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 fluctuation measurements in unseeded gas flows. This type of information is necessary for validating computational fluid dynamics and computational aeroacoustic codes. Dynamic temperature 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 temperature 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 through a 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 preceding 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. A Fabry-Perot interferometer (see the figure on the left) operated in the static imaging mode is used to analyze the spectra of the laser light and the Rayleigh scattered light. The resulting circular fringe pattern contains spectral information about the light (see the figure on the right).
Left: Fabry-Perot interferometer; fC, collimating lens focal length; fL, fringe-forming lens focal length. Right: Dissection of Fabry-Perot fringe pattern into four annular regions. PMT, photomultiplier tube.
A series of mirrors angled with respect to one another are used to divide the interference fringe pattern containing spectral information into four concentric regions (see the final figure). Light from each of these regions is directed toward photomultiplier tubes and sampled at 10 kHz using photon-counting electronics. Monitoring the relative change in intensity within each region allows for measurement of the gas temperature. Independently monitoring the total scattered intensity provides a measure of gas density. In addition, this technique has the potential to simultaneously measure a single component of flow velocity by monitoring the spectral peak location. Gas temperature and density were measured using a low-speed heated air jet surrounded by an unheated air coflow.
Mielke, A.F.; and Elam, K.A.: Molecular Rayleigh Scattering Diagnostic for Measurement of High Frequency Temperature Fluctuations. Proceedings of SPIE Optics and Photonics Conference, vol. 5880, 2005.
Mielke, Amy F., et al.: Time-Average Measurement of Velocity, Density, Temperature, and Turbulence Velocity Fluctuations Using Rayleigh and Mie Scattering. Exp. Fluids,vol. 39, no. 2, 2005, pp. 441-454.
Panda, J., et al.: Time-Averaged Velocity, Temperature and Density Surveys of Supersonic Free Jets. Proceedings of the ASME Heat Transfer/Fluids Engineering Summer Conference 2004, Charlotte, NC, 2004, pp. 811-817.
Panda, J., et al.: Effect of Heating on Turbulent Density Fluctuations and Noise Generation From High Speed Jets. AIAA-2004-3016, 2004.
Mielke, A.; and Seasholtz, R.: Time-Average Molecular Rayleigh Scattering Technique for Measurement of Velocity, Density, Temperature, and Turbulence Intensity in High Speed Nozzle Flows. AIAA-2004-0706, 2004.
Panda, J.; Seasholtz, R.; and Elam, K.:Further Progress in Noise Source Identification in High Speed Jets Via Causality Principle. AIAA-2003-3126, 2003.
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.
Glenn contact: Amy F. Mielke, 216-433-6757, Amy.F.Mielke@nasa.govLast updated: October 6, 2006
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