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Technical Paper

Reconstruction of Noise Source in a Ducted Fan Using a Generalized Nearfield Acoustical Holography

The identification of the propulsion noise of turbofan engines plays an important role in the design of low-noise aircraft. The noise generation mechanisms of a typical turbofan engine are very complicated and it is not practical, if not impossible, to identify these noise sources efficiently and accurately using numerical or experimental techniques alone. In addition, a major practical concern for the measurement of acoustic pressure inside the duct of a turbofan is the placement of microphones and their supporting frames which will change the flow conditions under normal operational conditions. The measurement of acoustic pressures on the surface of the duct using surface-mounted microphones eliminates this undesirable effect. In this paper, a generalized acoustical holography (GAH) method that is capable of estimating aeroacoustic sources using surface sound pressure is developed.
Technical Paper

Model Order Reduction Using Basis Expansions for Near field Acoustic Holography

The identification/localization of propulsion noise in turbo machinery plays an important role in its design and in noise mitigation techniques. Near field acoustic holography (NAH) is the process by which all aspects of the sound field can be reconstructed based on sound pressure measurements in the near field domain. Identification of noise sources, particularly in turbo-machinery applications, efficiently and accurately is difficult due to complex noise generation mechanisms. Backward prediction of the sound field closer to the source than the measurement plane is typically an unstable “ill-posed” inverse problem due to the presence of measurement noise. Therefore regularized inversion techniques are typically implemented for noise source reconstruction. Another major source of ill-posedness in NAH inverse problems is a larger number of unknowns (sources) than available pressure measurements. A model reduction technique is proposed in this paper to address this issue.
Journal Article

High Frequency Vibro-Acoustic Analysis Using Energy Finite Element Method

As an alternative method to Statistical Energy Analysis (SEA), Energy Finite Element Method (EFEM) offers several unique advantages for vibro-acoustic analysis of structural-acoustic systems. In this paper, the theory of the energy finite element method is overviewed. The main developments of a recently available EFEM code are presented. This is followed by the investigation of several example problems using EFEM; (a) the acoustic pressure computation in an acoustical duct, (b) the sound transmission loss of an automotive dash, and (c) the vibro-acoustic analysis of a truck cab. The EFEM predictions are compared to the analytical solutions, SEA predictions or test data and good correlations are observed. Further, the advantages of EFEM in the solution of high and middle frequency vibro-acoustic problems are discussed.
Technical Paper

Vibro-acoustic Analysis Using a Hybrid Energy Finite Element /Boundary Element Method

A hybrid method is developed by combining energy finite element method (EFEM) and energy boundary element method (EBEM) to predict interior noise of structural-acoustic systems at high frequencies. In the hybrid EFEM/EBEM method, the structural domain of the system is modeled by structural finite elements, and the acoustic domain is modeled by acoustic boundary elements. The structural vibration response is computed from EFEM. The interior sound pressure level in the acoustic domain is recovered using EBEM. To validate the hybrid method, the interior noise levels in simplified airplane cabin and van models are computed and compared with that of EFEM only model. Good correlations are observed.
Technical Paper

Sound Power Sensitivity Analysis and Design Optimization Using BEM

A general numerical formulation based on the Boundary Element Method (BEM) for computing radiated sound power sensitivity is presented in this paper. The total radiated sound power is computed using surface acoustic pressure and velocity information. Explicit analytical differentiation of the sound power with respect to acoustic normal velocities is performed on the boundary integral equations to obtain sound power sensitivity information. The formulation is applicable to structures with arbitrary geometries, free edges, openings, and multiple connections. Acoustic absorption materials applied on the structure surface can also be modeled as impedance boundary conditions. The developed formulation is validated and its application is demonstrated.
Technical Paper

A Fast Algorithm For Evaluating BEM-based Nearfield Acoustic Holography Transfer Functions

Formulation of a fast algorithm for evaluating the Nearfield Acoustic Holography (NAH) transfer functions based on boundary integral equations is presented. The new algorithm overcomes some of the well known numerical difficulties exhibited in the traditional Boundary Element Method (BEM) based NAH transfer function formulations such as excessive computational cost, non-uniqueness problem, and regularization of hypersingular integrals. The formulation is based on deriving transfer functions between acoustic field pressure and surface layer potentials using modified Helmholtz/Kirchhoff integral equations. Example problems are analyzed using both traditional and new algorithms. Solution time and NAH reconstruction results are compared to validate and demonstrate the effectiveness of the new algorithm. Finally, the acoustic holography reconstruction is applied for an automotive powerplant.
Technical Paper

Noise Source Identification in an Automotive Powerplant

An application involving noise source reconstruction on a full automotive powerplant including the engine, manifolds and the transmission is considered herein, to demonstrate the versatility of modern generalized acoustical holography. The complex source geometry necessitates measurements on non-conforming surfaces. The acoustic pressures were experimentally acquired at three different engine excitations. Accelerometers were mounted at select locations on the powerplant in order to study the accuracy of the reconstructed vibrations from acoustical holography. Through a series of synthetically generated holograms with added random noise, it is conclusively demonstrated that the error margins in the reconstructed vibrations on the powerplant are consistent with errors in reconstructed vibrations from numerically synthesized holograms of a similar Signal to Noise Ratio (SNR).
Technical Paper

Effect of Seating Foam on the Acoustic Behavior of an Earth-Moving Cab

The designs of vehicle seats have significant impact on interior acoustic modes as well as sound pressure level inside the vehicles. Seats trimmed with elastic porous materials are especially critical to the acoustic behavior of the vehicles due to the sound absorption of the materials. This paper demonstrates how seating materials affect the acoustic performance of an earh-moving cab. To accurarely simulate the sound absorption of the seat, the seat was modeled as a bulk reactive absorber instead of a local reactive absorber.
Technical Paper

Noise Source Identification in a Cab Interior

Nearfield Acoustical Holography (NAH) has traditionally been utilized in the identification of noise sources on separable geometry of the wave equation. Recent advances have utilized the Boundary Element Method (BEM) to extend the source identification to noise sources with arbitrary geometry. However, this generalized NAH leads to the solution of a discrete ill-posed problem that requires solution through Singular Value Decomposition (SVD) in conjunction with numerical regularization. Robust numerical regularization schemes have recently been implemented in commercial software COMET/Acoustics® [1, 2] so as to fully automate the noise source identification procedure, and render it applicable to complex, practical problems. An application involving noise source identification on the interior of an earthmoving equipment cab is presented to demonstrate the capability of generalized NAH. The NAH reconstructed velocities on the surface of the cab are compared with the input velocities.