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Leonardo Times JUNE 2013INTRODUCTION
Air traffi c has been increasing steadily in the last decades, which had adverse ef-fects on community noise near airports. Noise mitigation measures are frequently studied based on contour maps showing a long-term averaged noise metric like the LDEN. Examining eff ects of noise mitiga-tion measures based on such an averaged metric will exclusively allow determina-tion of the average eff ects rather than lo-cal eff ects.
The local eff ect of a noise mitigation mea-sure can be studied with more realism by using a high-fi delity simulation model that predicts the actual sound signature, instead of a noise metric, at a particular position. Developments to that end, pre-dicting the audible sound at a reason-able computational time, are ongoing at both the National Aeronautics and Space Administration (NASA) and the Dutch
Na-tional Aerospace Laboratory (NLR). The simulation environment at the NLR is called the Virtual Community Noise Simulator (VCNS). In 2007, the NLR ob-tained the VCNS from NASA. At NASA the idea to presents audible fl yover results emerged. They combined this with a vir-tual reality environment as to present the actual fl yover results in a realistic setting. In the VCNS, a test subject can experience fl yover noise in a virtual reality environ-ment. The system is based on the basis of seperately modeling the source noise and atmospheric propagation eff ects. In this article a quick introduction into the mod-elling steps, necessary for the functioning of the VCNS, is given.
SOURCE NOISE PREDICTION
An aircraft has diff erent noise generat-ing parts. The two most prominent ones can be classifi ed as the engine (jet
mix-ing and fan noise) and the airframe noise. The source is represented in the VCNS as a compact source, i.e. all sound is assumed to be emitted from one position. This is a far-fi eld assumption that is valid for the distances considered in typical fl yover situations.
Airframe noise
Airframe noise is generated by the tur-bulent wake coming from the gears, wings and high-lift devices. In general it is broadband of nature, i.e. containing a wide range of frequencies. Cavities on an aircraft, for instance the area behind an extended slat, are however known to pro-duce tones as well. As cavities are diff erent for every aircraft, airframe noise is usually very aircraft specifi c. Experimental and theoretical research is on-going in these areas and empirical models are made based upon these results.
Jet noise
The engine is modeled from two individ-ual noise generating components, the jet mixing noise and the fan noise. Jet mix-ing noise is generated by the mixmix-ing of the turbulent airstreams leaving the gas turbine. Sir James Lighthill was the fi rst to describe this complex phenomenon from a theoretical perspective in 1952 thereby, single-handedly, starting the research fi eld of aero-acoustics! Interested students are referred to [1] as to see what it takes to start a new research fi eld.
One of his breakthrough results was that a jet exhausts emits noise that scales pro-portional to the velocity diff erence of the
Aircraft noise imposes restrictions on possible growth of airports. Noise mitigation
measures can be based long-term predictive models but would benefi t from high-fi delity
simulation of the audible eff ects. To this end the NLR uses its Virtual Community Noise
Simulator (VCNS). Several modelling steps and an application will be demonstrated in
this article as to show the promising future of aircraft noise synthesis.
TEXT Michael Arntzen, PhD student aviation acoustics at the Air Transport & Operations department, executing his research at the NLR.
NOISE SYNTHESIS
Figure 1. The directivity pattern typical for a gasturbine engine in take-off conditions. The angle is measured from the inlet, i.e. 0 deg. means forward and 180 deg. means aft radiated sound.
for the Virtual Community Noise Simulator
MICHAEL
JUNE 2013 Leonardo Times
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jet and the ambient atmosphere to the eighth power. This is why today’s high-bypass engines are quieter than their straight-jet counterparts.
Fan noise
Whereas jet mixing noise is generally of a broadband nature, the fan emits both broadband as well as tonal components. The main tonal component is caused by fl uctuating aerodynamic forces resulting from unsteady wakes fl owing of the fan rotor vanes onto the stator vanes. If the fan tip speed exceeds the speed of sound, shock waves occur on the fan rotor blades that result in tonal noise as well. For this noise generating mechanism, popularly referred to as Buzz-Saw noise, the fan must be spinning at a high speed. Consequent-ly, this sound is usually present at take-off conditions and radiates almost exclusively forward. An example on the directivity of fan noise is presented by Figure 1.
SYNTHESIS AND PROPAGATION
In noise synthesis, a distinction is made between the synthesis of the noise at the source and propagation phenomena. Both are treated separately in the VCNS.
Synthesis
Having a source noise prediction is the start of the actual sound synthesis. Tonal components can be synthesized by ap-plying the correct frequency and ampli-tude characteristics to a basic cosine wave form. This is called additive synthesis. For broadband noise a diff erent method has to be used since a wide range of fre-quencies are present. It is assumed that broadband noise is similar to white-noise, i.e. containing random phase and equal intensity for all frequencies. Since this is easily generated in the frequency domain, the predicted noise spectra for airframe and engine noise can easily be applied through convolution. Using an inverse Fourier transform, the acoustic signal in the time-domain is re-constructed.
Propagation
If the synthesized signal at the source is known, it has to be propagated to the ground through the atmosphere. Using acoustic ray tracing, the eff ects of wind and temperature on the produced sound fi eld are calculated. For instance, aircraft usually take off in headwind conditions. This has an eff ect on the acoustic fi eld as well. Ray tracing allows calculating how an acoustic wave-front propagates through the atmosphere. Due to wind and tem-perature eff ects the commonly assumed straight line path is not valid, as is visual-ized in Figure 2.
From Figure 2 it becomes clear that curved sound rays cause areas on the ground where no sound is present. This is accen-tuated by the red ground surface. In these acoustic “shadow” zones, less sound is present than would be predicted by a tra-ditional straight-ray approach. Modifi ca-tions like this are audible and have to be taken into account in noise synthesis. Ex-amples of these calculations can be found in [2].
APPLICATION
It is hard to show results of noise synthesis. Probably the best way to visualize the data is to produce a spectrogram. A spectro-gram shows, as a function of time (aircraft passage), the Sound Pressure Level (SPL) at diff erent frequencies. Results of a recent synthesis eff ort are shown in Figure 3. In Figure 3, the distinctive line at 2600 Hz is tonal noise that is both present in the synthesis as well as the measurement. This tone is from fan rotor-stator wake in-teraction and starts around 2600 Hz after which it is Doppler shifted towards 1900 Hz as the aircraft fl ies over. Some diff er-ences remain between the synthesis and the measurement. The main elements are however nicely captured and confi rm the promising possibilities of aircraft noise synthesis. Eff orts are currently underway to assess where diff erences emanate from and which further improvements can be
made and will be published in the near future.
Furthermore, we would like to point out a recent study by NASA. [3] Rather than syn-thesizing regular aircraft they successfully synthesized a blended-wing-body to com-pare its noise reduction to a regular (Boe-ing 777) aircraft. These (audible) results are available on the internet together with movies of the virtual environment. [4]
CONCLUSIONS
Aircraft noise synthesis allows study-ing new procedures or aircraft designs without taking measurements. As such it becomes possible to hear future aircraft designs that are still on the drawing table or to evaluate noise mitigation measures without an extensive test campaign. Fu-ture research is directed towards making more realistic predictions and to further study atmospheric eff ects that modify the aircraft sound. If you have further ideas or want to contribute to this research as a graduate student, contact the author for further information. MICHAEL ARNTZEN MICHAEL ARNTZEN NLR References
[1] Lighthill, M.J., “On sound generated aerodynamically I: General Theory”, Proceedings of the Royal Society of London, Series A: Mathematical and physical sciences, pp. 564-572, 1952. [2] Arntzen, M., Rizzi, S.A., Visser, H.G., and Simons, D.G., “A framework for sim-ulation of aircraft fl yover noise through a non-standard atmosphere”, 18th AIAA/ CEAS Aeroacoustics Conference, AIAA 2012-2079, Colorado Springs, CO. [3] S.A. Rizzi, A. Aumann, L.V. Lopes, C.L. Burley, “ Auralization of Hybrid Wing Body Aircraft Flyover Noise from System Noise Predictions”, 51st AIAA ASM meet-ing, AIAA-2013-0542, Grapevine, Texas. [4] Aircraft fl yover simulation, http:// stabserv.larc.nasa.gov/fl yover/, NASA, 2013.
Figure 3. The synthesized signal of an aircraft (top picture) compared to a recording (lower picture).
Figure 2. Sound rays emanating from a source (fl ying left to right) in head-wind con-ditions. For clarity, the rays directly below the source have been omitted; the varying color refers to a diff erent initial direction of the sound. The red part of the ground, where no rays reach the ground, is referred to as an acoustic shadow zone.
