Measurement of Temperature-Dependent Bulk Viscosities of Nitrogen, Oxygen and Air From Spontaneous Rayleigh-Brillouin Scattering

In this paper, the spontaneous Rayleigh-Brillouin scattering spectra of air are simulated to study the effect of uncertainties of pressure, temperature, scattering angle and the characteristic parameter uncertainty of the Fabry-Perot interferometer on the accurate measurement of the bulk viscosity. It is found that those uncertainties have an obvious impact on the bulk viscosity measurement deviation and the bulk viscosity can be measured accurately under higher pressures (≥3.0 bar). In order to obtain the accurate bulk viscosity of nitrogen, oxygen and air, the spontaneous Rayleigh-Brillouin scattering spectra are measured with the wavelength of 532 nm under pressure of 4.0-7.0 bar and at temperature from 289.0 K to 400.0 K. The linear relation between the measured bulk viscosity and temperature is established with R² being above 0.99 for nitrogen, oxygen and air respectively. By comparison, it is found that our measured bulk viscosities mostly agree with the reported values obtained by spontaneous Rayleigh-Brillouin scattering, coherent Rayleigh-Brillouin scattering, ultrasonic determination or theoretical calculation for nitrogen, oxygen and air within 3σ results at the same temperature. The factors arousing the differences between them are attributed to the obvious measurement error and the measured uncertainty of the bulk viscosity under low pressures and the defects in the theoretical model itself. The empirical formula for calculating the bulk viscosity for air from pure components is proposed and it can match our measured results well.