Measuring Flow Using Spectrally Resolved Rayleigh Scattering

Lewis Research Center, Cleveland, Ohio

An experimental study has been conducted to demonstrate the feasibility of using spectrally resolved Rayleigh scattering to nonintrusively measure the instantaneous properties of the flow in a small supersonic wind tunnel. Unlike conventional probe flow measurements, Rayleigh scattering of a laser beam does not perturb the flow. Because the Rayleigh scattering uses the actual gas molecules that make up the flow under study, it is not necessary to seed the flow. Another important advantage of this technique is that properties of the flow can be determined simultaneously at multiple locations along the laser beam; moreover, if the laser beam is pulsed, then these flow properties can be obtained at multiple locations with a single laser pulse.

The principle of operation can be explained with the optical setup used in the feasibility study, as illustrated schematically in the figure. The main light beam for measuring the flow is generated by a pulsed, injection-seeded, frequency-doubled Nd:YAG laser, which is tuned to an absorption spectral band of iodine for the purpose of subsequent filtering, as explained below. By use of a series of lenses and mirrors, the beam is focused to a line, introduced into the wind tunnel, and directed upstream along the test section of the wind tunnel. As described thus far, this arrangement provides for Rayleigh scattering measurements from a number of regions along the illuminated downstream-to-upstream line. If the laser were sufficiently powerful, the beam-forming optics could be set to expand the laser beam to a sheet in the test section, making it possible to measure Rayleigh scattering in hundreds of regions within the test section.

The CCD Camera Is Aimed at the Measurement Volume through a Fabry-Perot interferometer and an iodine absorption cell to obtain an image of the measurement volume in Rayleigh-scattered light. The measurement volume can be any convenient region along the laser beam in the test section of the wind tunnel.

The Rayleigh-scattering spectrum is directly related to the velocity distribution of the illuminated molecules; it contains information about the temperature, bulk velocity, and density of the fluid in the measurement volume. Light from the measurement volume is focused into an intensified charge-coupled-device (CCD) camera via a Fabry-Perot interferometer. The width of recorded spectrum is related to the gas temperature, the shift of the spectral peak is proportional to one component of the bulk velocity, and the total intensity is proportional to the gas density. However, the recorded image also includes unwanted light that is not the result of Rayleigh scattering, but is caused by spurious laser scattering from windows and internal wind-tunnel surfaces. This unwanted laser light is at the laser frequency. Because the Fabry-Perot is not selective enough to eliminate this non-Rayleigh scattered light, an iodine absorption cell is placed in front of the Fabry-Perot to block light at the laser frequency.

Success in the use of the iodine absorption cell depends on knowledge of the Nd:YAG-laser frequency for each measurement. In this setup, part of the CCD is used to record simultaneously the Fabry-Perot interference-fringe patterns of (1) the unshifted light from the Nd:YAG laser, (2) light from a frequency-stabilized HeNe laser, and (3) light collected from the tunnel and filtered through the iodine cell. The frequency of the light from the Nd:YAG laser can be determined by analysis of the fringe patterns.

The apparatus performed well in the feasibility experiments. One disadvantage of this technique is the need for postprocessing of data to perform the fringe-pattern analysis to determine the frequency of the Nd:YAG laser; an on-line laser-frequency readout subsystem would be desirable in future implementations. A problem in this particular experiment was the relatively low mach number (about 2.0), which necessitated the assumption of adiabatic flow to avoid indeterminacy between velocity and temperature in the data-reduction process; however, at higher mach numbers, such indeterminacy would not occur, and therefore velocity and temperature could be determined independently of each other. The feasibility study also revealed the desirability of several improvements, including frequency stabilization of the Nd:YAG laser, vibration isolation of the Fabry-Perot interferometer, automatic (instead of time-consuming manual) alignment of the Fabry-Perot, and on-line data reduction.

This work was done by Richard G. Seasholtz and Alvin E. Buggele, of Lewis Research Center and Mark F. Reeder, a National Research Council Associate. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 176 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

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