The Acoustics Branch at the NASA Glenn Research Center conducts research to understand the mechanisms responsible for noise generation and assembles computational capability to predict and simulate the noise produced by aerospace systems. The results of these efforts are tools and technologies that allow products, such as aircraft, to have reduced noise emissions. Part of this effort involves the study of innovative noise-reduction technologies, at the fundamental level, to understand their effects on the entire system and to assess potential noise reductions.
One technology being studied is wake management using trailing edge blowing. When an airfoil, in this case a fan blade, slices through the air to generate the thrust that allows an aircraft to fly, wakes are created behind the airfoil much like those made by a boat in the water. These wakes travel downstream in the airflow and interact with downstream components, especially the downstream stationary airfoils called exit guide vanes. This wake-vane interaction generates one of the most significant noise sources in today’s aircraft, being rivaled only by the exhaust jet noise.
Cross section of model showing path for air (the arrows represent airflow) injected through the fan blade and exiting the trailing edge, resulting in wake filling. This reduces downstream wake strength and turbulence where the wake encounters the exit guide vane, reducing a major noise source in turbofan engines.
A fan with internal passages to carry air to the trailing edge and fill in the wakes was tested in a wind tunnel at Glenn, and acoustic, aerodynamic, and flow-field velocity data were acquired. Some operating configurations of this technology showed 2 dB of overall noise reduction--a meaningful reduction. Data inspection revealed details of how the technology works and possibly allows optimization of the injected flow to minimize system penalties. Computational aerodynamic and acoustic prediction tools were exercised using these data, and improvements in the codes may be implemented to better model advanced concepts.
The fan was designed using advanced three-dimensional computational tools to define the internal passages that carry air, efficiently and at specific flow rates, through the blade and to the trailing edge where it is injected into the flow stream. This fills the wake momentum deficit and reduces noise since the wake-vane interaction is lessened. The illustration shows the passages and injection at the trailing edge for wake filling.
The model was tested in Glenn’s 9- by 15-ft acoustic test section, where the acoustic performance was measured using a traversing microphone, and performance and flow velocity data were acquired to determine the impact on system efficiency and to gain insight into the mechanics of the wake filling. Foremost, the data will be used to assess the potential viability of wake filling as a noise-reduction concept and to determine system impacts. The dataset also can be used for validating and improving analysis codes used to predict the performance of complex concepts. The data indicated reductions in tone noise (peaks), but more importantly, reductions were also observed at nearly all frequencies through 20 kHz for a substantial reduction in broadband noise. The graph shows the overall acoustic merit of the concept.
The data show noise reduction using 2 percent of the fan flow injected along the trailing edge.
Together these tone and broadband reductions provide a 2-dB overall reduction, which is a significant reduction for the initial design iteration of the technology. In future efforts, NASA hopes to refine the concept and maximize the benefits while minimizing or eliminating overall system penalties.
Find out more about this research at Glenn’s Acoustics Branch: http://www.grc.nasa.gov/WWW/Acoustics/
Glenn contacts:
E. Brian Fite, 216-433-3892, Eric.B.Fite@nasa.gov
Richard P. Woodward, 216-433-3923, Richard.P.Woodward@nasa.gov
Gary G. Podboy, 216-433-3916, Gary.G.Podboy@nasa.gov
Author:
E. Brian Fite
Headquarters program office:
Aeronautics Research Mission Directorate
Programs/projects:
Subsonic Fixed Wing
Last updated: December 14, 2007
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