Rayleigh Scattering for Measuring Flow in a Nozzle Testing Facility

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