Turbine Noise: Its Significance in the Civil Aircraft Noise Problem

The authors show the presence of noise from the turbine of a turbojet or turbofan engine to be a significant contributor the overall engine noise. They review currently available information from both full-scale engines and model turbines and correlate it along lines following those previously developed for fans and compressors.

Analytical Prediction of Fan/Compressor Noise

The authors present an analytical treatment of the blade passing frequency noise generated in fans and compressors and describe, briefly, a theoretical model of the noise generation mechanisms, based on unsteady aerodynamics coupled with spinning mode theory. They show how the blade passing frequency noise is basically related to the aerodynamics and the geometry of the turbomachinery and present a numerical solution of the three-dimensional wave equation. The results of a study on sound transmission in a turbomachinery duct indicate that the length of the inlet/exhaust duct, as well as the duct configuration, can be of primary importance relative to sound transmission for certain areas of engine operation. The theoretical model is successfully checked against the experimental results obtained on a number of General Electric and NASA fans and compressors.

Lifting Fan Noise Studies

The paper reviews the results of an experimental investigation of noise radiated from a VTOL lifting fan inlet under static inflow conditions. The results are discussed in the light of recent theoretical and experimental conclusions on compressor noise radiation, combined with relevant data on the aerodynamic characteristics of the fan. The presence of high level discrete tones, up to four harmonics of the blade passing frequency superimposed over a broadband noise extending from 200 to 15,000 cps, is attributed to close spacing between rotor and stator blades, Low levels of broadband noise were observed to be caused by low levels of turbulence at the intake. Analysis of the limited results shows the variation of broadband noise as the sixth power of the velocity of the flow relative to the blade.

Sound Attenuation in Acoustically Treated Turbomachinery Ducts

The paper presents a theoretical development, accompanied by an experimental data check, of the attenuation in acoustically lined ducts with no airflow. The results indicate that the number of spinning lobes, as well as the generation frequency, have a major effect on the performance of the acoustic material. It also was indicated that the impedance components of the acoustic lining can be optimized to give a maximum sound energy reduction over a given range of engine speeds.

Discrete Frequency Noise Generation due to Fluid Flow Over Blades, Supporting Spokes, and Similar Bodies

The author reviews, briefly, previous work on the excitation of powerful acoustic resonances by periodicity in the wakes shed by blades, flat plates, spokes, or other similar bodies in a fluid flow. He presents further experimental data, from low-speed wind-tunnel tests, that add to the understanding of the phenomenon, but there is still a great deal that is not understood regarding the mechanism of excitation. While mechanical vibration can be excited, it has been established conclusively that this does not play any part in generating the intense discrete frequency noise observed in many cases. In the present tests, the amplitude of forced vibration of a plate due to the pressure fluctuations at an acoustic resonance was an order of magnitude greater than the amplitude of the directly excited mechanical resonance.

Low-Noise Propulsion Systems for Subsonic Transports

The authors present a brief review of the preliminary design studies that led to a definition of specifications for a low-noise output turbofan engine for use on long-range subsonic transport aircraft. Data on low-speed fans, with and without acoustic treatment in the fan ducting, indicate that overall noise output of four-engine long-range transport aircraft can be reduced 20 perceived noise decibels by appropriate fan design and the use of nacelle acoustic treatment.

Procedure for Optimum Design in Relation to Noise for Low-Speed Ducted Fans

A previously published ducted fan design theory is used to design a series of related fans in which certain parameters are varied systematically. A previously published equation for the calculation of fan noise is then used to estimate the noise generated by each of these fan configurations. The results suggest how the various design parameters influence noise generation. Due to a lack of experimental data, the fan noise equation has not yet been fully developed and verified. Nevertheless, it could already prove useful as a qualitative guide in selecting fan design parameters for minimum noise.