Figure 1 schematically depicts an experimental setup in which Rayleigh scattering from molecules of a flowing gas is used to measure the temperature and one component of the velocity of the gas. The Rayleigh-scattering apparatus in this setup is capable of operation in the harsh environments (varying temperatures and intense vibrations and sound) commonly found in aerospace test facilities.
The power of Rayleigh-scattered light is proportional to the gas density, the spectral width of the scattered light is related to the gas temperature, and the shift in the frequency of the spectral peak is proportional to one component of the bulk velocity. Because the molecules of the flowing gas of interest are Rayleigh scatterers, no seeding of the flow is necessary. The concept of using molecular Rayleigh scattering to measure temperature and velocity has been reported previously. The novel aspect of the present Rayleigh-scattering apparatus lies in (1) the manner in which its delicate optical and electronic equipment are protected from the harsh flow-test environment and (2) the manner in which the Rayleigh-scattering spectrum is acquired and analyzed.
At the protected location, light from an argon-ion laser is focused by lens 1 into an input optical fiber, which delivers the light to the test location. There, the light is collimated by lens 2, then directed through a polarizing beam splitter. The light reflected by the beam splitter is directed into a light trap (not shown in the figure); this light must be trapped because even though it would not contribute to the Rayleigh-scattering signal, it would contribute to detected stray light if it became depolarized when scattered from surfaces near the probe volume (the small volume from which Rayleigh scattering is to be measured). The beam transmitted by the beam splitter is focused into the probe volume by lens 3. Rayleigh-scattered light is collected by lens 4 and focused by lens 5 into an output optical fiber, which delivers the light back to the protected location.
The light coming out of the output optical fiber at the protected location is collimated by lens 7, then passed through a planar mirror Fabry-Perot interferometer operated in the static, imaging mode, then focused by lens 8 into a charge-coupled-device (CCD) camera cooled by liquid nitrogen.
Additional optics are included to provide for a reference Fabry-Perot image of non-Rayleigh-scattered laser light. These optics include a mirror that intercepts laser light coming out of the polarizing beam splitter. The light reflected by the mirror is focused by lens 6 through a neutral-density filter (if needed to reduce the intensity of the reference light) and a diffuser located in front of the output optical fiber. The diffusely scattered laser light then enters the fiber. The mirror, diffuser, and neutral-density filter are mounted on remotely controlled pneumatic linear actuators so that they can be moved into position for acquiring the reference image or moved out of the way for Rayleigh-scattering measurements. A prism assembly can be placed in the path between the Fabry-Perot interferometer and the CCD camera to direct light into a standard video camera.
The Fabry-Perot interferometer is aligned automatically on the reference image, then locked in alignment for acquisition of Rayleigh-scattering Fabry-Perot images. A five-parameter mathematical model of the instrument function is fitted to the digitized reference image. Thereafter, each Rayleigh-scattering Fabry-Perot image is digitized, then processed by an algorithm that, using the instrument-function parameters, finds the best fit of the image to a five-parameter mathematical model of the Rayleigh-scattering Fabry-Perot image; the fitting parameters are the amplitude of Rayleigh scattering, velocity, temperature, amplitude of stray laser light, and uniform background (see Figure 2).
In a test on a subsonic free air jet, velocities determined by Rayleigh scattering were in good agreement (within 5 m/s) with velocities calculated from isentropic-flow relations, but temperatures determined by Rayleigh scattering were not in equally good agreement. In a test on slowly flowing heated air, the velocities determined by Rayleigh scattering again agreed within 5 m/s, but the temperatures determined by Rayleigh scattering exhibited systematic errors on the order of 10 to 15 percent. Thus, it appears that the apparatus is capable of accurate velocity measurements, but that additional development of the temperature-measurement capability is needed.
This work was done by Richard G. Seasholtz and Lawrence C. Greer III of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
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