Page 18 - Volume 9, Issue 3

much as 3 dB OASPL. Calculation of the changes in wave packet behavior as a function of the actuator jet forcing fre- quency and amplitude should guide strategies for producing greater coupling between different wave packets and larger sound reductions.
Jet Noise Reduction through Corrugations and Fluidic Inserts
Instead of modifying the flow at the nozzle exit as in fluid injection, other attempts to reduce the jet noise are employed prior to the nozzle exit. Tactical aircraft engines involve supersonic, heated jets exhausting from convergent- divergent nozzles with flaps and seals to permit changes in
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the nozzle area ratio. Seiner et al. demonstrated that the
replacement of the seals by corrugated inserts could reduce the noise of these engines on take-off when the engines are generally operating in an over-expanded mode (the nozzle exit pressure is less than the ambient, causing the familiar diamond shock cells visible in Figure 1 to form). The corru- gations, seen in an engine nozzle schematic in Figure 14, are thought to reduce noise through two mechanisms. First, the corrugations change the effective nozzle area ratio, thereby weakening the shock cell strength and reducing broadband shock-associated noise, which primarily radiates in the for- ward direction. Second, jet mixing noise in the aft direction is reduced by the generation of streamwise vortices on the corrugated surfaces, which breaks up the LSS turbulence responsible for the noise radiation in the peak noise direc- tion. The change in effective area ratio causes the jet to be
closer to an ideally-expanded condition, which actually improves engine performance. Because of the noise reduc- tion yielded by the previous corrugations, the U.S. Naval Air Systems Command has recently further tasked the National Center for Physical Acoustics to develop and test additional
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and corrugations is being developed by researchers at The Pennsylvania State University (PSU). Because the mechanical corrugations were designed for take-off conditions, noise increase and engine performance degradation at other oper- ating points are possible. Consequently, PSU researchers63 are building on this noise reduction concept by replacing the corrugations with fluidic inserts. These inserts, which have an advantage in that they would be able to be actively altered as needed, are created by injecting air into the divergent sec- tion of the nozzle. At present, two injectors are used for each fluidic insert, as shown in the schematic in Figure 14. The pressures and total mass flow rates required for the injection are relatively low and could be accommodated by available engine air. Figure 14 shows the effectiveness of three fluidic inserts on the radiated noise as a function of polar angle from the jet axis for two different azimuthal angles. The OASPL in the peak emission direction is reduced by 5-6 dB and the broadband shock-associated noise at larger angles to the jet downstream axis is almost eliminated. Furthermore, comple- mentary computational fluid dynamics simulations have shown an increase in thrust for this jet operating and injector conditions. Although the use of only three inserts results in
corrugation designs.
An approach that combines ideas from fluid injections
Acoustical Society of America Books,
Paper Colletions, Demos,Videos
Highlighting the collection is the Scientific Papers of Lord Rayleigh written from 1869-1919 on topics such as sound, mathematics, general mechanics, hydro-dynamics, and properties of gasses and Collected Works of Distinguished Acousticians - Isadore Rudnick with over 100 papers covering Rudnick’s research in physical acoustics. The ASA collection includes Auditory Demonstrations containing demos of various characteristics of hearing and Measuring Speech Production demonstrations for use in teaching courses on speech acoustics, physiology, and instrumentation.
Historical works include: Study of Speech and Hearing at Bell Telephone Laboratories containing
historical documents from AT&T archives. Technical Memoranda issued by the Acoustics Research Laboratory-Harvard University between 1946 and 1971 on topics such as bubbles, cavitation, and properties of solids, liquids, and gasses. Proceedings of the 1994 Sabine Centennial Symposium including papers covering virtually every topic in architectural acoustics and the Proceedings of the ASA’s 75th Anniversary.
The VHS videos in the collection are Speech Perception presented by Patricia K. Kuhl and Fifty Years of Speech Communication with lectures by distinguished researchers covering the development of the field of Speech Communication.
To view Tables of Contents and Prefaces and to purchase these publications please visit www.abdi-ecommerce10.com/asa/.
Jet Noise from Military Aircraft 17