77 Noise Control and Noise Evaluation in Aircraft Engines Tatsuya ISHII, Aircraft Propulsion Research Center, E-mail: [email protected] Keywords: Noise reduction, active noise cancellation, turbo-machinery noise, source location. 1. Introduction Aircraft engine noise emitted for example by the jet exhaust, fan, compressor, turbine and combustor is the predominant factor in total aircraft noise during take-off and landing. As a result of enormous efforts to alleviate engine noise, noise levels have been improved by more than 20 dB compared to the first generation of airliners. However, the growing volume of air transport means that further noise reduction is still required. With this background, we decided to concentrate on two technical aspects of fulfilling this demand for further noise reduction. One is active noise cancellation technology (ANC) which has become more probable noise -proofing treatment in the last two decades. In comparison with passive methods, ANC has the potential to optimize reduction performance by adapting to the noise source. Fundamental research on ANC started around ten years ago and demonstrated noise reduction with a ducted fan. Application of ANC to existing turbo-machines is now the next theme. Another technical aspect of noise reduction is noise measurement technology which would provide precise information on both acoustic modal components and source locations. Experimental procedure based on microphone measurement is studied as the measurement technology. This report summarizes recent ANC and new noise measurement research work. 2. Active Noise Cancellation The active cancellation of spinning acoustic modes, if appropriately combined with existing devices, contributes to reducing the size and weight of silencers, improving reduction efficiency and cutting total costs. The practical application of ANC to turbo-machinery noise is illustrated in Fig.1. We have already completed fundamental experiments with a ducted fan, aimed at verifying the modal cancellation using secondary modes. Several technical concerns have been investigated; measurement of the spinning modes, anti-phased acoustic mode generation and piezoelectric devices for secondary sources and error sensors. Our experimental results have shown that the anti-phased mode, which is driven by 16 loudspeakers, suppresses the primary mode by more than 5 dB with little spillover of secondary sound. The piezoelectric devices, used in sound [Passive Noise Control] Acoustic lining [Active Noise Control] Error sensor Anti-phased sound source Spinning modes D ucted fan (Noise Source) Attenuated modes Fig.1: Application of ANC to a turbo-machine. error microphone acoustic lining small jet engine loudspeaker PZT driver Fig.2: Experimental setup with a small jet engine. sources and error sensors, provided as much noise reduction as that offered by the loudspeakers and microphones. In the next phase of putting this noise cancellation technology into practice, a small jet engine was employed as a noise source with a higher noise level and frequency. Sound pressure levels and frequency band were investigated to design noise control devices as shown in Fig.2. In front of the engine inlet, 8 loudspeakers together with 8 piezoelectric sound sources are installed on a duct. 3. Noise Measurement This document is provided by JAXA. 78 Experimental technique for modal detection and source location is a powerful tool in forecasting precisely the characteristics of noise and allows countermeasures to be taken with less time and cost. In 2001, development on th e simultaneous multiple-channel measurement and data analysis was carried out through two measurement programs. One measurement was taken using a YJ69 turbojet engine. A set o f microphones was fixed at around 3 m from the nozzle exit. An example of this measurement is plotted in the contour map in Fig.3. The concerning noise is jet noise at a relatively low frequency, and the operating condition was 50 % - rating. Although there were certain limitations to accuracy due to the number and position of microphones, the noise source was roughly determined just downstream of the nozzle exit. Similar results were obtained under other conditions. The other measurement is a wind tunnel test with a scaled fuselage model coupled to a small jet engine. A 32 -microphone section is mounted about 1.5 m below the fuselage and engine model as shown in Fig.4. An example of jet noise measurement is plotted in Fig.5. The rotation speed of the small jet engine is 110,000 rpm. The mean flow rate around the model and the attack angle of the model are 40 m/s and 0 deg respectively. A noise source of around 3260 Hz was detected downstream of the engine nozzle. The compressor noise of the engine, at a higher frequency band, is also successfully detected immediately upstream of the engine. Fig.3: Example of measurement with the YJ69. Microphone Small jet engine Fig.4: Microphones and a fuselage model combined with a small jet engine. Array-Type-1(G), N=110,000rpm, V_LWT=40m/s, fc=3260Hz 0.5 References 1. T. Ishii, et. al, “Modal Technique for Active Control of Tones Radiated from a Ducted Fan”, Journal of Aircraft, 1998, 35-5, pp.754-760. 2. T. Ishii et al. “Active Control of Spinning Modes Caused by a Ducted Fan”, Trans. JSME, 2000, B66-641, pp.133-140. 3. T. Ishii, K. Nagai, H. Oinuma, and K. Takeda, “Research on Control and Evaluation of Noise Radiated from Aircraft Engines”, NAL -SP -52, 2001, pp.59-62 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 X (m) . Fig.5: Example of measurement with the fuselage and jet engine model. This document is provided by JAXA.