A nonintrusive optical point-wise measurement technique utilizing the principles of molecular Rayleigh scattering was developed at the NASA Glenn Research Center to obtain turbulent temperature, velocity, and density fluctuation measurements in unseeded gas flows. This type of information is necessary for validating computational fluid dynamics and computational aeroacoustic codes. Dynamic property measurements allow the calculation of statistical quantities such as power spectra and mean square fluctuations. These types of measurements are used for jet noise studies in which sound pressure fluctuations are correlated with flow property fluctuations to ascertain the sources of noise in free jet flows.
Rayleigh scattering spectrum.
Molecular Rayleigh scattering is the result of elastic light scattering from gas molecules. When light from a single-frequency laser beam passes througha gas, the scattered light is shifted in frequency by the Doppler effect because of the bulk motion of the molecules. The optical frequency spectrum of Rayleigh-scattered light contains information about the gas density, bulk velocity, and temperature. The graph shows a Rayleigh scattering spectrum containing the narrow laser line and the broadened Rayleigh spectral peak. If the gas composition is known, the total intensity of the Rayleigh spectrum is directly proportional to the gas density. The frequency shift between the laser peak and the Rayleigh peak is proportional to the bulk flow velocity. The width of the spectral peak is broadened by thermal motion of the molecules and, hence, is related to gas temperature. The spectra of the laser light and the Rayleigh-scattered light are analyzed using a Fabry-Perot interferometer operated in the static imaging mode. The resulting circular fringe pattern contains spectral information about the light (see the following illustration).
Dissection of Fabry-Perot fringe pattern into four annular regions; PMT, photomultiplier tube.
Concentric angled mirror system used to dissect the Fabry-Perot fringe pattern as shown in the preceding figure.
The interference fringe pattern is divided into four concentric regions (see the preceding illustration) using a series of mirrors angled with respect to one another (see the preceding photograph). Light from each of these regions is directed toward photomultiplier tubes and sampled at rates up to 16 kHz using photon-counting electronics. Monitoring the width of the spectrum allows for measurement of the gas temperature. Monitoring the spectral peak location provides a measure of a single component of the flow velocity. Independently monitoring the total scattered intensity provides a measure of the gas density. Gas temperature, velocity, and density were measured in a low-speed heated air jet (left graph) as well as in an acoustically modulated nozzle flow (right graph).
Left: Temperature fluctuation profiles measured by Rayleigh scattering and constant current anemometry at an axial location four diameters downstream of the heated jet exit. Right: Velocity power spectra calculated from Rayleigh and hotwire velocity data acquired at a 16-kHz sampling rate in acoustically modulated nozzle flow at the given excitation frequencies.
Upon completion of additional development and validation tests, this measurement system will be implemented in the Aero-Acoustic Propulsion Laboratory and other facilities at Glenn for use in jet noise research. These tests are related to milestones under the Supersonics and Subsonic Fixed Wing thrust areas.
Mielke, Amy F.; and Elam, Kristie A.: Molecular Rayleigh Scattering Diagnostic for Measurement of High Frequency Temperature Fluctuations. Proc. SPIE Int. Soc. Opt. Eng., vol. 5880, 2005, pp. 1-12.
Mielke, A.; Elam, K.; and Sung, C.: Molecular Rayleigh Scattering Diagnostic for Dynamic Temperature, Velocity, and Density Measurements. AIAA-2006-2969, 2006.
Mielke, A.; Elam, K.; and Sung, C.: Rayleigh Scattering Diagnostic for Measurement of Temperature, Velocity, and Density Fluctuation Spectra. AIAA-2006-0837, 2006.
Panda, J.; and Seasholtz, R.G.: Experimental Investigation of Density Fluctuations in High-Speed Jets and Correlation With Generated Noise. J. Fluid Mech., vol. 450, 2002, pp. 97-130.
Seasholtz, R.G.; Panda, J.; and Elam, K.A.: Rayleigh Scattering Diagnostic for Measurement of Velocity and Density Fluctuation Spectra. AIAA-2002-0827, 2002.
Find out more about the research of Glenn’s Optical Instrumentation & NDE Branch: http://www.grc.nasa.gov/WWW/OptInstr/
Glenn contact:
Amy F. Mielke, 216-433-6757, Amy.F.Mielke@nasa.gov
Authors:
Amy F. Mielke and Kristie A. Elam
Headquarters program office:
Aeronautics Research Mission Directorate
Programs/projects:
Supersonics, Hypersonics, Subsonic Fixed Wing, Independent Research and Development
Last updated: December 14, 2007
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