An interferometric Rayleigh scattering system was developed to enable the measurement of multiple, orthogonal velocity components at several points within very-high-speed or high-temperature flows. The velocity of a gaseous flow can be optically measured by sending laser light into the gas flow, and then measuring the scattered light signal that is returned from matter within the flow. Scattering can arise from either gas molecules within the flow itself, known as Rayleigh scattering, or from particles within the flow, known as Mie scattering. Measuring Mie scattering is the basis of all commercial laser Doppler and particle imaging velocimetry systems, but particle seeding is problematic when measuring high-speed and high-temperature flows.
The velocimeter is designed to measure the Doppler shift from only Rayleigh scattering, and does not require, but can also measure, particles within the flow. The system combines a direct-view, largeoptic interferometric setup that calculates the Doppler shift from fringe patterns collected with a digital camera, and a subsystem to capture and re-circulate scattered light to maximize signal density. By measuring two orthogonal components of the velocity at multiple positions in the flow volume, the accuracy and usefulness of the flow measurement increase significantly over single or non-orthogonal component approaches.
The subject architecture can be combined with CARS (coherent anti-Stokes Raman spectroscopy) to provide temperature and composition of the measured flow. The system is also capable of characterizing high-velocity flames, up to 2,400 K, which is useful in analyzing high-speed combustion in fighter jet engines, scramjet engines, and even potentially in gas turbines.
This work was done by Paul M. Danehy and Joseph W. Lee of Langley Research Center and Daniel Bivolaru of The George Washington University — Hampton, VA. LAR-17235-1