AIAA/CEAS Aeroacoustics Conference 

About: AIAA/CEAS Aeroacoustics Conference is an academic conference. The conference publishes majorly in the area(s): Noise & Jet (fluid). Over the lifetime, 5371 publications have been published by the conference receiving 62362 citations.

CLEAN Based on Spatial Source Coherence

Abstract: To obtain higher resolution acoustic source plots from microphone array measurements, deconvolution techniques are becoming increasingly popular. Deconvolution algorithms aim at identifying Point Spread Functions (PSF) in source plots, and may therefore fall short when actual beam patterns of measured noise sources are not similar to synthetically obtained PSF's. To overcome this, a new version of the classical deconvolution method CLEAN is proposed here: CLEAN-SC. By this new method, which is based on spatial source coherence, side lobes can be removed of actually measured beam patterns. Essentially, CLEAN-SC iteratively removes the part of the source plot which is spatially coherent with the peak source. A feature of CLEAN-SC is its ability to extract absolute sound power levels from the source plots. The merits of CLEAN-SC were demonstrated using array measurements of airframe noise on a scale model of the Airbus A340 in the 8×6 m2 closed test section of DNW-LLF.

A Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) Determined from Phased Microphone Arrays

Abstract: Current processing of acoustic array data is burdened with considerable uncertainty. This study reports an original methodology that serves to demystify array results, reduce misinterpretation, and accurately quantify position and strength of acoustic sources. Traditional array results represent noise sources that are convolved with array beamform response functions, which depend on array geometry, size (with respect to source position and distributions), and frequency. The Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) method removes beamforming characteristics from output presentations. A unique linear system of equations accounts for reciprocal influence at different locations over the array survey region. It makes no assumption beyond the traditional processing assumption of statistically independent noise sources. The full rank equations are solved with a new robust iterative method. DAMAS is quantitatively validated using archival data from a variety of prior high-lift airframe component noise studies, including flap edge/cove, trailing edge, leading edge, slat, and calibration sources. Presentations are explicit and straightforward, as the noise radiated from a region of interest is determined by simply summing the mean-squared values over that region. DAMAS can fully replace existing array processing and presentations methodology in most applications. It appears to dramatically increase the value of arrays to the field of experimental acoustics.

Extensions of DAMAS and Benefits and Limitations of Deconvolution in Beamforming

Abstract: The DAMAS deconvolution algorithm represents a breakthrough in phased array imaging for aeroacoustics, potentially eliminating sidelobles and array resolution effects from beamform maps . DAMAS is an iterative non-negative least squares solver. The original algorithm is too slow and lacks an explicit regularization method to prevent noise amplification. Two extensions are proposed, DAMAS2 and DAMAS3. DAMAS2 provides a dramatic speedup of each iteration and adds regularization by a low pass filter. DAMAS3 also provides fast iterations, and additionally, reduces the required number of iterations. It uses a different regularization technique from DAMAS2, and is partially based on the Wiener filter. Both DAMAS2 and DAMAS3 restrict the point spread function to a translationally-invariant, convolutional, form. This is a common assumption in optics and radio astronomy, but may be a serious limitation in aeroacoustic beamforming. This limitation is addressed with a change of variables from (x,y,z) to a new set, (u,v,w). The concepts taken together, along with appropriate array design, may permit practical 3D beamforming in aeroacoustics.

Effect of Directional Array Size on the Measurement of Airframe Noise Components

Abstract: A study was conducted to examine the effects of overall size of directional (or phased) arrays on the measurement of aeroacoustic components An airframe model was mounted in the potential core of an open-jet windtunnel, with the directional arrays located outside the flow in an anechoic environment Two array systems were used; one with a solid measurement angle that encompasses 316 degrees of source directivity and a smaller one that encompasses 72 degrees The arrays, and sub-arrays of various sizes, measured noise from a calibrator source and flap edge model setups In these cases, noise was emitted from relatively small, but finite size source regions, with intense levels compared to other sources Although the larger arrays revealed much more source region detail, the measured source levels were substantially reduced due to finer resolution compared to that of the smaller arrays To better understand the measurements quantitatively, an analytical model was used to define the basic relationships between array to source region sizes and measured output level Also, the effect of noise scattering by shear layer turbulence was examined using the present data and those of previous studies Taken together, the two effects were sufficient to explain spectral level differences between arrays of different sizes An important result of this study is that total (integrated) noise source levels are retrievable and the levels are independent of the array size as long as certain experimental and processing criteria are met The criteria for both open and closed tunnels are discussed The success of special purpose diagonal-removal processing in obtaining integrated results is apparently dependent in part on source distribution Also discussed is the fact that extended sources are subject to substantial measurement error, especially for large arrays

Impedance Models in Time Domain including the Extended Helmholtz Resonator Model

Abstract: The problem of translating a frequency domain impedance boundary condition to time domain involves the Fourier transform of the impedance function. This requires extending the definition of the impedance not only to all real frequencies but to the whole complex frequency plane. Not any extension, however, is physically possible. The problemshouldremain causal, the variables real, andthe wall passive. This leads to necessary conditions for an impedance function. Various methods of extending the impedance that are available in the literature are discussed. A most promising one is the so-called z-transform by Ozyoruk & Long, which is nothing but an impedance that is functionally dependent on a suitable complex exponent e −iωκ . By choosing κ a multiple of the time step of the numerical algorithm, this approach fits very well with the underlying numerics, because the impedance becomes in time domain a delta-comb function and gives thus an exact relation on the grid points. An impedance function is proposed which is based on the Helmholtz resonator model, called Extended Helmholtz Resonator Model. This has the advantage that relatively easily the mentioned necessary conditions can be satisfied in advance. At a given frequency, the impedance is made exactly equal to a given design value. Rules of thumb are derived to produce an impedance which varies only moderately in frequency near design conditions. An explicit solution is given of a pulse reflecting in time domain at a Helmholtz resonator impedance wall that provides some insight into the reflection problem in time domain and at the same time may act as an analytical test case for numerical implementations, like is presented at this conference by the companion paper AIAA-2006-2569 by N. Chevaugeon, J.-F. Remacle and X. Gallez. The problem of the instability, inherent with the Ingard-Myers limit with mean flow, is discussed. It is argued that this instability is not consistent with the assumptions of the Ingard-Myers limit and may well be suppressed.