NASA Glenn researchers are using specialized labs to lessen jet engine noise. This work aims to create quieter flight conditions for communities living near major airports.
The 17,000-wedge acoustic shield in the Aero-Acoustic Propulsion Laboratory
The Aero-Acoustic Propulsion Laboratory utilizes a massive 65-foot radius geodesic dome to conduct aeroacoustic tests on aircraft engine components. To ensure that testing environments remain free of echoes, the facility employs a sophisticated absorption system. According to the NASA Glenn Research Center report, the interior of the dome is outfitted with 17,000 custom-designed, 2-foot thick fiberglass wedges mounted to the walls and floor to absorb sound and prevent reflections.
This controlled environment allows researchers to configure the space as either a hemi-anechoic or anechoic chamber.. The laboratory serves as a central hub for testing various facilities, including:
- The 9-by-15-Foot Low Speed Wind Tunnel
- The Small Hot Jet Acoustic Rig (SHJAR)
- The Nozzle Acoustic Test Rig (NATR)
- The DGEN AeroPropulsion Research Turbofan
- Rayleigh Scattering and Raman Scattering for optical measurements
- Digital Schleiren imaging
- Real-time Particle Image Velocimetry (PIV) to track turbulent velocities
Mach 2.0 testing at the Small Hot Jet Acoustic Rig
The Small Hot Jet Acoustic Rig (SHJAR) provides a critical link between nozzle geometry and the resulting noise produced by jet flow. This rig is capable of testing single stream nozzles up to 5 cm in diameter at speeds of Mach 2.0 and temperatures of 800 K. By simulating these extreme conditions, scientists can better understand the complex physics of turbulence.
To map the sources of noise, researchers have deployed advanced measurement techniques that allow for high-fidelity data collection. As the report details, these methods include:
These tools have enabled breakthroughs, such as the 2006 milestone where researchers used time-resolved PIV to measure space-time correlations in hot jets—a key statistic for noise production.
The 2019 upgrades to the 9-by-15-Foot Low Speed Wind Tunnel
The 9-by-15-Foot Low Speed Wind Tunnel is specifically engineered to study aircraft engine aerodynamics during the critical takeoff and landing phases. Because these phases are often the loudest, the tunnel's ability to provide a controlled environment is essential for reducing the noise impact on nearby residential communities. To enhance this capability, major modifications were completed in 2019 to improve airflow and lower the facility's background noise levels.
Recent technical improvements also include the deployment of a new 5-sensor traversing microphone rake. This hardware allows for more precise measurements of sound produced by scale experimental propulsors, providing the granular data necessary to refine engine designs before they reach full-scale production.
The gap between SHJAR scale models and full-scale engines
While the research conducted at NASA Glenn is groundbreaking , several questions remain regarding the transition from laboratory success to commercial reality. Who will ultimately implement these new noise prediction codes in the manufacturing of commercial jetliners? The source notes that better codes are needed to help engineers design quieter nozzles, but it does not specify if these tools are being shared with private aerospace giants like Boeing or Airbus.
Additionally, the report does not clarify how much of the noise reduction will come from nozzle design versus the "jet-surface interaction" identified in SHJAR tests. While researchers have found that the interaction between a jet plume and an aircraft's wings or flaps creates a fundamentally different noise mechanism, it is still unknown how much of this interaction can be mitigated through standard engine modifications.
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