Small Hot Jet Acoustic Rig (SHJAR)
SHJAR is one of three rigs installed inside the anechoic dome of the Aeroacoustic Propulsion Lab. It can be used to test single stream nozzles up to 5 cm in diameter, Mach 2.0 and 800 K. SHJAR is a key tool in connecting the dots between nozzle geometry, jet flow, and noise. Creative nozzle designs are inexpensively created for this rig, and their impact on flow and noise is measured with world-leading measurement techniques. Researchers at NASA Glenn have been using SHJAR to develop advanced flow measurement techniques like Rayleigh Scattering, Raman Scattering, digital Schleiren, and real-time Particle Image Velocimetry. Three-dimensional microphone arrays have been deployed to map the sources of the noise. These techniques hold the promise of unlocking some of the mysteries of how the turbulence caused by the nozzle creates noise. The data from this rig is used to validate the computer codes that are being developed to calculate jet noise. Better noise prediction codes will help engineers design quieter nozzles.
Among the many firsts in turbulent jet research produced on SHJAR, time-resolved measurements of turbulent temperature were recently made using Raman-based optical methods, achieving high-frequency measurements of thermodynamic temperature fluctuations needed to complete turbulence models used in heat transfer as well as jet noise. In 2006, researchers had used the first time-resolved particle image velocimetry to measured space-time correlations of turbulent velocities in hot jets, a flow statistic key to the production of noise. Time-resolved velocimetry was also used to map convection speeds of turbulence, another flow metric required by jet noise theory but previously unavailable in hot jets.
The SHJAR is not only a good place to develop advanced diagnostic techniques, it has been a good testbed for exploring canonical problems in jet noise such as how the installation of the jet on an aircraft changes the noise produced by the jet. Jet-surface interaction, where the plume of the jet interacts with the wings, flaps, and tail of an aircraft, was identified as a fundamentally different noise mechanism in SHJAR tests.
Most of these ground-breaking measurement techniques and practical noise insights first developed on the relatively simple SHJAR have been deployed on the high-fidelity Nozzle Acoustic Test Rig (NATR), co-located in the Aero-Acoustic Propulsion Lab.