Beamforming for measurements under disturbed propagation conditions using numerically calculated Green’s functions

Beamforming methods for sound source localization are usually based on free-field Green’s functions to model the sound propagation between source and microphones. This assumption is known to be incorrect for many industrial applications and the beamforming results can suffer from this inconsistency regarding both, main lobe width and dynamic range. The aim of this paper is to investigate whether the use of numerically calculated Green’s functions, which include the diffraction and reflection of the sound path between source and microphones, can improve the results of beamforming measurements. The current test cases of numerical and experimental investigations consist of a source placed in a short rectangular duct. The measurements are performed outside the duct in a semi-anechoic chamber. A typical example for this kind of installation is a fan with a heat exchanger. The Green’s functions for this test case are calculated numerically using the boundary element method. These tailored Green’s functions are used to calculate the corresponding beamforming steering vectors. Beamforming measurements are performed in this paper using a loudspeaker mounted in a disc as a reference source in the heat exchanger duct. The measurements are performed both with stationary and rotating disc. The stationary measurements are evaluated in the frequency domain. For the evaluation of the rotating measurements, a new beamforming method in the time domain is presented. This method also uses the stationary Green’s functions, which were calculated numerically in the frequency domain. It is also shown how the weighting of these tailored Green’s functions can be done for time domain beamforming. By means of different validation criteria it can be shown that the results with the numerical calculated Green’s functions are improved compared to free field beamforming. This is true both in the stationary and rotating case

Sound shielding simulation by coupled discontinuous Galerkin and fast boundary element methods.

A code coupling has been established for performing efficient fan tone shielding simulations of aerial vehicles with unconventional engine installations. In particular, the Fast Multipole Boundary Element Method (FM-BEM) which is formulated to solve a surface integral based on the Kirchhoff-Helmholtz wave equation for large geometries is combined with a volume resolving Discontinuous Galerkin (DG) method which is well suited for the compact region around a jet engine intake where strong mean flow gradients are present. The Möhring-Howe acoustic analogy is utilised during the back- ward data exchange process for derivation of acoustic velocities in presence of a mean flow. The method can help to overcome a major difficulty related to computational complexity when solving an noise shielding and scattering problems for a complete aircraft geometry

Validation of DLR's sound shielding prediction tool using a novel sound source

This paper is concerned with an experimental validation methodology for DLR’s acoustic shielding prediction boundary element and raytracing codes BEMPAR and SHADOW respectively. These codes in turn will be integrated into the overall aircraft noise prediction tool PANAM of DLR. The presented validation concept is based on a novel laser-based sound source. Almost perfect monopole-type test signals may be produced with a frequency content up to roughly 100kHz in combination with a very small source extension. These characteristics make this technique especially attractive for shielding/installation tests, which typically have to be performed at relativey small scale. BEMPAR is a boundary element code (BEM) which solves for the scattered pressure field. Three generic test cases are evaluated, a circular plate, a long cylinder and DLR’s low noise aircraft (LNA-1) nacelle model. The outcome of this investigation clearly demonstrate the potential of BEMPAR for the prediction of installation effects

State-of-the-art CAA approaches

Stochastic approaches for airframe noise predictio

Monopole Diffraction by Generic Plane Geometries

The aircraft industry is facing growing complaints on aircraft noise from the residents around airports. Therefore novel aircraft architectures ("Low Noise Aircraft" - LNA) with unconventional engine positioning are considered to benefit from engine noise reduction through shielding effects. During a one year cooperation between DLR and Airbus France, an effort was undertaken to select the best methodology for the assessment of noise shielding effects for LNA configurations. The main achievements of this cooperation were (i) the benchmarking of already available tools for the prediction of noise shielding effects and (ii) the generation of an experimental database for validation. In a "Rear-Fuselage-Nacelle" configuration the engines are mounted above the rear part of the fuselage. For such a configuration, a shielding effect is expected from the fuselage and the empennage. In this article, we will focus on the shielding effect by the empennage. For an initial estimation of noise shielding, the overall aircraft contour can be simulated by simplified geometries and the engines be represented by idealized compact noise sources