Molecular-based optical diagnostics techniques capable of obtaining simultaneous measurements of multiple fluid properties are critically important for characterizing hypersonic air-breathing engines, such as scramjet engines and scramjet-rocket combined cycle engines. Correlations between those properties lead to a more detailed understanding of complex flow behavior, and aid in the development of multiparameter turbulence models required for supersonic combustion engine flow path predictions.

A novel approach to analyze Rayleigh scattered light from a measurement volume in a gas flow using planar Fabry-Perot interferometers is described. The signal is analyzed both with high and low spectral resolution to determine the gas velocity, density, and temperature. Additional optical components recover and increase the signal used for spectral analysis by more than one order of magnitude compared with normal configurations.

An effective means of referencing the measurements of density and temperature is embedded into the system for simultaneous measurement of these parameters at multiple points together with velocity. A dual-cavity interferometric spectral analysis design significantly improves the measurement of velocity and increases the dynamic range and the accuracy of measuring velocity.

This invention refers to a method and apparatus for simultaneous measurement of velocity, density, temperature, and their spatial and temporal derivatives in gas flow using elastic light scattering from molecules. In particular, the invention refers to an optical system using a narrow-band dual laser for probing, planar interferometers for spectral analysis, and critical software components to extract spectra, to measure, and to compute multiple parameters in a variety of gas media including supersonic/hypersonic combustion or non-reacting flows, steady or unsteady, and methods of efficient utilization of the collected radiation in interferometric Rayleigh scattering applications. The measurements are spatially and temporally resolved at scales of hundreds of microns or less and tens of nanoseconds or less, respectively.

The technique disclosed is a substantial advancement of prior art of performing simultaneous and instantaneous measurements at multiple points of gas velocity, density, and temperature in unseeded flows. It is novel in that it uses a pulsed dual laser for probing; a dual-cavity interferometric spectral analysis design; uses known property Rayleigh spectra for reference; uses an advanced signal recirculating system; and implements a novel software routine for image processing. This novel use of a pulsed dual laser for probing enables the measurement of all properties and computation of their time derivatives (or time correlations) that is extremely important for the understanding of turbulent combustion physics.

The use of reference Rayleigh spectra (from a reference gas at standard pressure and temperature), combined with the signal, dramatically increases the accuracy and reliability of measuring the gas density and temperature. The use of novel signal recirculation enables the maximization of the signals, greatly improving the signal-to-noise ratio during measurements, resulting in better accuracy without the requirement of increasing the probing laser energy.

This work was done by Paul M. Danehy of Langley Research Center, with Andrew D. Cutler and Daniel Bivolaru of the George Washington University. LAR-17779-1

NASA Tech Briefs Magazine

This article first appeared in the April, 2015 issue of NASA Tech Briefs Magazine.

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