A molecular Rayleigh scattering based air density measurement system was built in a large nozzle and engine component test facility for surveying supersonic plumes from jet-engine exhaust. The facility (see Figure 1) can test nozzles up to 8.75 in. (22.2-cm) in diameter. It is enclosed in a 7.5-ft (2.3- m) diameter tank where ambient pressure is adjusted to simulate engine operation up to an altitude of 48,000 ft (14,630 m). The measurement technique depends on the light scattering by gas molecules present in the air; no artificial seeding is required. Commercially available particle-based techniques, such as laser Doppler velocimetry and particle image velocimetry, were avoided for such reasons as requirement of extremely large volume of seed particles; undesirable coating of every flow passages, model, and test windows with seed particles; and measurement errors from seed particles not following the flow. The molecular Rayleigh-scattering-based technique avoids all of these problems; however, a different set of obstacles associated with cleaning of dust particles, avoidance of stray light, and protection of the optical components from the facility vibration need to be addressed
To avoid a problem with facility vibration, light from a single-mode continuous-wave laser was transmitted into the vacuum tank by the use of an optical fiber. It was then collimated and passed through the plume. Rayleigh-scattered light from various points along the collimated beam was collected by a set of collection lenses placed outside the vacuum tank and measured by a photomultiplier tube (PMT). Large glass windows on the tank provided optical access. The collimator for the transmitted beam and the light-collection optics were placed on two synchronized traversing units to enable a survey over a cross-section of the nozzle plume. Although the technique is suitable to measure velocity, temperature, and density, in this first entry only air density was measured by monitoring intensity of the scattered light. Excellent comparison between theoretically predicted variation and the measured data along the centerline of a highly under expanded supersonic jet provided validation to the measurement technique (see Figure 2).
This work was done by Carlos R. Gomez of Glenn Research Center and Jayanta Panda of the Ohio Aerospace Institute. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. Inquiries concerning rights for the commercial use of this invention should be addressed to
: NASA Glenn Research Center Innovative Partnerships Office Attn: Steve Fedor Mail Stop 4–8 21000 Brookpark Road Cleveland
Refer to LEW-17627-1.