Dramatic reductions in civilian aircraft noise have been achieved since the introduction of the turbojet and low-bypass turbofan commercial engines in the late 1950s. A major contributing factor to this decline in noise has been major NASA-sponsored noise-reduction research programs. Over the past two decades, cooperative NASA/industry technology programs - such as the Advanced Subsonic Technology Program, the High-Speed Research Program, and the Vehicle Systems Program - have supported engine noise-reduction research. Recently, a robust, generalized method for predicting aircraft engine core noise was developed and coded into NASA's computer programs used for aircraft noise prediction.

Primary emphasis has historically been directed to the principal turbofan engine noise sources: the jet and the fan. With NASA's 20-year goal to develop technologies for reducing aircraft noise by 20 EPNdB (effective perceived noise in decibels) relative to the 1997 state of the art, this focus has been appropriate. However, as fan and jet noise are reduced, other engine noise sources, albeit lesser, are "uncovered" and become important.

The noise of the engine "core," classified principally as noise emanating from the unsteady heat release of the combustion process, is perhaps the most important of these other propulsion-related sources. Now that many fan and jet noise-reduction technologies have been successful, core noise needs to be addressed in order to meet the Agency's aggressive noise-reduction goal, especially with the advent of modern, high-pressure and high-temperature cores. It is, therefore, important that core noise be better understood and more accurately predicted.

Therefore, NASA sponsored GE Aircraft Engines to conduct acoustic engine measurements and subsequent analytical core noise modeling under a Revolutionary Aerospace Engine Research task order (ref. 1). In this task, which was awarded and monitored by the NASA Glenn Research Center, GE collected static acoustic data from four turbofan engines - the CF34, the CFM56, the CF6, and the GE90 - representing an extremely wide span of thrust classes (see the photograph). Modern Technologies Corporation was subcontracted to develop semiempirical core-noise relationships for each of the engine classes.

The results from the preliminary corenoise models were encouraging, but since correlating core noise is quite complex, there was a need for further improvement. For example, the calibration coefficients and the spectral character of the initial model changed from engine to engine, resulting in a noise-prediction method that was valid only for the individual engine type, rather than a general method for engines of all types and thrust classes. Modern Technologies was subsequently contracted to perform a follow-on task funded by NASA's Quiet Aircraft Technology project (ref. 2). In this effort, Modern Technologies discerned the internal geometric combustor design factors and thermodynamic parameters that influence the core's spectral content and levels. This resulted in a robust, generalized, and more physics-based core-noise-prediction method, which was subsequently coded into NASA's computer programs used for aircraft noise prediction.

References:
1. Data-Based Core Noise Prediction Models by Modern Technologies Corporation. GEAE-NASA RASER Task Order 21: Modern Propulsion System Core Noise Evaluation, 2005 (to be published as a NASA CR).
2. Stone, J.R.; Krejsa, E.A.; and Clark, B.J.: Enhanced Core Noise Modeling for Turbofan Engines. NASA Contract NAS3-00178, Task Order 15, 2005 (to be published as a NASA CR).

Glenn Author and Contact:
Jeffrey J. Berton, 216-977-7031, Jeff.Berton@grc.nasa.gov

Programs/Projects:
Quiet Aircraft Technology Project