ´ = scattering efe ciency of laser-induced grating µ = full angle between beams · = thermal conductivity 3 = grating spacing ¸ = laser wavelength π = dynamic viscosity Ω = e uid density ?rot = rotational relaxation time ?vib = vibrational relaxation time