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.

https://iopscience.iop.org/article/10.1088/0957-0233/12/5/201/pdf

Laser Rayleigh scattering

A compact, highly robust airborne High Spectral Resolution Lidar (HSRL) that provides measurements of aerosol backscatter and extinction coefficients and aerosol depolarization at two wavelengths has been developed, tested, and deployed on nine field experiments (over 650 flight hours). A unique and advantageous design element of the HSRL system is the ability to radiometrically calibrate the instrument internally, eliminating any reliance on vicarious calibration from atmospheric targets for which aerosol loading must be estimated. This paper discusses the design of the airborne HSRL, the internal calibration and accuracy of the instrument, data products produced, and observations and calibration data from the first two field missions: the Joint Intercontinental Chemical Transport Experiment--Phase B (INTEX-B)/Megacity Aerosol Experiment--Mexico City (MAX-Mex)/Megacities Impacts on Regional and Global Environment (MILAGRO) field mission (hereafter MILAGRO) and the Gulf of Mexico Atmospheric Composition and Climate Study/Texas Air Quality Study II (hereafter GoMACCS/TexAQS II).

Airborne High Spectral Resolution Lidar for profiling aerosol optical properties

Filtered Rayleigh scattering is an optical diagnostic technique that allows for simultaneous planar measurement of velocity, temperature, and pressure in unseeded flows. An overview of the major components of a filtered Rayleigh scattering system is presented. In particular, a detailed theoretical model is developed and discussed with associated model parameters and related uncertainties. Based on this model, results for two experimental conditions are presented: ambient room air and a Mach 2 freejet. These results include two-dimensional, spatially resolved measurements of velocity, temperature, and pressure derived from time-averaged spectra. ILTERED Rayleigh scattering (FRS), a recently developed flow diagnostic technique,1'2 achieves large suppression of background scattering allowing planar flowfield visualization and obtains quantitative measurements of velocity, temperature, and density in unseeded gaseous flows. This technique makes use of Rayleigh scattering from molecules in the flow and is driven by a high-power, narrow linewidth, tunable, injection seeded laser. When imaging the scattered light onto a charge-coupled device (CCD) camera, unwanted background scattering from stationary objects may be filtered out by tuning the frequency of the narrow linewidth laser to coincide with an atomic or molecular absorption line and by placing a cell containing the atomic or molecular species between the camera and the flow. This cell acts as a notch filter, absorbing all background scatter at the laser frequency. Scattered light that is Doppler shifted, however, passes through the filter and is imaged on the camera. Quantitative measure of flow properties is achieved by measuring the total intensity, Doppler shift, and spectral profile of the Rayleigh scattered light. The total intensity is directly proportional to density; the Doppler shift is directly proportional to velocity; and the spectral profile is a function of temperature and pressure. The scattering intensity, Doppler shift, and spectral profile are determined by passing the scattered light through the notch absorption filter and then by imaging it onto an intensified CCD camera. Because the filter absorbs light in a narrow frequency band, it converts the spectral information contained in the Doppler shift and Rayleigh profile into intensity information at the camera. By collecting data (camera pixel intensity) for varying conditions, v, T, and P may be determined. Previous work has concentrated on the use of this technique for background suppression when visualizing flows and for the measurement of velocity. The background suppression feature of FRS has been used to image flowfields that otherwise would be completely obscured by the strong scattering from wind-tunnel surfaces. The authors have used this technique to image the flowfield inside a Mach 3 inlet and to generate volumetric images of the crossing shocks and boundary layer.3 Elliott et al.4 have also used this technique to observe structures in compressible mixing layers. The use of FRS to measure velocity was initially demonstrated using scattering

Demonstration and characterization of filtered Rayleigh scattering for planar velocity measurements

An airborne high spectral resolution lidar (HSRL) based on an iodine absorption filter and a high-power frequency-doubled Nd:YAG laser has been developed to measure backscatter and extinction coefficients of aerosols and clouds. The instrument was operated aboard the Falcon 20 research aircraft of the German Aerospace Center (DLR) during the Saharan Mineral Dust Experiment in May-June 2006 to measure optical properties of Saharan dust. A detailed description of the lidar system, the analysis of its data products, and measurements of backscatter and extinction coefficients of Saharan dust are presented. The system errors are discussed and airborne HSRL results are compared to ground-based Raman lidar and sunphotometer measurements.

/pdf/airborne-high-spectral-resolution-lidar-for-measuring-13a01euu24.pdf

Airborne high spectral resolution lidar for measuring aerosol extinction and backscatter coefficients

High Spectral Resolution Lidar

We analyze in detail, in the kinetic regime, the behavior of a model kinetic equation previously used by us for the description of the density fluctuation spectrum in molecular gases. We find that ...

On the Kinetic Model Description of Rayleigh–Brillouin Scattering from Molecular Gases

Within the framework of the Wang Chang–Uhlenbeck kinetic equation, we propose a model description of molecular gases. The model is related to the models earlier discussed by Hanson and Morse. In ou...

Kinetic models and Brillouin scattering in a molecular gas

Using a method closely akin to the Gross–Jackson–Sirovich procedure, we present a modeling theory of the linear Waldmann–Snider collision operator. The resulting model kinetic equations are applica...

Kinetic Theory of Molecular Gases I: Models of the Linear Waldmann–Snider Collision Operator

Kinetic model equations, based on the Waldmann–Snider equation and valid over a wide range of wavelength and frequency, are applied to the study of time dependent correlation functions in a dilute molecular gas. In particular, the density autocorrelation function is calculated and its double Fourier transform compared with the polarized Rayleigh–Brillouin light scattering. Similarly, the tensor polarization autocorrelation function is studied and its spectrum compared to the depolarized Rayleigh light scattering. Thus, both scalar and tensorial phenomena are studied from a unified viewpoint. Furthermore, the asymptotic form of the theory is used to study in detail the behavior of the tensor polarization correlation function in the pressure broadening region.

G. Tenti

Papers

On the Kinetic Model Description of Rayleigh–Brillouin Scattering from Molecular Gases

Kinetic models and Brillouin scattering in a molecular gas

Kinetic Theory of Molecular Gases I: Models of the Linear Waldmann–Snider Collision Operator

Kinetic Theory of Molecular Gases II: Calculation of Time Dependent Correlation Functions