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The main goal of this paper is to develop an efficient numerical algorithm to compute the radiated far field noise provided by an unsteady flow field from bodies in arbitrary motion. The method computes a turbulent flow field in the near fields using a high-order spectral difference method coupled with large-eddy simulation approach. The unsteady equations are solved by advancing in time using a second-order backward difference formulae scheme. The nonlinear algebraic system arising from the time discretization is solved with the nonlinear lower-upper symmetric Gauss-Seidel algorithm. In the second step, the method calculates the far field sound pressure based on the acoustic source information provided by the first step simulation. The method is based on the Ffowcs Williams-Hawkings approach, which provides noise contributions for monopole, dipole and quadrupole acoustic sources. This paper will focus on the validation and assessment of this hybrid approach using different test cases. The test cases used are: a laminar flow over a two-dimensional (2D) open cavity at Re = 1.5 × 103 and M = 0.15 and a laminar flow past a 2D square cylinder at Re = 200 and M = 0.5. In order to show the application of the numerical method in industrial cases and to assess its capability for sound field simulation, a three-dimensional turbulent flow in a muffler at Re = 4.665 × 104 and M = 0.05 has been chosen as a third test case. The flow results show good agreement with numerical and experimental reference solutions. Comparison of the computed noise results with those of reference solutions also shows that the numerical approach predicts noise accurately. [ABSTRACT FROM AUTHOR]

Choosing the right number and type of elements in modern commercial finite element tools is a challenging task. It requires a broad knowledge about the theory behind or much experience by the user. Benchmark tests are a common method to prove the element performance against analytical solutions. However, these tests often analyze the performance only for single elements. When investigating the complete mesh of an arbitrary structure, the comparison of the element's performance is quite challenging due to the lack of closed or fully converged solutions. The purpose of this paper is to show a high-precision comparison of eigenfrequencies of a real structure between experimental and numerical results in the context of an element performance check with respect to a converged solution. Additionally, the authors identify the practically relevant accuracy of simulation and experiment. Finally, the influence of accuracy with respect to the number of elements per standing structural bending wave is shown. [ABSTRACT FROM AUTHOR]

Modal back-scattering matrix is the key component for reverberation predicting and also important for understanding the mechanism of bottom back-scattering. To extract the modal back-scattering matrix from reverberation data is a challening topic. So far, inversion of modal back-scattering matrix from reverberation data is based on some a priori assumptions. A new method for extracting modal back-scattering matrix from reverberation data in shallow-water waveguide is proposed recently [Shang et al., 2002]. In this paper numerical simulation of extracting the modal back-scattering matrix has been performed, it is shown that the inversion by changing the source depth works very well for lower frequency. Alternatively, the inversion by using the reverberation data at different ranges is proposed and it is shown that this approach is suitable on certain conditions. [ABSTRACT FROM AUTHOR]

This paper investigates the prediction of the far-field performances of high frequency projectors using the second source array method (SSAM). The far-field parameters can be calculated accurately using the complex acoustic pressure data of two very close parallel planes which lie in the near-field region of the projector. The paper simulates the feasibility of predicting the far-field parameters such as transmitting voltage response and the far-field directivity pattern. The predicting results are compared with that calculated using boundary element method (BEM). It shows very good agreement between the two methods. A planar high frequency projector is measured using the near-field method. In order to verify the predicting results, the far-field measurement is performed for the same projector. The comparison of the results shows that the near-field method is capable to precisely predict the far-field parameters of the projector. [ABSTRACT FROM AUTHOR]

A numerical study on the active control of a machine suspension system supported on a cylindrical shell aiming to reduce the sound radiation is presented in this paper. In this system, a rigid-body machine is supported on a simply-supported elastic cylindrical shell through four active isolators. A theoretical model is employed and four types of active control strategies including kinetic energy minimization strategy, power flow minimization strategy, squared acceleration minimization strategy and acoustic power minimization strategy are considered, with corresponding active control force obtained by linear quadratic optimal method. Numerical simulations are conducted and detailed results were presented. Active control performance under these four control strategies is compared and analyzed in terms of radiated sound power, and the effect of the number of active actuators is discussed by numerical analysis. The results show that acoustic power minimization strategy has the best performance to reduce the sound power radiated from supporting shell in general. Through numerical simulations, some comprehensive design principles of active control system are discussed at the end. [ABSTRACT FROM AUTHOR]

In this paper, four three-node triangular finite element models which can readily be incorporated into the standard finite element program framework are devised via a multi-field variational functional for the bounded plane Helmholtz problem. In the models, boundary and domain fields are independently assumed. The former is constructed by nodal interpolation and the latter comprises nonsingular solutions of the Helmholtz equation. The equality of the two fields are enforced along the element boundary. Among the four devised models, the most accurate one is 1/3 to 1/2 less erroneous than the conventional single-field model in most examples. [ABSTRACT FROM AUTHOR]

This paper demonstrates an application of computational aeroacoustics to the prediction of noise generated by a round nozzle jet flow. In this study, the nozzle internal flow and the free jet flow outside are computed simultaneously by a high-order accurate, multi-block, large-eddy simulation (LES) code with overset grid capability. To simulate the jet flow field and its radiated noise, we solve the governing equations on approximately 370 million grid points using high-fidelity numerical schemes developed for computational aeroacoustics. Projection of the near-field noise to the far-field is accomplished by coupling the LES data with the Ffowcs Williams-Hawkings method. The main emphasis of these simulations is to compute the jet flow in sufficient detail to accurately capture the physical processes that lead to noise generation. Two separate simulations are performed using turbulent and laminar inflow conditions at the jet nozzle inlet. Simulation results are compared with the corresponding experimental measurements. Results show that nozzle inflow conditions have an influence on the jet flow field and far-field noise. [ABSTRACT FROM AUTHOR]

A three-dimensional wave equation has been introduced by Lee–Schultz–Saad (LSS equation). This equation has been discretized into a difference equation. A stable numerical solution has been introduced to solve this difference equation. A computer model, FOR3D, implements this numerical solution. The FOR3D has been used for various applications in the acoustic community. It is important and of great interest to find out whether or not the LSS equation is energy-conserving. This paper will discuss fully the energy-conserving property of the LSS equation. [ABSTRACT FROM AUTHOR]

This paper considers how the accuracy of range-dependent propagation modeling affects the results of matched-field inversion (MFI) for seabed geoacoustic parameters. In MFI, the forward problem of computing the acoustic fields associated with candidate geoacoustic models is solved a large number of times. Given significant mismatch due to measurement and theory errors, together with the computationally intensive nature of MFI, the appropriate tradeoff between modeling accuracy and computational speed is not obvious. This tradeoff is considered here in terms of the degradation in information content for the geoacoustic parameters that results from inaccurate propagation modeling. The information content is quantified using the marginal posterior probability distributions of the geoacoustic parameters, as computed from a fast Gibbs sampling approach to Bayesian inversion. A synthetic example of this analysis is presented in which the parabolic equation is used to model acoustic fields for a shallow-water, upslope environment, with different levels of modeling accuracy/speed controlled by the range and depth step sizes of the computational grid. [ABSTRACT FROM AUTHOR]

In predicting wave propagations in either direction, the size of the angle of propagation plays an important role; thus, the concept of wide-angle is introduced. Most existing acoustic propagation prediction models do have the capability of treating the wide-angle but the treatment, in practice, is vertical. This is desirable for solving two-dimensional (range and depth) problems. In extending the two-dimensional treatment to 3 dimensions, even though the wide-angle capability is maintained in most 3D models, it is still vertical. Owing to the need of a wide-angle capability in the azimuth direction, this paper formulates an azimuthal wide-angle wave equation whose theoretical development is presented. An illustrative example is included to demonstrate the need for such azimuthal wide-angle capability. Also, a comparison is shown between results using narrow-angle and wide-angle equations separately. [ABSTRACT FROM AUTHOR]

A new deconvolution algorithm for retrieving a sparse reflectivity series from noisy seismic traces is proposed. The problem is formulated as a constrained minimization, taking the approximation zero norm of reflectivity as the objective function. The resulting minimization is solved efficiently by the trust-region based sequential quadratic programming (SQP) method, which provides global convergence and local quadratic convergence rates under suitable assumptions. The null space decomposition method and the de-biasing method are employed to reduce computational complexity and further improve the calculation accuracy. Synthetic simulations indicate that the spikes on the reflectivity, both their positions and amplitudes, are recovered effectively by the proposed approach. [ABSTRACT FROM AUTHOR]

Acoustic liners are widely used in commercial aero-engine to suppress noise. In theoretical investigations, the liner geometry is often assumed as an array of symmetric micro resonator with orifice or slit at the center. However, in real application, orifices or slits distributed in micro resonator are offset. For better understanding of sound absorption mechanism of micro resonator with offset slits under high incident sound pressure level (SPL), direct numerical simulations (DNS) using high order low dispersion and low dissipation computational aeroacoustics (CAA) method are carried out. The simulations are first validated by experimental data, showing good agreement and establishing the relevance of the simulation methodology. Numerical simulations of resonators with single offset slit or two slits are then conducted. The two sound absorption mechanisms, namely viscous dissipation and vortex shedding, are discussed with detailed numerical data and analysis, which lead to quantitative parametric description of the energy partition between the two mechanisms as a function of both frequency and geometry. It is shown that offset slit can reduce vortex shedding and results in less sound absorption. The effects of more than one slit are, however, opposite; more vortex shedding occurs with more slits so that sound absorption is enhanced. This may potentially help guide liner design in practical applications. [ABSTRACT FROM AUTHOR]

This work studies the acoustic imaging problem with compressed sensing (CS) and microphone arrays. The CS algorithm with Basis Pursuit (BP) algorithm has shown satisfying results in acoustic imaging, the maps of which are characterized by super-resolution. However, the performance of the CS algorithm with the BP algorithm is limited to Restricted Isometry Property (RIP), and the algorithm has a long CPU-time. We propose a new CS algorithm with Orthogonal Matching Pursuit (OMP) algorithm for acoustic imaging. The performance of the OMP algorithm with regard to RIP is examined through numerical simulation in this work. The simulation results and CPU-time for OMP algorithm are compared with those of the BP algorithm and the conventional beamformer (CBF). When the RIP does not hold, satisfying results can still be obtained by the OMP algorithm, and the CPU-time for OMP algorithm is far less than BP algorithm. In order to validate the feasibility of the OMP algorithm in acoustic imaging, an experiment is also conducted in a semi-anechoic room. Two mobile phones are served as sound sources. We investigate the mobile phones sources and compare the experimental results with those of BP algorithm and CBF method. The OMP algorithm can locate the main sources at low frequencies, while the CBF method can just give a rough indication and fails for low frequencies due to the width of its main lobe. Due to many reconstructed sources outside of the expected source positions existing on the map, the BP algorithm fails to locate the main sources at low frequencies. [ABSTRACT FROM AUTHOR]

In this work, acoustic performances of a liner concept based on perforated screens backed by air cavities are investigated numerically for circular ducts with mean flow. Dimensions of the cavity are chosen to be of the order or bigger than the wavelength so acoustic waves within the liner can propagate parallel to the duct surface. In this case, the liner becomes nonlocally reacting and this gives rise to additional resonance effects which renders the attenuation more effective over a broader frequency range. In order to predict the mufflers' acoustic performances, a special boundary integral method is presented. Using a tailored Green's function for hard wall circular ducts containing uniform mean flow, the numerical technique only requires the discretization of the acoustic velocity potential on both sides of the perforated screen separating the central channel from the air cavities. Comparisons with finite element results show that the proposed method allows accurate results for a relatively modest computational cost. Influence of the mean flow in the central airway, the dimensions of the cavity as well as the nature of the incident field on acoustic performances are also shown and discussed. [ABSTRACT FROM AUTHOR]

A new algorithm involving flank array geometry calibration under strong multipath conditions is proposed to address the problem of installation errors and the shell deformation. The derived linear mapping relationship between geometric error of sensors and signal eigenvector reduces the calculating difficulty in near calibration mode. By regarding the reflection as coherent visual sources in known position, the compensation strategy of strong multipath matching is put forward. Cramer-Rao bound (CRB) analysis for the calibration mode is employed. Numerical simulations and lake trials verify the efficiency of the proposed algorithm and illustrate the performance improvement under strong multipath conditions. [ABSTRACT FROM AUTHOR]

Photoacoustic generation using an optical fiber is a promising approach to meet the challenges in most advanced ultrasonic non-destructive testing (NDT) applications. The energy-absorption layer coated on the fiber, endface plays an important role in converting the laser energy into heat and exciting the acoustic wave. The selection of the absorption material and the optimization of the structure dimension are the keys for achieving high energy-conversion efficiency. A 2D-axisymmetric finite element analysis (FEA) model was established to simulate the photoacoustic generation process using a newly developed absorption material, polydimethylsiloxane/gold nanoparticle (PDMS/Au NPs) nanocomposites, with different thicknesses and laser pulse durations. The experimental results obtained using a similar set-up show a similar trend predicted by this FEA simulation. The FEA results provide practical clues to the design of the fiber-optic photoacoustic generator. [ABSTRACT FROM AUTHOR]

A full three-dimensional parabolic equation model (MONM3D) has been developed that incorporates techniques that reduce the required number of model grid points and reduces computation time. The concept of tessellation is implemented in MONM3D, which allows the number of radial paths in the model grid to vary with range from the source, reducing the number of computational points in the horizontal plane. This design establishes a grid layout that is both numerically and computationally desirable. A benchmark test case is used to illustrate the accuracy and efficiency of the model. [ABSTRACT FROM AUTHOR]

We present physical/mathematical models base on partial differential equations (PDEs) and efficient numerical simulation schemes based on the Finite Element (FE) method for multi-field problems, where the acoustic field is the field of main interest. Acoustics, the theory of sound, is an emerging scientific field including disciplines from physics over engineering to medical science. We concentrate on the following three topics: vibro-acoustics, aero-acoustics and high intensity focused ultrasound. For each topic, we discuss the physical/mathematical modeling, efficient numerical schemes and provide practical applications. [ABSTRACT FROM AUTHOR]

A parallelized algorithm based on an existing 3D wide-angle parabolic equation model is developed to perform numerical simulations of underwater acoustic propagation on a massively parallel computer. The parallelization method used is a suitable two-level procedure: A frequency decomposition and a spatial decomposition of the calculations, which are respectively dedicated to reduce CPU times for broadband and cw signal propagation. The high-performance of the parallelized algorithm is examined for the 3D extension of the classical ASA wedge benchmark. CPU times are reported and both speedup and efficiency are analyzed. An investigation of significant 3D effects at higher frequencies and at longer propagation ranges than in earlier works [F. Sturm, J. Acoust. Soc. Am.117(3) (2005) 1058-1079] is performed with reasonable CPU times by using the new parallel algorithm. Further, the feasability of the procedure applied to a realistic oceanic environment problem involving both real sound speed profiles and bathymetry data sets is also illustrated. [ABSTRACT FROM AUTHOR]