Laser Rayleigh scattering

Abstract

Rayleigh scattering is a powerful diagnostic tool for the study of gases and is particularly useful for aiding in the understanding of complex flow fields and combustion phenomena. Although the mechanism associated with the scattering, induced electric dipole radiation, is conceptually straightforward, the features of the scattering are complex because of the anisotropy of molecules, collective scattering from many molecules and inelastic scattering associated with rotational and vibrational transitions. These effects cause the scattered signal to be depolarized and to have spectral features that reflect the pressure, temperature and internal energy states of the gas. The very small scattering cross section makes molecular Rayleigh scattering particularly susceptible to background interference. Scattering from very small particles also falls into the Rayleigh range and may dominate the scattering from molecules if the particle density is high. This particle scattering can be used to enhance flow visualization and velocity measurements, or it may be removed by spectral filtering. New approaches to spectral filtering are now being applied to both Rayleigh molecular scattering and Rayleigh particle scattering to extract quantitative information about complex gas flow fields. This paper outlines the classical properties of Rayleigh scattering and reviews some of the new advances in flow field imaging that have been achieved using the new filter approaches.

Coherent Rayleigh–Brillouin scattering measurements of bulk viscosity of polar and nonpolar gases, and kinetic theory

“…Asymmetric molecules typically also have significant dipole moments, the effects of which are described by the ED model. The RS model only describes an aspect of molecules consisting of larger numbers of atoms, such as CH 4 or H 2 S, but because of TABLE II. The four kinetic models considered, the property of gas molecules which they represent, their parameters, which of the gases under investigation in this work they can be applied to, and references which contain the expressions used for the bulk viscosity and the other transport coefficients.…”

“…Line shapes in spontaneous Rayleigh-Brillouin scattering ͑SRBS͒ have been predicted most succesfully [2][3][4] by the theoretical model developed by Boley et al 5 and Tenti et al 6 In this model, information regarding the Boltzmann collision integral is obtained from transport coefficients. Pan et al successfully extended the model of Boley et al 5 to predict coherent Rayleigh-Brillouin scattering ͑CRBS͒ and reported good agreement with experimental data on several monatomic and molecular gases.…”

New diagnostic methods for laser plasma- and microwave-enhanced combustion

“…sin(θ/2) , (2.1) where θ is the scattering angle defined relative to the laser propagation direction, λ is the laser wavelength in nanometres, T is the temperature in kelvin and P is the pressure in atmospheres [6]. The broadening of the Rayleigh line can be described using either sixth moment (Tenti, Pan) or seventh moment (Pan) models of the density fluctuation spectrum, derived from approximate solutions of the time-dependent Boltzmann equation [7,8].…”

“…For standard air (298 K, 1 atm) at 532 nm, F = 1.049 [6], so the depolarization ratio is 0.014. The depolarization ratio for pure oxygen is 0.027, and for nitrogen it is 0. .…”

Imaging Flow Velocimetry with Laser Mie Scattering

“…Optical flow velocity measurements based on Rayleigh scattering are described for instance in [11][12][13][14][15][16][17]. An overview article exists from Miles, 2001 [18]. The flow velocity measurements are based on the optical Doppler effect, which causes a shift in the observed scattered light frequency due to the scattering at the moving fluid particles.…”