Corrections for Mirror Sources in Phased Array Processing Techniques AIAA Paper 2003-3308

This technical report (NLR-TP-2003-123) was published by NLR (the National Aerospace Laboratory of the Netherlands) in 2003 and was written by P. Sijtsma and H. Holthusen. When an aero-acoustic source is close to a reflecting wall, results from conventional phased array beamforming techniques are disturbed by the nearby mirror source, which is coherent with the original source. The recalculated source spectrum tends to deviate from the spectrum of the same source obtained in an anechoic environment. A periodic modulation of the spectrum occurs, which is most prominent at low frequencies. In this paper, a number of non-conventional beamforming techniques to correct for this spectral modulation is investigated. First, a technique is discussed which adds the mirror source to the transmission model. It was found that this technique is not very suitable because of its lack of robustness. Then, a more robust beamforming technique is proposed that minimises the influence of a given mirror source. By this technique, much better results were found. Nevertheless, at low frequencies the method still suffers from lack of robustness. Finally, a modification to this minimisation technique is proposed which preserves the robustness. Using this "controlled minimisation" technique, the best agreement was found between the recalculated spectra of a source close to a wall and the same source in anechoic conditions. The beamforming techniques were applied to array measurements on a calibration source in the DNW Low Speed Wind Tunnel LST. [Taken from abstract]. The full text is available as a PDF file.

Damage Risk from Impulse Noise

This web site provides access to a Research and Technology Organization (RTO) Educational Note, RTO-EN-011, dated September 2000. The document was produced by the Human Factors and Medicine Panel (HFM) and the Consultant and Exchange Programme of RTO in support of a Lecture Series presented on 5-6 June 2000 in Maryland, USA and on 15-16 June 2000 in Meppen, Germany. This publication comprises papers from an RTO Lecture Series on Damage Risk From Impulse Noise. High-level impulse noise (weapons noise) can cause auditory as well as non-auditory damage, which may limit combat effectiveness and may result in communication impairments as a consequence of noise-induced hearing loss. Recent research has shown that the present damage risk criteria have to be adjusted. This has major implications for the protective measures that have to be taken when using weapon systems. Protection equipment can be very effective when properly used, but everyday practice shows that the results in the field fall short of what could be achieved. In addition, hearing protection may interfere with communication. New developments in the design of hearing protectors: level dependent, active noise reduction... show how the protection and communication requirements can be combined and satisfied. Educational programs, emphasizing the new developments, may help to improve the effectiveness of hearing conservation and reduce the number of non-auditory accidents. Topics covered by individual papers are: • techniques and procedures for the measurement of impulse noise • a draft ANSI standard on auditory risk criteria • performance of hearing protectors • communication and localisation with hearing protectors • individual susceptibility to noise-induced hearing loss • new perspectives in the treatment of acute noise trauma • cost effectiveness of hearing conservation programmes • non-auditory damage risk assessment for impulse noise. Bibliographic and abstract details are available in HTML format. A table of contents, and the full text of the document (9 Mbytes) can be accessed online in PDF format. The document is contained in the RTO's Full Text Publication Library.

Determination of Absolute Contributions of Aircraft Noise Components using Fly-Over Array Measurements

This technical report (NLR-TP-2004-167) was published by NLR (the National Aerospace Laboratory of the Netherlands) in 2004 and was written by P. Sijtsma and R. W. Stoker. For wind tunnel array measurements, the source power integration technique has proved to be a valuable technique to determine absolute source levels. This paper describes the extension of the source power integration technique to moving sources, and the application to fly-over array measurements on landing aircraft at Amsterdam Airport Schiphol. The technique is applied in combination with a modified version of the Delay-and-Sum beamforming technique, which includes microphone- and frequency-dependent weight factors. These weight factors are used to correct for microphone spatial density and to account for the effects of coherence loss. This beamforming technique works well in combination with the array design, which consists of a number of concentric rings of microphones, with increasing density towards the center. In this paper, it is demonstrated that the extended source power integration technique is able to determine absolute levels from fly-over array measurements, when it is used in combination with the special beamforming technique and the Schiphol array design. Thus, absolute quantification of difference source regions on an aircraft is feasible. Microphone auto-correlations must be included in the beamforming process in order to obtain correct integrated levels, although acoustic images look better when beamforming is done without auto-correlations. The source power integration technique is applied to a Boeing 737-400, and to an Airbus A340. [Taken from abstract]. This is a PDF file, so Adobe Acrobat software will be required in order to read it.

Determination of Absolute Levels from Phased Array Measurements Using Spatial Source Coherence

This technical report (NLR-TP-2002-226) was published by NLR (the National Aerospace Laboratory of the Netherlands) in 2002 and was written by S. Oerlemans and P. Sijtsma. The phased array technique is a valuable tool in acoustic testing for its capability to distinguish between different source locations. However, the interpretation of phased array measurements is still difficult due to the simultaneous occurrence of several effects: the size and level of a spot in a conventional acoustic 'source plot' may be affected by a combination of (1) the limited resolution of the array (2) coherence loss during propagation to the array (3) the spatial extent of the source region. This ambiguity complicates the determination of absolute source levels from phased array measurements. The current paper addresses this problem for a noise source that is extended mainly in one direction, i.e. trailing-edge noise. Simulations are done for a line source, and the influence of array resolution and source coherence length on the array output is investigated. Furthermore, an array processing technique is presented which determines the coherence level between different sources in the scan plane. As a first application, the technique is used to identify mirror sources in a closed wind tunnel. The new method is then applied to trailing-edge noise measurements in NLR's Small Anechoic Wind Tunnel, in order to estimate the spanwise coherence length. In conjunction with the simulations this enables an improved determination of absolute trailing-edge noise from phased array measurements. [Taken from abstract]. The full text is available as a PDF file.

Experimental Techniques for Identification and Characterisation of Noise Sources

This technical report (NLR-TP-2004-165) was published by NLR (the National Aerospace Laboratory of the Netherlands) in 2004 and was written by P. Sijtsma. An introduction is given of phased array beamforming with microphone arrays. Beamforming is considered both in the time-domain and in the frequency-domain. Conventional beamforming techniques are treated, but also more advanced techniques. Special attention is paid to the localisation of moving sources. Applications are shown of wind tunnel measurements and aircraft fly-over measurements. [Taken from abstract]. This is a PDF file, so Adobe Acrobat software will be required in order to read it.

Identification of Noise Sources on Civil Aircraft in Approach using a Phased Array of Microphones

This technical report (NLR-TP-2004-166) was published by NLR (the National Aerospace Laboratory of the Netherlands) in 2004 and was written by Pieter Sijtsma and Jenk M. M. van der Wal. In September 2002, microphone array measurements were performed on landing aircraft at Amsterdam Airport Schiphol. During three days of measurements, 484 fly-over events were recorded successfully. Many aircraft types were included. Most fly-overs were recorded with a large array of 243 microphones. Using a convenient source description, which includes the effect of Doppler frequency shift, acoustic images were obtained through Delay-and-Sum beamforming. The array consisted of a number of concentric rings of microphones. Maximum resolution was achieved by applying weight factors that correct for microphone spatial density. Furthermore, frequency-dependent spatial windowing was applied to account for the effects of coherence loss. Herewith, the lobe widths were constant for a large range of frequencies. The speed and the altitude of the airplanes were determined by a set of light sensors. This light sensor technique enabled an efficient, automatic determination of speed and height, without the cumbersome manual processing that is needed with for instance video cameras. [Taken from abstract]. This is a PDF file, so Adobe Acrobat software will be required in order to read it.

Impact of Acoustic Loads on Aircraft Structures

This is Research and Technology Organization (RTO) AGARD-CP-549, dated September 1994. A broad band of different activities was addressed in the Specialists' Meeting held by the Structures and Materials Panel of AGARD in May 1. Bibliographic and abstract details are available in HTML format. A table of contents, and the full text (152.88MB) of the document can be accessed online in PDF format. The document is contained in the RTO's Full Text Publication Library.

In-Situ Acoustic Impedance Measurements on Air-Injected Liners Under Grazing Flow

This technical report (NLR-TP-2004-146) was published by NLR (the National Aerospace Laboratory of the Netherlands) in 2004 and was written by E. R. Rademaker and E. A. Demmenie. Acoustic liners, that are used in aero-engines and that have an adaptable impedance for different phases of flight, have a strong appeal to the aerospace industry that seeks further ways to reduce noise. A way to control the impedance of a liner is by injecting air into it. This idea - proposed by Tester and Dean in the seventies - has gained renewed interest for application in the nacelle and at the inlet-lip - combined with anti-icing - of turbofans. This has led to the current study that was part of the European “SILENCE(R)” project. The research was aimed at (i) applying an in-situ multiple microphone impedance measurement technique on 3DOF air-injected liners under grazing flow conditions and (ii) improving a prediction model. At low cell or through flow rates, tests in an impedance tube showed good agreement between measured impedances using tube instrumentation and liner in-situ instrumentation. At high flow velocities however, significant differences were found in the measured resistance due to pressure distortions, picked-up by in-situ instrumentation and related to high flow velocities at perforated septa. Results can improve when the diameter of the septum orifices and liner cross-area are reduced. In a wind tunnel, measurements on air-injected liners in grazing flow showed that the through-flow could change the (facing sheet) resistance significantly. An increase or even decrease is possible depending on the ratio of orifice velocity and skin friction velocity (proportional to respectively cell flow and grazing flow Mach number). The results were used to raise an improved, though simple prediction model. [Taken from abstract]. This is a PDF file, so Adobe Acrobat software will be required in order to read it.

Inventory of acoustic fatigue test facilities in the NATO countries

This is Research and Technology Organization (RTO) AGARD-R-584 , dated August 1971. This recent survey carried out by a small specialist group converted by B.L Clarkson, the AGARD Structures and Materials Panel Co-ordinator for Acoustic Fatigue, concluded that there is now active interest and significant work in acoustic fatigue in six of the NATO countries. As a result of this initial survey of interest, a collaborative programme of data collection and compilation has been proposed. Arrangements have been made for the Engineering Sciences Data Unit to undertake the collation of this data and the preparation of design data sheets. Bibliographic and abstract details are available in HTML format. A table of contents, and the full text (2.69MB) of the document can be accessed online in PDF format. The document is contained in the RTO's Full Text Publication Library.

Local Search Methods for Design in Aeronautics

The material in this publication was assembled to support a Lecture Series under the sponsorship of the Applied Vehicle Technology Panel (AVT) and the Consultant and Exchange Programme of RTA presented on 9-10 September 2010 in Berlin, Germany. Published September 2010

Numerical optimisation aims at locating the minima of a regular function (called objective function) on a finite-dimensional design space, while satisfying a certain number of constraints (expressed as inequality verified by the so-called constraint functions). More precisely, local optimisation aims at finding a local optimum in the neighborhood of an initial guess, whereas global optimisation aims at finding the global optimum on the whole design space. These problems are, of course, the mathematical counterparts of mechanical optimisation problems - like drag minimisation of an aircraft or total pressure maximisation of a supersonic aircraft air intake - as soon as (a) a mesh and a simulation tool are available ; (b) the solid shape has been parameterised/a remeshing tool is available to propagate its deformation to the whole mesh ; (c) the objective and constraints have been expressed as functions of the geometry and state variables.

Numerical optimisation for airplane design was used almost as soon as simulation codes appeared. The aerodynamic optimisations carried out by G.N. Van der Plaats at NASA in the mid 70’s illustrate this early interest in optimisation [1]. At that time, 2D and simple 3D configurations were considered, simplex or descent methods were used and the gradients required by descent methods were estimated by the finite-differences. Since then, the framework of aerospace optimisation has known at least three drastic extensions:

1. (1) several global optimisation methods have been defined and intensively used (evolutionary algorithm, particle swarm, ant colony, simulated annealing,...);
2. (2) surrogate functions (neural network, Kriging, polynomial regression, support vector machine,...) have been used for a part of the evaluation of the global optimisation methods leading to significant cost reductions;
3. (3) adjoint vector and direct differentiation method have been defined , studied and more and more often used to computate the gradients necessary for descent algorithms.